linux/kernel/rcutree_plugin.h

2378 lines
68 KiB
C
Raw Normal View History

rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
/*
* Read-Copy Update mechanism for mutual exclusion (tree-based version)
* Internal non-public definitions that provide either classic
* or preemptible semantics.
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*
* Copyright Red Hat, 2009
* Copyright IBM Corporation, 2009
*
* Author: Ingo Molnar <mingo@elte.hu>
* Paul E. McKenney <paulmck@linux.vnet.ibm.com>
*/
#include <linux/delay.h>
#include <linux/gfp.h>
#include <linux/oom.h>
#include <linux/smpboot.h>
rcu: Kick adaptive-ticks CPUs that are holding up RCU grace periods Adaptive-ticks CPUs inform RCU when they enter kernel mode, but they do not necessarily turn the scheduler-clock tick back on. This state of affairs could result in RCU waiting on an adaptive-ticks CPU running for an extended period in kernel mode. Such a CPU will never run the RCU state machine, and could therefore indefinitely extend the RCU state machine, sooner or later resulting in an OOM condition. This patch, inspired by an earlier patch by Frederic Weisbecker, therefore causes RCU's force-quiescent-state processing to check for this condition and to send an IPI to CPUs that remain in that state for too long. "Too long" currently means about three jiffies by default, which is quite some time for a CPU to remain in the kernel without blocking. The rcu_tree.jiffies_till_first_fqs and rcutree.jiffies_till_next_fqs sysfs variables may be used to tune "too long" if needed. Reported-by: Frederic Weisbecker <fweisbec@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> Cc: Chris Metcalf <cmetcalf@tilera.com> Cc: Christoph Lameter <cl@linux.com> Cc: Geoff Levand <geoff@infradead.org> Cc: Gilad Ben Yossef <gilad@benyossef.com> Cc: Hakan Akkan <hakanakkan@gmail.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Kevin Hilman <khilman@linaro.org> Cc: Li Zhong <zhong@linux.vnet.ibm.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Thomas Gleixner <tglx@linutronix.de>
2013-04-12 23:19:10 +00:00
#include <linux/tick.h>
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
#define RCU_KTHREAD_PRIO 1
#ifdef CONFIG_RCU_BOOST
#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
#else
#define RCU_BOOST_PRIO RCU_KTHREAD_PRIO
#endif
#ifdef CONFIG_RCU_NOCB_CPU
static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */
static bool have_rcu_nocb_mask; /* Was rcu_nocb_mask allocated? */
rcu: Make rcu_nocb_poll an early_param instead of module_param The as-documented rcu_nocb_poll will fail to enable this feature for two reasons. (1) there is an extra "s" in the documented name which is not in the code, and (2) since it uses module_param, it really is expecting a prefix, akin to "rcutree.fanout_leaf" and the prefix isn't documented. However, there are several reasons why we might not want to simply fix the typo and add the prefix: 1) we'd end up with rcutree.rcu_nocb_poll, and rather probably make a change to rcutree.nocb_poll 2) if we did #1, then the prefix wouldn't be consistent with the rcu_nocbs=<cpumap> parameter (i.e. one with, one without prefix) 3) the use of module_param in a header file is less than desired, since it isn't immediately obvious that it will get processed via rcutree.c and get the prefix from that (although use of module_param_named() could clarify that.) 4) the implied export of /sys/module/rcutree/parameters/rcu_nocb_poll data to userspace via module_param() doesn't really buy us anything, as it is read-only and we can tell if it is enabled already without it, since there is a printk at early boot telling us so. In light of all that, just change it from a module_param() to an early_setup() call, and worry about adding it to /sys later on if we decide to allow a dynamic setting of it. Also change the variable to be tagged as read_mostly, since it will only ever be fiddled with at most, once at boot. Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2012-12-20 21:19:22 +00:00
static bool __read_mostly rcu_nocb_poll; /* Offload kthread are to poll. */
static char __initdata nocb_buf[NR_CPUS * 5];
#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
/*
* Check the RCU kernel configuration parameters and print informative
* messages about anything out of the ordinary. If you like #ifdef, you
* will love this function.
*/
static void __init rcu_bootup_announce_oddness(void)
{
#ifdef CONFIG_RCU_TRACE
pr_info("\tRCU debugfs-based tracing is enabled.\n");
#endif
#if (defined(CONFIG_64BIT) && CONFIG_RCU_FANOUT != 64) || (!defined(CONFIG_64BIT) && CONFIG_RCU_FANOUT != 32)
pr_info("\tCONFIG_RCU_FANOUT set to non-default value of %d\n",
CONFIG_RCU_FANOUT);
#endif
#ifdef CONFIG_RCU_FANOUT_EXACT
pr_info("\tHierarchical RCU autobalancing is disabled.\n");
#endif
#ifdef CONFIG_RCU_FAST_NO_HZ
pr_info("\tRCU dyntick-idle grace-period acceleration is enabled.\n");
#endif
#ifdef CONFIG_PROVE_RCU
pr_info("\tRCU lockdep checking is enabled.\n");
#endif
#ifdef CONFIG_RCU_TORTURE_TEST_RUNNABLE
pr_info("\tRCU torture testing starts during boot.\n");
#endif
#if defined(CONFIG_TREE_PREEMPT_RCU) && !defined(CONFIG_RCU_CPU_STALL_VERBOSE)
pr_info("\tDump stacks of tasks blocking RCU-preempt GP.\n");
#endif
#if defined(CONFIG_RCU_CPU_STALL_INFO)
pr_info("\tAdditional per-CPU info printed with stalls.\n");
#endif
#if NUM_RCU_LVL_4 != 0
pr_info("\tFour-level hierarchy is enabled.\n");
#endif
if (rcu_fanout_leaf != CONFIG_RCU_FANOUT_LEAF)
pr_info("\tBoot-time adjustment of leaf fanout to %d.\n", rcu_fanout_leaf);
if (nr_cpu_ids != NR_CPUS)
pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%d.\n", NR_CPUS, nr_cpu_ids);
#ifdef CONFIG_RCU_NOCB_CPU
#ifndef CONFIG_RCU_NOCB_CPU_NONE
if (!have_rcu_nocb_mask) {
zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL);
have_rcu_nocb_mask = true;
}
#ifdef CONFIG_RCU_NOCB_CPU_ZERO
pr_info("\tOffload RCU callbacks from CPU 0\n");
cpumask_set_cpu(0, rcu_nocb_mask);
#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ZERO */
#ifdef CONFIG_RCU_NOCB_CPU_ALL
pr_info("\tOffload RCU callbacks from all CPUs\n");
cpumask_setall(rcu_nocb_mask);
#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
#endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
if (have_rcu_nocb_mask) {
cpulist_scnprintf(nocb_buf, sizeof(nocb_buf), rcu_nocb_mask);
pr_info("\tOffload RCU callbacks from CPUs: %s.\n", nocb_buf);
if (rcu_nocb_poll)
pr_info("\tPoll for callbacks from no-CBs CPUs.\n");
}
#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
}
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
#ifdef CONFIG_TREE_PREEMPT_RCU
struct rcu_state rcu_preempt_state =
RCU_STATE_INITIALIZER(rcu_preempt, 'p', call_rcu);
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
DEFINE_PER_CPU(struct rcu_data, rcu_preempt_data);
static struct rcu_state *rcu_state = &rcu_preempt_state;
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
static int rcu_preempted_readers_exp(struct rcu_node *rnp);
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
/*
* Tell them what RCU they are running.
*/
static void __init rcu_bootup_announce(void)
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
{
pr_info("Preemptible hierarchical RCU implementation.\n");
rcu_bootup_announce_oddness();
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
}
/*
* Return the number of RCU-preempt batches processed thus far
* for debug and statistics.
*/
long rcu_batches_completed_preempt(void)
{
return rcu_preempt_state.completed;
}
EXPORT_SYMBOL_GPL(rcu_batches_completed_preempt);
/*
* Return the number of RCU batches processed thus far for debug & stats.
*/
long rcu_batches_completed(void)
{
return rcu_batches_completed_preempt();
}
EXPORT_SYMBOL_GPL(rcu_batches_completed);
/*
* Force a quiescent state for preemptible RCU.
*/
void rcu_force_quiescent_state(void)
{
force_quiescent_state(&rcu_preempt_state);
}
EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
/*
* Record a preemptible-RCU quiescent state for the specified CPU. Note
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
* that this just means that the task currently running on the CPU is
* not in a quiescent state. There might be any number of tasks blocked
* while in an RCU read-side critical section.
rcu: refactor RCU's context-switch handling The addition of preemptible RCU to treercu resulted in a bit of confusion and inefficiency surrounding the handling of context switches for RCU-sched and for RCU-preempt. For RCU-sched, a context switch is a quiescent state, pure and simple, just like it always has been. For RCU-preempt, a context switch is in no way a quiescent state, but special handling is required when a task blocks in an RCU read-side critical section. However, the callout from the scheduler and the outer loop in ksoftirqd still calls something named rcu_sched_qs(), whose name is no longer accurate. Furthermore, when rcu_check_callbacks() notes an RCU-sched quiescent state, it ends up unnecessarily (though harmlessly, aside from the performance hit) enqueuing the current task if it happens to be running in an RCU-preempt read-side critical section. This not only increases the maximum latency of scheduler_tick(), it also needlessly increases the overhead of the next outermost rcu_read_unlock() invocation. This patch addresses this situation by separating the notion of RCU's context-switch handling from that of RCU-sched's quiescent states. The context-switch handling is covered by rcu_note_context_switch() in general and by rcu_preempt_note_context_switch() for preemptible RCU. This permits rcu_sched_qs() to handle quiescent states and only quiescent states. It also reduces the maximum latency of scheduler_tick(), though probably by much less than a microsecond. Finally, it means that tasks within preemptible-RCU read-side critical sections avoid incurring the overhead of queuing unless there really is a context switch. Suggested-by: Lai Jiangshan <laijs@cn.fujitsu.com> Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Peter Zijlstra <peterz@infradead.org>
2010-04-02 00:37:01 +00:00
*
* Unlike the other rcu_*_qs() functions, callers to this function
* must disable irqs in order to protect the assignment to
* ->rcu_read_unlock_special.
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
*/
static void rcu_preempt_qs(int cpu)
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
{
struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
rcu: refactor RCU's context-switch handling The addition of preemptible RCU to treercu resulted in a bit of confusion and inefficiency surrounding the handling of context switches for RCU-sched and for RCU-preempt. For RCU-sched, a context switch is a quiescent state, pure and simple, just like it always has been. For RCU-preempt, a context switch is in no way a quiescent state, but special handling is required when a task blocks in an RCU read-side critical section. However, the callout from the scheduler and the outer loop in ksoftirqd still calls something named rcu_sched_qs(), whose name is no longer accurate. Furthermore, when rcu_check_callbacks() notes an RCU-sched quiescent state, it ends up unnecessarily (though harmlessly, aside from the performance hit) enqueuing the current task if it happens to be running in an RCU-preempt read-side critical section. This not only increases the maximum latency of scheduler_tick(), it also needlessly increases the overhead of the next outermost rcu_read_unlock() invocation. This patch addresses this situation by separating the notion of RCU's context-switch handling from that of RCU-sched's quiescent states. The context-switch handling is covered by rcu_note_context_switch() in general and by rcu_preempt_note_context_switch() for preemptible RCU. This permits rcu_sched_qs() to handle quiescent states and only quiescent states. It also reduces the maximum latency of scheduler_tick(), though probably by much less than a microsecond. Finally, it means that tasks within preemptible-RCU read-side critical sections avoid incurring the overhead of queuing unless there really is a context switch. Suggested-by: Lai Jiangshan <laijs@cn.fujitsu.com> Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Peter Zijlstra <peterz@infradead.org>
2010-04-02 00:37:01 +00:00
rcu: Simplify quiescent-state accounting There is often a delay between the time that a CPU passes through a quiescent state and the time that this quiescent state is reported to the RCU core. It is quite possible that the grace period ended before the quiescent state could be reported, for example, some other CPU might have deduced that this CPU passed through dyntick-idle mode. It is critically important that quiescent state be counted only against the grace period that was in effect at the time that the quiescent state was detected. Previously, this was handled by recording the number of the last grace period to complete when passing through a quiescent state. The RCU core then checks this number against the current value, and rejects the quiescent state if there is a mismatch. However, one additional possibility must be accounted for, namely that the quiescent state was recorded after the prior grace period completed but before the current grace period started. In this case, the RCU core must reject the quiescent state, but the recorded number will match. This is handled when the CPU becomes aware of a new grace period -- at that point, it invalidates any prior quiescent state. This works, but is a bit indirect. The new approach records the current grace period, and the RCU core checks to see (1) that this is still the current grace period and (2) that this grace period has not yet ended. This approach simplifies reasoning about correctness, and this commit changes over to this new approach. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2011-06-27 07:17:43 +00:00
if (rdp->passed_quiesce == 0)
trace_rcu_grace_period("rcu_preempt", rdp->gpnum, "cpuqs");
rcu: Simplify quiescent-state accounting There is often a delay between the time that a CPU passes through a quiescent state and the time that this quiescent state is reported to the RCU core. It is quite possible that the grace period ended before the quiescent state could be reported, for example, some other CPU might have deduced that this CPU passed through dyntick-idle mode. It is critically important that quiescent state be counted only against the grace period that was in effect at the time that the quiescent state was detected. Previously, this was handled by recording the number of the last grace period to complete when passing through a quiescent state. The RCU core then checks this number against the current value, and rejects the quiescent state if there is a mismatch. However, one additional possibility must be accounted for, namely that the quiescent state was recorded after the prior grace period completed but before the current grace period started. In this case, the RCU core must reject the quiescent state, but the recorded number will match. This is handled when the CPU becomes aware of a new grace period -- at that point, it invalidates any prior quiescent state. This works, but is a bit indirect. The new approach records the current grace period, and the RCU core checks to see (1) that this is still the current grace period and (2) that this grace period has not yet ended. This approach simplifies reasoning about correctness, and this commit changes over to this new approach. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2011-06-27 07:17:43 +00:00
rdp->passed_quiesce = 1;
rcu: refactor RCU's context-switch handling The addition of preemptible RCU to treercu resulted in a bit of confusion and inefficiency surrounding the handling of context switches for RCU-sched and for RCU-preempt. For RCU-sched, a context switch is a quiescent state, pure and simple, just like it always has been. For RCU-preempt, a context switch is in no way a quiescent state, but special handling is required when a task blocks in an RCU read-side critical section. However, the callout from the scheduler and the outer loop in ksoftirqd still calls something named rcu_sched_qs(), whose name is no longer accurate. Furthermore, when rcu_check_callbacks() notes an RCU-sched quiescent state, it ends up unnecessarily (though harmlessly, aside from the performance hit) enqueuing the current task if it happens to be running in an RCU-preempt read-side critical section. This not only increases the maximum latency of scheduler_tick(), it also needlessly increases the overhead of the next outermost rcu_read_unlock() invocation. This patch addresses this situation by separating the notion of RCU's context-switch handling from that of RCU-sched's quiescent states. The context-switch handling is covered by rcu_note_context_switch() in general and by rcu_preempt_note_context_switch() for preemptible RCU. This permits rcu_sched_qs() to handle quiescent states and only quiescent states. It also reduces the maximum latency of scheduler_tick(), though probably by much less than a microsecond. Finally, it means that tasks within preemptible-RCU read-side critical sections avoid incurring the overhead of queuing unless there really is a context switch. Suggested-by: Lai Jiangshan <laijs@cn.fujitsu.com> Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Peter Zijlstra <peterz@infradead.org>
2010-04-02 00:37:01 +00:00
current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
}
/*
* We have entered the scheduler, and the current task might soon be
* context-switched away from. If this task is in an RCU read-side
* critical section, we will no longer be able to rely on the CPU to
* record that fact, so we enqueue the task on the blkd_tasks list.
* The task will dequeue itself when it exits the outermost enclosing
* RCU read-side critical section. Therefore, the current grace period
* cannot be permitted to complete until the blkd_tasks list entries
* predating the current grace period drain, in other words, until
* rnp->gp_tasks becomes NULL.
*
* Caller must disable preemption.
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
*/
static void rcu_preempt_note_context_switch(int cpu)
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
{
struct task_struct *t = current;
unsigned long flags;
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
struct rcu_data *rdp;
struct rcu_node *rnp;
rcu: protect __rcu_read_unlock() against scheduler-using irq handlers The addition of RCU read-side critical sections within runqueue and priority-inheritance lock critical sections introduced some deadlock cycles, for example, involving interrupts from __rcu_read_unlock() where the interrupt handlers call wake_up(). This situation can cause the instance of __rcu_read_unlock() invoked from interrupt to do some of the processing that would otherwise have been carried out by the task-level instance of __rcu_read_unlock(). When the interrupt-level instance of __rcu_read_unlock() is called with a scheduler lock held from interrupt-entry/exit situations where in_irq() returns false, deadlock can result. This commit resolves these deadlocks by using negative values of the per-task ->rcu_read_lock_nesting counter to indicate that an instance of __rcu_read_unlock() is in flight, which in turn prevents instances from interrupt handlers from doing any special processing. This patch is inspired by Steven Rostedt's earlier patch that similarly made __rcu_read_unlock() guard against interrupt-mediated recursion (see https://lkml.org/lkml/2011/7/15/326), but this commit refines Steven's approach to avoid the need for preemption disabling on the __rcu_read_unlock() fastpath and to also avoid the need for manipulating a separate per-CPU variable. This patch avoids need for preempt_disable() by instead using negative values of the per-task ->rcu_read_lock_nesting counter. Note that nested rcu_read_lock()/rcu_read_unlock() pairs are still permitted, but they will never see ->rcu_read_lock_nesting go to zero, and will therefore never invoke rcu_read_unlock_special(), thus preventing them from seeing the RCU_READ_UNLOCK_BLOCKED bit should it be set in ->rcu_read_unlock_special. This patch also adds a check for ->rcu_read_unlock_special being negative in rcu_check_callbacks(), thus preventing the RCU_READ_UNLOCK_NEED_QS bit from being set should a scheduling-clock interrupt occur while __rcu_read_unlock() is exiting from an outermost RCU read-side critical section. Of course, __rcu_read_unlock() can be preempted during the time that ->rcu_read_lock_nesting is negative. This could result in the setting of the RCU_READ_UNLOCK_BLOCKED bit after __rcu_read_unlock() checks it, and would also result it this task being queued on the corresponding rcu_node structure's blkd_tasks list. Therefore, some later RCU read-side critical section would enter rcu_read_unlock_special() to clean up -- which could result in deadlock if that critical section happened to be in the scheduler where the runqueue or priority-inheritance locks were held. This situation is dealt with by making rcu_preempt_note_context_switch() check for negative ->rcu_read_lock_nesting, thus refraining from queuing the task (and from setting RCU_READ_UNLOCK_BLOCKED) if we are already exiting from the outermost RCU read-side critical section (in other words, we really are no longer actually in that RCU read-side critical section). In addition, rcu_preempt_note_context_switch() invokes rcu_read_unlock_special() to carry out the cleanup in this case, which clears out the ->rcu_read_unlock_special bits and dequeues the task (if necessary), in turn avoiding needless delay of the current RCU grace period and needless RCU priority boosting. It is still illegal to call rcu_read_unlock() while holding a scheduler lock if the prior RCU read-side critical section has ever had either preemption or irqs enabled. However, the common use case is legal, namely where then entire RCU read-side critical section executes with irqs disabled, for example, when the scheduler lock is held across the entire lifetime of the RCU read-side critical section. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2011-07-18 04:14:35 +00:00
if (t->rcu_read_lock_nesting > 0 &&
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
(t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
/* Possibly blocking in an RCU read-side critical section. */
rdp = per_cpu_ptr(rcu_preempt_state.rda, cpu);
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
rnp = rdp->mynode;
raw_spin_lock_irqsave(&rnp->lock, flags);
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
t->rcu_blocked_node = rnp;
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
/*
* If this CPU has already checked in, then this task
* will hold up the next grace period rather than the
* current grace period. Queue the task accordingly.
* If the task is queued for the current grace period
* (i.e., this CPU has not yet passed through a quiescent
* state for the current grace period), then as long
* as that task remains queued, the current grace period
* cannot end. Note that there is some uncertainty as
* to exactly when the current grace period started.
* We take a conservative approach, which can result
* in unnecessarily waiting on tasks that started very
* slightly after the current grace period began. C'est
* la vie!!!
*
* But first, note that the current CPU must still be
* on line!
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
*/
WARN_ON_ONCE((rdp->grpmask & rnp->qsmaskinit) == 0);
WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
if ((rnp->qsmask & rdp->grpmask) && rnp->gp_tasks != NULL) {
list_add(&t->rcu_node_entry, rnp->gp_tasks->prev);
rnp->gp_tasks = &t->rcu_node_entry;
#ifdef CONFIG_RCU_BOOST
if (rnp->boost_tasks != NULL)
rnp->boost_tasks = rnp->gp_tasks;
#endif /* #ifdef CONFIG_RCU_BOOST */
} else {
list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
if (rnp->qsmask & rdp->grpmask)
rnp->gp_tasks = &t->rcu_node_entry;
}
trace_rcu_preempt_task(rdp->rsp->name,
t->pid,
(rnp->qsmask & rdp->grpmask)
? rnp->gpnum
: rnp->gpnum + 1);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
rcu: protect __rcu_read_unlock() against scheduler-using irq handlers The addition of RCU read-side critical sections within runqueue and priority-inheritance lock critical sections introduced some deadlock cycles, for example, involving interrupts from __rcu_read_unlock() where the interrupt handlers call wake_up(). This situation can cause the instance of __rcu_read_unlock() invoked from interrupt to do some of the processing that would otherwise have been carried out by the task-level instance of __rcu_read_unlock(). When the interrupt-level instance of __rcu_read_unlock() is called with a scheduler lock held from interrupt-entry/exit situations where in_irq() returns false, deadlock can result. This commit resolves these deadlocks by using negative values of the per-task ->rcu_read_lock_nesting counter to indicate that an instance of __rcu_read_unlock() is in flight, which in turn prevents instances from interrupt handlers from doing any special processing. This patch is inspired by Steven Rostedt's earlier patch that similarly made __rcu_read_unlock() guard against interrupt-mediated recursion (see https://lkml.org/lkml/2011/7/15/326), but this commit refines Steven's approach to avoid the need for preemption disabling on the __rcu_read_unlock() fastpath and to also avoid the need for manipulating a separate per-CPU variable. This patch avoids need for preempt_disable() by instead using negative values of the per-task ->rcu_read_lock_nesting counter. Note that nested rcu_read_lock()/rcu_read_unlock() pairs are still permitted, but they will never see ->rcu_read_lock_nesting go to zero, and will therefore never invoke rcu_read_unlock_special(), thus preventing them from seeing the RCU_READ_UNLOCK_BLOCKED bit should it be set in ->rcu_read_unlock_special. This patch also adds a check for ->rcu_read_unlock_special being negative in rcu_check_callbacks(), thus preventing the RCU_READ_UNLOCK_NEED_QS bit from being set should a scheduling-clock interrupt occur while __rcu_read_unlock() is exiting from an outermost RCU read-side critical section. Of course, __rcu_read_unlock() can be preempted during the time that ->rcu_read_lock_nesting is negative. This could result in the setting of the RCU_READ_UNLOCK_BLOCKED bit after __rcu_read_unlock() checks it, and would also result it this task being queued on the corresponding rcu_node structure's blkd_tasks list. Therefore, some later RCU read-side critical section would enter rcu_read_unlock_special() to clean up -- which could result in deadlock if that critical section happened to be in the scheduler where the runqueue or priority-inheritance locks were held. This situation is dealt with by making rcu_preempt_note_context_switch() check for negative ->rcu_read_lock_nesting, thus refraining from queuing the task (and from setting RCU_READ_UNLOCK_BLOCKED) if we are already exiting from the outermost RCU read-side critical section (in other words, we really are no longer actually in that RCU read-side critical section). In addition, rcu_preempt_note_context_switch() invokes rcu_read_unlock_special() to carry out the cleanup in this case, which clears out the ->rcu_read_unlock_special bits and dequeues the task (if necessary), in turn avoiding needless delay of the current RCU grace period and needless RCU priority boosting. It is still illegal to call rcu_read_unlock() while holding a scheduler lock if the prior RCU read-side critical section has ever had either preemption or irqs enabled. However, the common use case is legal, namely where then entire RCU read-side critical section executes with irqs disabled, for example, when the scheduler lock is held across the entire lifetime of the RCU read-side critical section. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2011-07-18 04:14:35 +00:00
} else if (t->rcu_read_lock_nesting < 0 &&
t->rcu_read_unlock_special) {
/*
* Complete exit from RCU read-side critical section on
* behalf of preempted instance of __rcu_read_unlock().
*/
rcu_read_unlock_special(t);
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
}
/*
* Either we were not in an RCU read-side critical section to
* begin with, or we have now recorded that critical section
* globally. Either way, we can now note a quiescent state
* for this CPU. Again, if we were in an RCU read-side critical
* section, and if that critical section was blocking the current
* grace period, then the fact that the task has been enqueued
* means that we continue to block the current grace period.
*/
local_irq_save(flags);
rcu_preempt_qs(cpu);
local_irq_restore(flags);
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
}
/*
* Check for preempted RCU readers blocking the current grace period
* for the specified rcu_node structure. If the caller needs a reliable
* answer, it must hold the rcu_node's ->lock.
*/
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
{
return rnp->gp_tasks != NULL;
}
rcu: Fix grace-period-stall bug on large systems with CPU hotplug When the last CPU of a given leaf rcu_node structure goes offline, all of the tasks queued on that leaf rcu_node structure (due to having blocked in their current RCU read-side critical sections) are requeued onto the root rcu_node structure. This requeuing is carried out by rcu_preempt_offline_tasks(). However, it is possible that these queued tasks are the only thing preventing the leaf rcu_node structure from reporting a quiescent state up the rcu_node hierarchy. Unfortunately, the old code would fail to do this reporting, resulting in a grace-period stall given the following sequence of events: 1. Kernel built for more than 32 CPUs on 32-bit systems or for more than 64 CPUs on 64-bit systems, so that there is more than one rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set to a number smaller than CONFIG_NR_CPUS.) 2. The kernel is built with CONFIG_TREE_PREEMPT_RCU. 3. A task running on a CPU associated with a given leaf rcu_node structure blocks while in an RCU read-side critical section -and- that CPU has not yet passed through a quiescent state for the current RCU grace period. This will cause the task to be queued on the leaf rcu_node's blocked_tasks[] array, in particular, on the element of this array corresponding to the current grace period. 4. Each of the remaining CPUs corresponding to this same leaf rcu_node structure pass through a quiescent state. However, the task is still in its RCU read-side critical section, so these quiescent states cannot be reported further up the rcu_node hierarchy. Nevertheless, all bits in the leaf rcu_node structure's ->qsmask field are now zero. 5. Each of the remaining CPUs go offline. (The events in step #4 and #5 can happen in any order as long as each CPU passes through a quiescent state before going offline.) 6. When the last CPU goes offline, __rcu_offline_cpu() will invoke rcu_preempt_offline_tasks(), which will move the task to the root rcu_node structure, but without reporting a quiescent state up the rcu_node hierarchy (and this failure to report a quiescent state is the bug). But because this leaf rcu_node structure's ->qsmask field is already zero and its ->block_tasks[] entries are all empty, force_quiescent_state() will skip this rcu_node structure. Therefore, grace periods are now hung. This patch abstracts some code out of rcu_read_unlock_special(), calling the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet() from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking task_quiet() from __rcu_offline_cpu() reports the quiescent state up the rcu_node hierarchy, fixing the bug. This ends up requiring a separate lock_class_key per level of the rcu_node hierarchy, which this patch also provides. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12589088301770-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-11-22 16:53:48 +00:00
/*
* Record a quiescent state for all tasks that were previously queued
* on the specified rcu_node structure and that were blocking the current
* RCU grace period. The caller must hold the specified rnp->lock with
* irqs disabled, and this lock is released upon return, but irqs remain
* disabled.
*/
rcu: Rename "quiet" functions The number of "quiet" functions has grown recently, and the names are no longer very descriptive. The point of all of these functions is to do some portion of the task of reporting a quiescent state, so rename them accordingly: o cpu_quiet() becomes rcu_report_qs_rdp(), which reports a quiescent state to the per-CPU rcu_data structure. If this turns out to be a new quiescent state for this grace period, then rcu_report_qs_rnp() will be invoked to propagate the quiescent state up the rcu_node hierarchy. o cpu_quiet_msk() becomes rcu_report_qs_rnp(), which reports a quiescent state for a given CPU (or possibly a set of CPUs) up the rcu_node hierarchy. o cpu_quiet_msk_finish() becomes rcu_report_qs_rsp(), which reports a full set of quiescent states to the global rcu_state structure. o task_quiet() becomes rcu_report_unblock_qs_rnp(), which reports a quiescent state due to a task exiting an RCU read-side critical section that had previously blocked in that same critical section. As indicated by the new name, this type of quiescent state is reported up the rcu_node hierarchy (using rcu_report_qs_rnp() to do so). Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Josh Triplett <josh@joshtriplett.org> Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com> Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12597846163698-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-12-02 20:10:13 +00:00
static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp, unsigned long flags)
rcu: Fix grace-period-stall bug on large systems with CPU hotplug When the last CPU of a given leaf rcu_node structure goes offline, all of the tasks queued on that leaf rcu_node structure (due to having blocked in their current RCU read-side critical sections) are requeued onto the root rcu_node structure. This requeuing is carried out by rcu_preempt_offline_tasks(). However, it is possible that these queued tasks are the only thing preventing the leaf rcu_node structure from reporting a quiescent state up the rcu_node hierarchy. Unfortunately, the old code would fail to do this reporting, resulting in a grace-period stall given the following sequence of events: 1. Kernel built for more than 32 CPUs on 32-bit systems or for more than 64 CPUs on 64-bit systems, so that there is more than one rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set to a number smaller than CONFIG_NR_CPUS.) 2. The kernel is built with CONFIG_TREE_PREEMPT_RCU. 3. A task running on a CPU associated with a given leaf rcu_node structure blocks while in an RCU read-side critical section -and- that CPU has not yet passed through a quiescent state for the current RCU grace period. This will cause the task to be queued on the leaf rcu_node's blocked_tasks[] array, in particular, on the element of this array corresponding to the current grace period. 4. Each of the remaining CPUs corresponding to this same leaf rcu_node structure pass through a quiescent state. However, the task is still in its RCU read-side critical section, so these quiescent states cannot be reported further up the rcu_node hierarchy. Nevertheless, all bits in the leaf rcu_node structure's ->qsmask field are now zero. 5. Each of the remaining CPUs go offline. (The events in step #4 and #5 can happen in any order as long as each CPU passes through a quiescent state before going offline.) 6. When the last CPU goes offline, __rcu_offline_cpu() will invoke rcu_preempt_offline_tasks(), which will move the task to the root rcu_node structure, but without reporting a quiescent state up the rcu_node hierarchy (and this failure to report a quiescent state is the bug). But because this leaf rcu_node structure's ->qsmask field is already zero and its ->block_tasks[] entries are all empty, force_quiescent_state() will skip this rcu_node structure. Therefore, grace periods are now hung. This patch abstracts some code out of rcu_read_unlock_special(), calling the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet() from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking task_quiet() from __rcu_offline_cpu() reports the quiescent state up the rcu_node hierarchy, fixing the bug. This ends up requiring a separate lock_class_key per level of the rcu_node hierarchy, which this patch also provides. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12589088301770-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-11-22 16:53:48 +00:00
__releases(rnp->lock)
{
unsigned long mask;
struct rcu_node *rnp_p;
if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) {
raw_spin_unlock_irqrestore(&rnp->lock, flags);
rcu: Fix grace-period-stall bug on large systems with CPU hotplug When the last CPU of a given leaf rcu_node structure goes offline, all of the tasks queued on that leaf rcu_node structure (due to having blocked in their current RCU read-side critical sections) are requeued onto the root rcu_node structure. This requeuing is carried out by rcu_preempt_offline_tasks(). However, it is possible that these queued tasks are the only thing preventing the leaf rcu_node structure from reporting a quiescent state up the rcu_node hierarchy. Unfortunately, the old code would fail to do this reporting, resulting in a grace-period stall given the following sequence of events: 1. Kernel built for more than 32 CPUs on 32-bit systems or for more than 64 CPUs on 64-bit systems, so that there is more than one rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set to a number smaller than CONFIG_NR_CPUS.) 2. The kernel is built with CONFIG_TREE_PREEMPT_RCU. 3. A task running on a CPU associated with a given leaf rcu_node structure blocks while in an RCU read-side critical section -and- that CPU has not yet passed through a quiescent state for the current RCU grace period. This will cause the task to be queued on the leaf rcu_node's blocked_tasks[] array, in particular, on the element of this array corresponding to the current grace period. 4. Each of the remaining CPUs corresponding to this same leaf rcu_node structure pass through a quiescent state. However, the task is still in its RCU read-side critical section, so these quiescent states cannot be reported further up the rcu_node hierarchy. Nevertheless, all bits in the leaf rcu_node structure's ->qsmask field are now zero. 5. Each of the remaining CPUs go offline. (The events in step #4 and #5 can happen in any order as long as each CPU passes through a quiescent state before going offline.) 6. When the last CPU goes offline, __rcu_offline_cpu() will invoke rcu_preempt_offline_tasks(), which will move the task to the root rcu_node structure, but without reporting a quiescent state up the rcu_node hierarchy (and this failure to report a quiescent state is the bug). But because this leaf rcu_node structure's ->qsmask field is already zero and its ->block_tasks[] entries are all empty, force_quiescent_state() will skip this rcu_node structure. Therefore, grace periods are now hung. This patch abstracts some code out of rcu_read_unlock_special(), calling the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet() from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking task_quiet() from __rcu_offline_cpu() reports the quiescent state up the rcu_node hierarchy, fixing the bug. This ends up requiring a separate lock_class_key per level of the rcu_node hierarchy, which this patch also provides. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12589088301770-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-11-22 16:53:48 +00:00
return; /* Still need more quiescent states! */
}
rnp_p = rnp->parent;
if (rnp_p == NULL) {
/*
* Either there is only one rcu_node in the tree,
* or tasks were kicked up to root rcu_node due to
* CPUs going offline.
*/
rcu: Rename "quiet" functions The number of "quiet" functions has grown recently, and the names are no longer very descriptive. The point of all of these functions is to do some portion of the task of reporting a quiescent state, so rename them accordingly: o cpu_quiet() becomes rcu_report_qs_rdp(), which reports a quiescent state to the per-CPU rcu_data structure. If this turns out to be a new quiescent state for this grace period, then rcu_report_qs_rnp() will be invoked to propagate the quiescent state up the rcu_node hierarchy. o cpu_quiet_msk() becomes rcu_report_qs_rnp(), which reports a quiescent state for a given CPU (or possibly a set of CPUs) up the rcu_node hierarchy. o cpu_quiet_msk_finish() becomes rcu_report_qs_rsp(), which reports a full set of quiescent states to the global rcu_state structure. o task_quiet() becomes rcu_report_unblock_qs_rnp(), which reports a quiescent state due to a task exiting an RCU read-side critical section that had previously blocked in that same critical section. As indicated by the new name, this type of quiescent state is reported up the rcu_node hierarchy (using rcu_report_qs_rnp() to do so). Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Josh Triplett <josh@joshtriplett.org> Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com> Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12597846163698-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-12-02 20:10:13 +00:00
rcu_report_qs_rsp(&rcu_preempt_state, flags);
rcu: Fix grace-period-stall bug on large systems with CPU hotplug When the last CPU of a given leaf rcu_node structure goes offline, all of the tasks queued on that leaf rcu_node structure (due to having blocked in their current RCU read-side critical sections) are requeued onto the root rcu_node structure. This requeuing is carried out by rcu_preempt_offline_tasks(). However, it is possible that these queued tasks are the only thing preventing the leaf rcu_node structure from reporting a quiescent state up the rcu_node hierarchy. Unfortunately, the old code would fail to do this reporting, resulting in a grace-period stall given the following sequence of events: 1. Kernel built for more than 32 CPUs on 32-bit systems or for more than 64 CPUs on 64-bit systems, so that there is more than one rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set to a number smaller than CONFIG_NR_CPUS.) 2. The kernel is built with CONFIG_TREE_PREEMPT_RCU. 3. A task running on a CPU associated with a given leaf rcu_node structure blocks while in an RCU read-side critical section -and- that CPU has not yet passed through a quiescent state for the current RCU grace period. This will cause the task to be queued on the leaf rcu_node's blocked_tasks[] array, in particular, on the element of this array corresponding to the current grace period. 4. Each of the remaining CPUs corresponding to this same leaf rcu_node structure pass through a quiescent state. However, the task is still in its RCU read-side critical section, so these quiescent states cannot be reported further up the rcu_node hierarchy. Nevertheless, all bits in the leaf rcu_node structure's ->qsmask field are now zero. 5. Each of the remaining CPUs go offline. (The events in step #4 and #5 can happen in any order as long as each CPU passes through a quiescent state before going offline.) 6. When the last CPU goes offline, __rcu_offline_cpu() will invoke rcu_preempt_offline_tasks(), which will move the task to the root rcu_node structure, but without reporting a quiescent state up the rcu_node hierarchy (and this failure to report a quiescent state is the bug). But because this leaf rcu_node structure's ->qsmask field is already zero and its ->block_tasks[] entries are all empty, force_quiescent_state() will skip this rcu_node structure. Therefore, grace periods are now hung. This patch abstracts some code out of rcu_read_unlock_special(), calling the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet() from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking task_quiet() from __rcu_offline_cpu() reports the quiescent state up the rcu_node hierarchy, fixing the bug. This ends up requiring a separate lock_class_key per level of the rcu_node hierarchy, which this patch also provides. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12589088301770-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-11-22 16:53:48 +00:00
return;
}
/* Report up the rest of the hierarchy. */
mask = rnp->grpmask;
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
raw_spin_lock(&rnp_p->lock); /* irqs already disabled. */
rcu: Rename "quiet" functions The number of "quiet" functions has grown recently, and the names are no longer very descriptive. The point of all of these functions is to do some portion of the task of reporting a quiescent state, so rename them accordingly: o cpu_quiet() becomes rcu_report_qs_rdp(), which reports a quiescent state to the per-CPU rcu_data structure. If this turns out to be a new quiescent state for this grace period, then rcu_report_qs_rnp() will be invoked to propagate the quiescent state up the rcu_node hierarchy. o cpu_quiet_msk() becomes rcu_report_qs_rnp(), which reports a quiescent state for a given CPU (or possibly a set of CPUs) up the rcu_node hierarchy. o cpu_quiet_msk_finish() becomes rcu_report_qs_rsp(), which reports a full set of quiescent states to the global rcu_state structure. o task_quiet() becomes rcu_report_unblock_qs_rnp(), which reports a quiescent state due to a task exiting an RCU read-side critical section that had previously blocked in that same critical section. As indicated by the new name, this type of quiescent state is reported up the rcu_node hierarchy (using rcu_report_qs_rnp() to do so). Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Josh Triplett <josh@joshtriplett.org> Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com> Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12597846163698-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-12-02 20:10:13 +00:00
rcu_report_qs_rnp(mask, &rcu_preempt_state, rnp_p, flags);
rcu: Fix grace-period-stall bug on large systems with CPU hotplug When the last CPU of a given leaf rcu_node structure goes offline, all of the tasks queued on that leaf rcu_node structure (due to having blocked in their current RCU read-side critical sections) are requeued onto the root rcu_node structure. This requeuing is carried out by rcu_preempt_offline_tasks(). However, it is possible that these queued tasks are the only thing preventing the leaf rcu_node structure from reporting a quiescent state up the rcu_node hierarchy. Unfortunately, the old code would fail to do this reporting, resulting in a grace-period stall given the following sequence of events: 1. Kernel built for more than 32 CPUs on 32-bit systems or for more than 64 CPUs on 64-bit systems, so that there is more than one rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set to a number smaller than CONFIG_NR_CPUS.) 2. The kernel is built with CONFIG_TREE_PREEMPT_RCU. 3. A task running on a CPU associated with a given leaf rcu_node structure blocks while in an RCU read-side critical section -and- that CPU has not yet passed through a quiescent state for the current RCU grace period. This will cause the task to be queued on the leaf rcu_node's blocked_tasks[] array, in particular, on the element of this array corresponding to the current grace period. 4. Each of the remaining CPUs corresponding to this same leaf rcu_node structure pass through a quiescent state. However, the task is still in its RCU read-side critical section, so these quiescent states cannot be reported further up the rcu_node hierarchy. Nevertheless, all bits in the leaf rcu_node structure's ->qsmask field are now zero. 5. Each of the remaining CPUs go offline. (The events in step #4 and #5 can happen in any order as long as each CPU passes through a quiescent state before going offline.) 6. When the last CPU goes offline, __rcu_offline_cpu() will invoke rcu_preempt_offline_tasks(), which will move the task to the root rcu_node structure, but without reporting a quiescent state up the rcu_node hierarchy (and this failure to report a quiescent state is the bug). But because this leaf rcu_node structure's ->qsmask field is already zero and its ->block_tasks[] entries are all empty, force_quiescent_state() will skip this rcu_node structure. Therefore, grace periods are now hung. This patch abstracts some code out of rcu_read_unlock_special(), calling the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet() from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking task_quiet() from __rcu_offline_cpu() reports the quiescent state up the rcu_node hierarchy, fixing the bug. This ends up requiring a separate lock_class_key per level of the rcu_node hierarchy, which this patch also provides. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12589088301770-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-11-22 16:53:48 +00:00
}
/*
* Advance a ->blkd_tasks-list pointer to the next entry, instead
* returning NULL if at the end of the list.
*/
static struct list_head *rcu_next_node_entry(struct task_struct *t,
struct rcu_node *rnp)
{
struct list_head *np;
np = t->rcu_node_entry.next;
if (np == &rnp->blkd_tasks)
np = NULL;
return np;
}
rcu: Fix grace-period-stall bug on large systems with CPU hotplug When the last CPU of a given leaf rcu_node structure goes offline, all of the tasks queued on that leaf rcu_node structure (due to having blocked in their current RCU read-side critical sections) are requeued onto the root rcu_node structure. This requeuing is carried out by rcu_preempt_offline_tasks(). However, it is possible that these queued tasks are the only thing preventing the leaf rcu_node structure from reporting a quiescent state up the rcu_node hierarchy. Unfortunately, the old code would fail to do this reporting, resulting in a grace-period stall given the following sequence of events: 1. Kernel built for more than 32 CPUs on 32-bit systems or for more than 64 CPUs on 64-bit systems, so that there is more than one rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set to a number smaller than CONFIG_NR_CPUS.) 2. The kernel is built with CONFIG_TREE_PREEMPT_RCU. 3. A task running on a CPU associated with a given leaf rcu_node structure blocks while in an RCU read-side critical section -and- that CPU has not yet passed through a quiescent state for the current RCU grace period. This will cause the task to be queued on the leaf rcu_node's blocked_tasks[] array, in particular, on the element of this array corresponding to the current grace period. 4. Each of the remaining CPUs corresponding to this same leaf rcu_node structure pass through a quiescent state. However, the task is still in its RCU read-side critical section, so these quiescent states cannot be reported further up the rcu_node hierarchy. Nevertheless, all bits in the leaf rcu_node structure's ->qsmask field are now zero. 5. Each of the remaining CPUs go offline. (The events in step #4 and #5 can happen in any order as long as each CPU passes through a quiescent state before going offline.) 6. When the last CPU goes offline, __rcu_offline_cpu() will invoke rcu_preempt_offline_tasks(), which will move the task to the root rcu_node structure, but without reporting a quiescent state up the rcu_node hierarchy (and this failure to report a quiescent state is the bug). But because this leaf rcu_node structure's ->qsmask field is already zero and its ->block_tasks[] entries are all empty, force_quiescent_state() will skip this rcu_node structure. Therefore, grace periods are now hung. This patch abstracts some code out of rcu_read_unlock_special(), calling the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet() from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking task_quiet() from __rcu_offline_cpu() reports the quiescent state up the rcu_node hierarchy, fixing the bug. This ends up requiring a separate lock_class_key per level of the rcu_node hierarchy, which this patch also provides. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12589088301770-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-11-22 16:53:48 +00:00
/*
* Handle special cases during rcu_read_unlock(), such as needing to
* notify RCU core processing or task having blocked during the RCU
* read-side critical section.
*/
void rcu_read_unlock_special(struct task_struct *t)
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
{
int empty;
int empty_exp;
rcu: Avoid RCU-preempt expedited grace-period botch Because rcu_read_unlock_special() samples rcu_preempted_readers_exp(rnp) after dropping rnp->lock, the following sequence of events is possible: 1. Task A exits its RCU read-side critical section, and removes itself from the ->blkd_tasks list, releases rnp->lock, and is then preempted. Task B remains on the ->blkd_tasks list, and blocks the current expedited grace period. 2. Task B exits from its RCU read-side critical section and removes itself from the ->blkd_tasks list. Because it is the last task blocking the current expedited grace period, it ends that expedited grace period. 3. Task A resumes, and samples rcu_preempted_readers_exp(rnp) which of course indicates that nothing is blocking the nonexistent expedited grace period. Task A is again preempted. 4. Some other CPU starts an expedited grace period. There are several tasks blocking this expedited grace period queued on the same rcu_node structure that Task A was using in step 1 above. 5. Task A examines its state and incorrectly concludes that it was the last task blocking the expedited grace period on the current rcu_node structure. It therefore reports completion up the rcu_node tree. 6. The expedited grace period can then incorrectly complete before the tasks blocked on this same rcu_node structure exit their RCU read-side critical sections. Arbitrarily bad things happen. This commit therefore takes a snapshot of rcu_preempted_readers_exp(rnp) prior to dropping the lock, so that only the last task thinks that it is the last task, thus avoiding the failure scenario laid out above. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2011-09-21 21:41:37 +00:00
int empty_exp_now;
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
unsigned long flags;
struct list_head *np;
#ifdef CONFIG_RCU_BOOST
struct rt_mutex *rbmp = NULL;
#endif /* #ifdef CONFIG_RCU_BOOST */
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
struct rcu_node *rnp;
int special;
/* NMI handlers cannot block and cannot safely manipulate state. */
if (in_nmi())
return;
local_irq_save(flags);
/*
* If RCU core is waiting for this CPU to exit critical section,
* let it know that we have done so.
*/
special = t->rcu_read_unlock_special;
if (special & RCU_READ_UNLOCK_NEED_QS) {
rcu_preempt_qs(smp_processor_id());
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
}
/* Hardware IRQ handlers cannot block. */
softirq,rcu: Inform RCU of irq_exit() activity The rcu_read_unlock_special() function relies on in_irq() to exclude scheduler activity from interrupt level. This fails because exit_irq() can invoke the scheduler after clearing the preempt_count() bits that in_irq() uses to determine that it is at interrupt level. This situation can result in failures as follows: $task IRQ SoftIRQ rcu_read_lock() /* do stuff */ <preempt> |= UNLOCK_BLOCKED rcu_read_unlock() --t->rcu_read_lock_nesting irq_enter(); /* do stuff, don't use RCU */ irq_exit(); sub_preempt_count(IRQ_EXIT_OFFSET); invoke_softirq() ttwu(); spin_lock_irq(&pi->lock) rcu_read_lock(); /* do stuff */ rcu_read_unlock(); rcu_read_unlock_special() rcu_report_exp_rnp() ttwu() spin_lock_irq(&pi->lock) /* deadlock */ rcu_read_unlock_special(t); Ed can simply trigger this 'easy' because invoke_softirq() immediately does a ttwu() of ksoftirqd/# instead of doing the in-place softirq stuff first, but even without that the above happens. Cure this by also excluding softirqs from the rcu_read_unlock_special() handler and ensuring the force_irqthreads ksoftirqd/# wakeup is done from full softirq context. [ Alternatively, delaying the ->rcu_read_lock_nesting decrement until after the special handling would make the thing more robust in the face of interrupts as well. And there is a separate patch for that. ] Cc: Thomas Gleixner <tglx@linutronix.de> Reported-and-tested-by: Ed Tomlinson <edt@aei.ca> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2011-07-19 22:32:00 +00:00
if (in_irq() || in_serving_softirq()) {
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
local_irq_restore(flags);
return;
}
/* Clean up if blocked during RCU read-side critical section. */
if (special & RCU_READ_UNLOCK_BLOCKED) {
t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
/*
* Remove this task from the list it blocked on. The
* task can migrate while we acquire the lock, but at
* most one time. So at most two passes through loop.
*/
for (;;) {
rnp = t->rcu_blocked_node;
raw_spin_lock(&rnp->lock); /* irqs already disabled. */
if (rnp == t->rcu_blocked_node)
break;
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
}
empty = !rcu_preempt_blocked_readers_cgp(rnp);
empty_exp = !rcu_preempted_readers_exp(rnp);
smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
np = rcu_next_node_entry(t, rnp);
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
list_del_init(&t->rcu_node_entry);
t->rcu_blocked_node = NULL;
trace_rcu_unlock_preempted_task("rcu_preempt",
rnp->gpnum, t->pid);
if (&t->rcu_node_entry == rnp->gp_tasks)
rnp->gp_tasks = np;
if (&t->rcu_node_entry == rnp->exp_tasks)
rnp->exp_tasks = np;
#ifdef CONFIG_RCU_BOOST
if (&t->rcu_node_entry == rnp->boost_tasks)
rnp->boost_tasks = np;
/* Snapshot/clear ->rcu_boost_mutex with rcu_node lock held. */
if (t->rcu_boost_mutex) {
rbmp = t->rcu_boost_mutex;
t->rcu_boost_mutex = NULL;
rcu: Fix RCU_BOOST race handling current->rcu_read_unlock_special The RCU_BOOST commits for TREE_PREEMPT_RCU introduced an other-task write to a new RCU_READ_UNLOCK_BOOSTED bit in the task_struct structure's ->rcu_read_unlock_special field, but, as noted by Steven Rostedt, without correctly synchronizing all accesses to ->rcu_read_unlock_special. This could result in bits in ->rcu_read_unlock_special being spuriously set and cleared due to conflicting accesses, which in turn could result in deadlocks between the rcu_node structure's ->lock and the scheduler's rq and pi locks. These deadlocks would result from RCU incorrectly believing that the just-ended RCU read-side critical section had been preempted and/or boosted. If that RCU read-side critical section was executed with either rq or pi locks held, RCU's ensuing (incorrect) calls to the scheduler would cause the scheduler to attempt to once again acquire the rq and pi locks, resulting in deadlock. More complex deadlock cycles are also possible, involving multiple rq and pi locks as well as locks from multiple rcu_node structures. This commit fixes synchronization by creating ->rcu_boosted field in task_struct that is accessed and modified only when holding the ->lock in the rcu_node structure on which the task is queued (on that rcu_node structure's ->blkd_tasks list). This results in tasks accessing only their own current->rcu_read_unlock_special fields, making unsynchronized access once again legal, and keeping the rcu_read_unlock() fastpath free of atomic instructions and memory barriers. The reason that the rcu_read_unlock() fastpath does not need to access the new current->rcu_boosted field is that this new field cannot be non-zero unless the RCU_READ_UNLOCK_BLOCKED bit is set in the current->rcu_read_unlock_special field. Therefore, rcu_read_unlock() need only test current->rcu_read_unlock_special: if that is zero, then current->rcu_boosted must also be zero. This bug does not affect TINY_PREEMPT_RCU because this implementation of RCU accesses current->rcu_read_unlock_special with irqs disabled, thus preventing races on the !SMP systems that TINY_PREEMPT_RCU runs on. Maybe-reported-by: Dave Jones <davej@redhat.com> Maybe-reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Reported-by: Steven Rostedt <rostedt@goodmis.org> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
2011-07-14 19:24:11 +00:00
}
#endif /* #ifdef CONFIG_RCU_BOOST */
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
/*
* If this was the last task on the current list, and if
* we aren't waiting on any CPUs, report the quiescent state.
rcu: Avoid RCU-preempt expedited grace-period botch Because rcu_read_unlock_special() samples rcu_preempted_readers_exp(rnp) after dropping rnp->lock, the following sequence of events is possible: 1. Task A exits its RCU read-side critical section, and removes itself from the ->blkd_tasks list, releases rnp->lock, and is then preempted. Task B remains on the ->blkd_tasks list, and blocks the current expedited grace period. 2. Task B exits from its RCU read-side critical section and removes itself from the ->blkd_tasks list. Because it is the last task blocking the current expedited grace period, it ends that expedited grace period. 3. Task A resumes, and samples rcu_preempted_readers_exp(rnp) which of course indicates that nothing is blocking the nonexistent expedited grace period. Task A is again preempted. 4. Some other CPU starts an expedited grace period. There are several tasks blocking this expedited grace period queued on the same rcu_node structure that Task A was using in step 1 above. 5. Task A examines its state and incorrectly concludes that it was the last task blocking the expedited grace period on the current rcu_node structure. It therefore reports completion up the rcu_node tree. 6. The expedited grace period can then incorrectly complete before the tasks blocked on this same rcu_node structure exit their RCU read-side critical sections. Arbitrarily bad things happen. This commit therefore takes a snapshot of rcu_preempted_readers_exp(rnp) prior to dropping the lock, so that only the last task thinks that it is the last task, thus avoiding the failure scenario laid out above. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2011-09-21 21:41:37 +00:00
* Note that rcu_report_unblock_qs_rnp() releases rnp->lock,
* so we must take a snapshot of the expedited state.
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
*/
rcu: Avoid RCU-preempt expedited grace-period botch Because rcu_read_unlock_special() samples rcu_preempted_readers_exp(rnp) after dropping rnp->lock, the following sequence of events is possible: 1. Task A exits its RCU read-side critical section, and removes itself from the ->blkd_tasks list, releases rnp->lock, and is then preempted. Task B remains on the ->blkd_tasks list, and blocks the current expedited grace period. 2. Task B exits from its RCU read-side critical section and removes itself from the ->blkd_tasks list. Because it is the last task blocking the current expedited grace period, it ends that expedited grace period. 3. Task A resumes, and samples rcu_preempted_readers_exp(rnp) which of course indicates that nothing is blocking the nonexistent expedited grace period. Task A is again preempted. 4. Some other CPU starts an expedited grace period. There are several tasks blocking this expedited grace period queued on the same rcu_node structure that Task A was using in step 1 above. 5. Task A examines its state and incorrectly concludes that it was the last task blocking the expedited grace period on the current rcu_node structure. It therefore reports completion up the rcu_node tree. 6. The expedited grace period can then incorrectly complete before the tasks blocked on this same rcu_node structure exit their RCU read-side critical sections. Arbitrarily bad things happen. This commit therefore takes a snapshot of rcu_preempted_readers_exp(rnp) prior to dropping the lock, so that only the last task thinks that it is the last task, thus avoiding the failure scenario laid out above. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2011-09-21 21:41:37 +00:00
empty_exp_now = !rcu_preempted_readers_exp(rnp);
if (!empty && !rcu_preempt_blocked_readers_cgp(rnp)) {
trace_rcu_quiescent_state_report("preempt_rcu",
rnp->gpnum,
0, rnp->qsmask,
rnp->level,
rnp->grplo,
rnp->grphi,
!!rnp->gp_tasks);
rcu: Rename "quiet" functions The number of "quiet" functions has grown recently, and the names are no longer very descriptive. The point of all of these functions is to do some portion of the task of reporting a quiescent state, so rename them accordingly: o cpu_quiet() becomes rcu_report_qs_rdp(), which reports a quiescent state to the per-CPU rcu_data structure. If this turns out to be a new quiescent state for this grace period, then rcu_report_qs_rnp() will be invoked to propagate the quiescent state up the rcu_node hierarchy. o cpu_quiet_msk() becomes rcu_report_qs_rnp(), which reports a quiescent state for a given CPU (or possibly a set of CPUs) up the rcu_node hierarchy. o cpu_quiet_msk_finish() becomes rcu_report_qs_rsp(), which reports a full set of quiescent states to the global rcu_state structure. o task_quiet() becomes rcu_report_unblock_qs_rnp(), which reports a quiescent state due to a task exiting an RCU read-side critical section that had previously blocked in that same critical section. As indicated by the new name, this type of quiescent state is reported up the rcu_node hierarchy (using rcu_report_qs_rnp() to do so). Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Josh Triplett <josh@joshtriplett.org> Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com> Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12597846163698-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-12-02 20:10:13 +00:00
rcu_report_unblock_qs_rnp(rnp, flags);
} else {
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
#ifdef CONFIG_RCU_BOOST
/* Unboost if we were boosted. */
if (rbmp)
rt_mutex_unlock(rbmp);
#endif /* #ifdef CONFIG_RCU_BOOST */
/*
* If this was the last task on the expedited lists,
* then we need to report up the rcu_node hierarchy.
*/
rcu: Avoid RCU-preempt expedited grace-period botch Because rcu_read_unlock_special() samples rcu_preempted_readers_exp(rnp) after dropping rnp->lock, the following sequence of events is possible: 1. Task A exits its RCU read-side critical section, and removes itself from the ->blkd_tasks list, releases rnp->lock, and is then preempted. Task B remains on the ->blkd_tasks list, and blocks the current expedited grace period. 2. Task B exits from its RCU read-side critical section and removes itself from the ->blkd_tasks list. Because it is the last task blocking the current expedited grace period, it ends that expedited grace period. 3. Task A resumes, and samples rcu_preempted_readers_exp(rnp) which of course indicates that nothing is blocking the nonexistent expedited grace period. Task A is again preempted. 4. Some other CPU starts an expedited grace period. There are several tasks blocking this expedited grace period queued on the same rcu_node structure that Task A was using in step 1 above. 5. Task A examines its state and incorrectly concludes that it was the last task blocking the expedited grace period on the current rcu_node structure. It therefore reports completion up the rcu_node tree. 6. The expedited grace period can then incorrectly complete before the tasks blocked on this same rcu_node structure exit their RCU read-side critical sections. Arbitrarily bad things happen. This commit therefore takes a snapshot of rcu_preempted_readers_exp(rnp) prior to dropping the lock, so that only the last task thinks that it is the last task, thus avoiding the failure scenario laid out above. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2011-09-21 21:41:37 +00:00
if (!empty_exp && empty_exp_now)
rcu_report_exp_rnp(&rcu_preempt_state, rnp, true);
rcu: Fix grace-period-stall bug on large systems with CPU hotplug When the last CPU of a given leaf rcu_node structure goes offline, all of the tasks queued on that leaf rcu_node structure (due to having blocked in their current RCU read-side critical sections) are requeued onto the root rcu_node structure. This requeuing is carried out by rcu_preempt_offline_tasks(). However, it is possible that these queued tasks are the only thing preventing the leaf rcu_node structure from reporting a quiescent state up the rcu_node hierarchy. Unfortunately, the old code would fail to do this reporting, resulting in a grace-period stall given the following sequence of events: 1. Kernel built for more than 32 CPUs on 32-bit systems or for more than 64 CPUs on 64-bit systems, so that there is more than one rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set to a number smaller than CONFIG_NR_CPUS.) 2. The kernel is built with CONFIG_TREE_PREEMPT_RCU. 3. A task running on a CPU associated with a given leaf rcu_node structure blocks while in an RCU read-side critical section -and- that CPU has not yet passed through a quiescent state for the current RCU grace period. This will cause the task to be queued on the leaf rcu_node's blocked_tasks[] array, in particular, on the element of this array corresponding to the current grace period. 4. Each of the remaining CPUs corresponding to this same leaf rcu_node structure pass through a quiescent state. However, the task is still in its RCU read-side critical section, so these quiescent states cannot be reported further up the rcu_node hierarchy. Nevertheless, all bits in the leaf rcu_node structure's ->qsmask field are now zero. 5. Each of the remaining CPUs go offline. (The events in step #4 and #5 can happen in any order as long as each CPU passes through a quiescent state before going offline.) 6. When the last CPU goes offline, __rcu_offline_cpu() will invoke rcu_preempt_offline_tasks(), which will move the task to the root rcu_node structure, but without reporting a quiescent state up the rcu_node hierarchy (and this failure to report a quiescent state is the bug). But because this leaf rcu_node structure's ->qsmask field is already zero and its ->block_tasks[] entries are all empty, force_quiescent_state() will skip this rcu_node structure. Therefore, grace periods are now hung. This patch abstracts some code out of rcu_read_unlock_special(), calling the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet() from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking task_quiet() from __rcu_offline_cpu() reports the quiescent state up the rcu_node hierarchy, fixing the bug. This ends up requiring a separate lock_class_key per level of the rcu_node hierarchy, which this patch also provides. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12589088301770-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-11-22 16:53:48 +00:00
} else {
local_irq_restore(flags);
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
}
}
#ifdef CONFIG_RCU_CPU_STALL_VERBOSE
/*
* Dump detailed information for all tasks blocking the current RCU
* grace period on the specified rcu_node structure.
*/
static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
{
unsigned long flags;
struct task_struct *t;
raw_spin_lock_irqsave(&rnp->lock, flags);
if (!rcu_preempt_blocked_readers_cgp(rnp)) {
raw_spin_unlock_irqrestore(&rnp->lock, flags);
return;
}
t = list_entry(rnp->gp_tasks,
struct task_struct, rcu_node_entry);
list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
sched_show_task(t);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
/*
* Dump detailed information for all tasks blocking the current RCU
* grace period.
*/
static void rcu_print_detail_task_stall(struct rcu_state *rsp)
{
struct rcu_node *rnp = rcu_get_root(rsp);
rcu_print_detail_task_stall_rnp(rnp);
rcu_for_each_leaf_node(rsp, rnp)
rcu_print_detail_task_stall_rnp(rnp);
}
#else /* #ifdef CONFIG_RCU_CPU_STALL_VERBOSE */
static void rcu_print_detail_task_stall(struct rcu_state *rsp)
{
}
#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_VERBOSE */
#ifdef CONFIG_RCU_CPU_STALL_INFO
static void rcu_print_task_stall_begin(struct rcu_node *rnp)
{
pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
rnp->level, rnp->grplo, rnp->grphi);
}
static void rcu_print_task_stall_end(void)
{
pr_cont("\n");
}
#else /* #ifdef CONFIG_RCU_CPU_STALL_INFO */
static void rcu_print_task_stall_begin(struct rcu_node *rnp)
{
}
static void rcu_print_task_stall_end(void)
{
}
#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_INFO */
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
/*
* Scan the current list of tasks blocked within RCU read-side critical
* sections, printing out the tid of each.
*/
static int rcu_print_task_stall(struct rcu_node *rnp)
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
{
struct task_struct *t;
int ndetected = 0;
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
if (!rcu_preempt_blocked_readers_cgp(rnp))
return 0;
rcu_print_task_stall_begin(rnp);
t = list_entry(rnp->gp_tasks,
struct task_struct, rcu_node_entry);
list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
pr_cont(" P%d", t->pid);
ndetected++;
}
rcu_print_task_stall_end();
return ndetected;
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
}
/*
* Check that the list of blocked tasks for the newly completed grace
* period is in fact empty. It is a serious bug to complete a grace
* period that still has RCU readers blocked! This function must be
* invoked -before- updating this rnp's ->gpnum, and the rnp's ->lock
* must be held by the caller.
*
* Also, if there are blocked tasks on the list, they automatically
* block the newly created grace period, so set up ->gp_tasks accordingly.
*/
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));
if (!list_empty(&rnp->blkd_tasks))
rnp->gp_tasks = rnp->blkd_tasks.next;
WARN_ON_ONCE(rnp->qsmask);
}
#ifdef CONFIG_HOTPLUG_CPU
/*
* Handle tasklist migration for case in which all CPUs covered by the
* specified rcu_node have gone offline. Move them up to the root
* rcu_node. The reason for not just moving them to the immediate
* parent is to remove the need for rcu_read_unlock_special() to
* make more than two attempts to acquire the target rcu_node's lock.
rcu: Fix grace-period-stall bug on large systems with CPU hotplug When the last CPU of a given leaf rcu_node structure goes offline, all of the tasks queued on that leaf rcu_node structure (due to having blocked in their current RCU read-side critical sections) are requeued onto the root rcu_node structure. This requeuing is carried out by rcu_preempt_offline_tasks(). However, it is possible that these queued tasks are the only thing preventing the leaf rcu_node structure from reporting a quiescent state up the rcu_node hierarchy. Unfortunately, the old code would fail to do this reporting, resulting in a grace-period stall given the following sequence of events: 1. Kernel built for more than 32 CPUs on 32-bit systems or for more than 64 CPUs on 64-bit systems, so that there is more than one rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set to a number smaller than CONFIG_NR_CPUS.) 2. The kernel is built with CONFIG_TREE_PREEMPT_RCU. 3. A task running on a CPU associated with a given leaf rcu_node structure blocks while in an RCU read-side critical section -and- that CPU has not yet passed through a quiescent state for the current RCU grace period. This will cause the task to be queued on the leaf rcu_node's blocked_tasks[] array, in particular, on the element of this array corresponding to the current grace period. 4. Each of the remaining CPUs corresponding to this same leaf rcu_node structure pass through a quiescent state. However, the task is still in its RCU read-side critical section, so these quiescent states cannot be reported further up the rcu_node hierarchy. Nevertheless, all bits in the leaf rcu_node structure's ->qsmask field are now zero. 5. Each of the remaining CPUs go offline. (The events in step #4 and #5 can happen in any order as long as each CPU passes through a quiescent state before going offline.) 6. When the last CPU goes offline, __rcu_offline_cpu() will invoke rcu_preempt_offline_tasks(), which will move the task to the root rcu_node structure, but without reporting a quiescent state up the rcu_node hierarchy (and this failure to report a quiescent state is the bug). But because this leaf rcu_node structure's ->qsmask field is already zero and its ->block_tasks[] entries are all empty, force_quiescent_state() will skip this rcu_node structure. Therefore, grace periods are now hung. This patch abstracts some code out of rcu_read_unlock_special(), calling the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet() from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking task_quiet() from __rcu_offline_cpu() reports the quiescent state up the rcu_node hierarchy, fixing the bug. This ends up requiring a separate lock_class_key per level of the rcu_node hierarchy, which this patch also provides. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12589088301770-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-11-22 16:53:48 +00:00
* Returns true if there were tasks blocking the current RCU grace
* period.
*
rcu: Fix TREE_PREEMPT_RCU CPU_HOTPLUG bad-luck hang If the following sequence of events occurs, then TREE_PREEMPT_RCU will hang waiting for a grace period to complete, eventually OOMing the system: o A TREE_PREEMPT_RCU build of the kernel is booted on a system with more than 64 physical CPUs present (32 on a 32-bit system). Alternatively, a TREE_PREEMPT_RCU build of the kernel is booted with RCU_FANOUT set to a sufficiently small value that the physical CPUs populate two or more leaf rcu_node structures. o A task is preempted in an RCU read-side critical section while running on a CPU corresponding to a given leaf rcu_node structure. o All CPUs corresponding to this same leaf rcu_node structure record quiescent states for the current grace period. o All of these same CPUs go offline (hence the need for enough physical CPUs to populate more than one leaf rcu_node structure). This causes the preempted task to be moved to the root rcu_node structure. At this point, there is nothing left to cause the quiescent state to be propagated up the rcu_node tree, so the current grace period never completes. The simplest fix, especially after considering the deadlock possibilities, is to detect this situation when the last CPU is offlined, and to set that CPU's ->qsmask bit in its leaf rcu_node structure. This will cause the next invocation of force_quiescent_state() to end the grace period. Without this fix, this hang can be triggered in an hour or so on some machines with rcutorture and random CPU onlining/offlining. With this fix, these same machines pass a full 10 hours of this sort of abuse. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <20091015162614.GA19131@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-10-15 16:26:14 +00:00
* Returns 1 if there was previously a task blocking the current grace
* period on the specified rcu_node structure.
*
* The caller must hold rnp->lock with irqs disabled.
*/
rcu: Fix TREE_PREEMPT_RCU CPU_HOTPLUG bad-luck hang If the following sequence of events occurs, then TREE_PREEMPT_RCU will hang waiting for a grace period to complete, eventually OOMing the system: o A TREE_PREEMPT_RCU build of the kernel is booted on a system with more than 64 physical CPUs present (32 on a 32-bit system). Alternatively, a TREE_PREEMPT_RCU build of the kernel is booted with RCU_FANOUT set to a sufficiently small value that the physical CPUs populate two or more leaf rcu_node structures. o A task is preempted in an RCU read-side critical section while running on a CPU corresponding to a given leaf rcu_node structure. o All CPUs corresponding to this same leaf rcu_node structure record quiescent states for the current grace period. o All of these same CPUs go offline (hence the need for enough physical CPUs to populate more than one leaf rcu_node structure). This causes the preempted task to be moved to the root rcu_node structure. At this point, there is nothing left to cause the quiescent state to be propagated up the rcu_node tree, so the current grace period never completes. The simplest fix, especially after considering the deadlock possibilities, is to detect this situation when the last CPU is offlined, and to set that CPU's ->qsmask bit in its leaf rcu_node structure. This will cause the next invocation of force_quiescent_state() to end the grace period. Without this fix, this hang can be triggered in an hour or so on some machines with rcutorture and random CPU onlining/offlining. With this fix, these same machines pass a full 10 hours of this sort of abuse. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <20091015162614.GA19131@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-10-15 16:26:14 +00:00
static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
struct rcu_node *rnp,
struct rcu_data *rdp)
{
struct list_head *lp;
struct list_head *lp_root;
int retval = 0;
struct rcu_node *rnp_root = rcu_get_root(rsp);
struct task_struct *t;
if (rnp == rnp_root) {
WARN_ONCE(1, "Last CPU thought to be offlined?");
rcu: Fix TREE_PREEMPT_RCU CPU_HOTPLUG bad-luck hang If the following sequence of events occurs, then TREE_PREEMPT_RCU will hang waiting for a grace period to complete, eventually OOMing the system: o A TREE_PREEMPT_RCU build of the kernel is booted on a system with more than 64 physical CPUs present (32 on a 32-bit system). Alternatively, a TREE_PREEMPT_RCU build of the kernel is booted with RCU_FANOUT set to a sufficiently small value that the physical CPUs populate two or more leaf rcu_node structures. o A task is preempted in an RCU read-side critical section while running on a CPU corresponding to a given leaf rcu_node structure. o All CPUs corresponding to this same leaf rcu_node structure record quiescent states for the current grace period. o All of these same CPUs go offline (hence the need for enough physical CPUs to populate more than one leaf rcu_node structure). This causes the preempted task to be moved to the root rcu_node structure. At this point, there is nothing left to cause the quiescent state to be propagated up the rcu_node tree, so the current grace period never completes. The simplest fix, especially after considering the deadlock possibilities, is to detect this situation when the last CPU is offlined, and to set that CPU's ->qsmask bit in its leaf rcu_node structure. This will cause the next invocation of force_quiescent_state() to end the grace period. Without this fix, this hang can be triggered in an hour or so on some machines with rcutorture and random CPU onlining/offlining. With this fix, these same machines pass a full 10 hours of this sort of abuse. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <20091015162614.GA19131@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-10-15 16:26:14 +00:00
return 0; /* Shouldn't happen: at least one CPU online. */
}
/* If we are on an internal node, complain bitterly. */
WARN_ON_ONCE(rnp != rdp->mynode);
/*
* Move tasks up to root rcu_node. Don't try to get fancy for
* this corner-case operation -- just put this node's tasks
* at the head of the root node's list, and update the root node's
* ->gp_tasks and ->exp_tasks pointers to those of this node's,
* if non-NULL. This might result in waiting for more tasks than
* absolutely necessary, but this is a good performance/complexity
* tradeoff.
*/
if (rcu_preempt_blocked_readers_cgp(rnp) && rnp->qsmask == 0)
retval |= RCU_OFL_TASKS_NORM_GP;
if (rcu_preempted_readers_exp(rnp))
retval |= RCU_OFL_TASKS_EXP_GP;
lp = &rnp->blkd_tasks;
lp_root = &rnp_root->blkd_tasks;
while (!list_empty(lp)) {
t = list_entry(lp->next, typeof(*t), rcu_node_entry);
raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
list_del(&t->rcu_node_entry);
t->rcu_blocked_node = rnp_root;
list_add(&t->rcu_node_entry, lp_root);
if (&t->rcu_node_entry == rnp->gp_tasks)
rnp_root->gp_tasks = rnp->gp_tasks;
if (&t->rcu_node_entry == rnp->exp_tasks)
rnp_root->exp_tasks = rnp->exp_tasks;
#ifdef CONFIG_RCU_BOOST
if (&t->rcu_node_entry == rnp->boost_tasks)
rnp_root->boost_tasks = rnp->boost_tasks;
#endif /* #ifdef CONFIG_RCU_BOOST */
raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
}
rnp->gp_tasks = NULL;
rnp->exp_tasks = NULL;
#ifdef CONFIG_RCU_BOOST
rnp->boost_tasks = NULL;
rcu: Improve boost selection when moving tasks to root rcu_node The rcu_preempt_offline_tasks() moves all tasks queued on a given leaf rcu_node structure to the root rcu_node, which is done when the last CPU corresponding the the leaf rcu_node structure goes offline. Now that RCU-preempt's synchronize_rcu_expedited() implementation blocks CPU-hotplug operations during the initialization of each rcu_node structure's ->boost_tasks pointer, rcu_preempt_offline_tasks() can do a better job of setting the root rcu_node's ->boost_tasks pointer. The key point is that rcu_preempt_offline_tasks() runs as part of the CPU-hotplug process, so that a concurrent synchronize_rcu_expedited() is guaranteed to either have not started on the one hand (in which case there is no boosting on behalf of the expedited grace period) or to be completely initialized on the other (in which case, in the absence of other priority boosting, all ->boost_tasks pointers will be initialized). Therefore, if rcu_preempt_offline_tasks() finds that the ->boost_tasks pointer is equal to the ->exp_tasks pointer, it can be sure that it is correctly placed. In the case where there was boosting ongoing at the time that the synchronize_rcu_expedited() function started, different nodes might start boosting the tasks blocking the expedited grace period at different times. In this mixed case, the root node will either be boosting tasks for the expedited grace period already, or it will start as soon as it gets done boosting for the normal grace period -- but in this latter case, the root node's tasks needed to be boosted in any case. This commit therefore adds a check of the ->boost_tasks pointer against the ->exp_tasks pointer to the list that prevents updating ->boost_tasks. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2012-07-31 21:09:49 +00:00
/*
* In case root is being boosted and leaf was not. Make sure
* that we boost the tasks blocking the current grace period
* in this case.
*/
raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
if (rnp_root->boost_tasks != NULL &&
rcu: Improve boost selection when moving tasks to root rcu_node The rcu_preempt_offline_tasks() moves all tasks queued on a given leaf rcu_node structure to the root rcu_node, which is done when the last CPU corresponding the the leaf rcu_node structure goes offline. Now that RCU-preempt's synchronize_rcu_expedited() implementation blocks CPU-hotplug operations during the initialization of each rcu_node structure's ->boost_tasks pointer, rcu_preempt_offline_tasks() can do a better job of setting the root rcu_node's ->boost_tasks pointer. The key point is that rcu_preempt_offline_tasks() runs as part of the CPU-hotplug process, so that a concurrent synchronize_rcu_expedited() is guaranteed to either have not started on the one hand (in which case there is no boosting on behalf of the expedited grace period) or to be completely initialized on the other (in which case, in the absence of other priority boosting, all ->boost_tasks pointers will be initialized). Therefore, if rcu_preempt_offline_tasks() finds that the ->boost_tasks pointer is equal to the ->exp_tasks pointer, it can be sure that it is correctly placed. In the case where there was boosting ongoing at the time that the synchronize_rcu_expedited() function started, different nodes might start boosting the tasks blocking the expedited grace period at different times. In this mixed case, the root node will either be boosting tasks for the expedited grace period already, or it will start as soon as it gets done boosting for the normal grace period -- but in this latter case, the root node's tasks needed to be boosted in any case. This commit therefore adds a check of the ->boost_tasks pointer against the ->exp_tasks pointer to the list that prevents updating ->boost_tasks. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
2012-07-31 21:09:49 +00:00
rnp_root->boost_tasks != rnp_root->gp_tasks &&
rnp_root->boost_tasks != rnp_root->exp_tasks)
rnp_root->boost_tasks = rnp_root->gp_tasks;
raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
#endif /* #ifdef CONFIG_RCU_BOOST */
rcu: Fix TREE_PREEMPT_RCU CPU_HOTPLUG bad-luck hang If the following sequence of events occurs, then TREE_PREEMPT_RCU will hang waiting for a grace period to complete, eventually OOMing the system: o A TREE_PREEMPT_RCU build of the kernel is booted on a system with more than 64 physical CPUs present (32 on a 32-bit system). Alternatively, a TREE_PREEMPT_RCU build of the kernel is booted with RCU_FANOUT set to a sufficiently small value that the physical CPUs populate two or more leaf rcu_node structures. o A task is preempted in an RCU read-side critical section while running on a CPU corresponding to a given leaf rcu_node structure. o All CPUs corresponding to this same leaf rcu_node structure record quiescent states for the current grace period. o All of these same CPUs go offline (hence the need for enough physical CPUs to populate more than one leaf rcu_node structure). This causes the preempted task to be moved to the root rcu_node structure. At this point, there is nothing left to cause the quiescent state to be propagated up the rcu_node tree, so the current grace period never completes. The simplest fix, especially after considering the deadlock possibilities, is to detect this situation when the last CPU is offlined, and to set that CPU's ->qsmask bit in its leaf rcu_node structure. This will cause the next invocation of force_quiescent_state() to end the grace period. Without this fix, this hang can be triggered in an hour or so on some machines with rcutorture and random CPU onlining/offlining. With this fix, these same machines pass a full 10 hours of this sort of abuse. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <20091015162614.GA19131@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-10-15 16:26:14 +00:00
return retval;
}
#endif /* #ifdef CONFIG_HOTPLUG_CPU */
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
/*
* Check for a quiescent state from the current CPU. When a task blocks,
* the task is recorded in the corresponding CPU's rcu_node structure,
* which is checked elsewhere.
*
* Caller must disable hard irqs.
*/
static void rcu_preempt_check_callbacks(int cpu)
{
struct task_struct *t = current;
if (t->rcu_read_lock_nesting == 0) {
rcu_preempt_qs(cpu);
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
return;
}
rcu: protect __rcu_read_unlock() against scheduler-using irq handlers The addition of RCU read-side critical sections within runqueue and priority-inheritance lock critical sections introduced some deadlock cycles, for example, involving interrupts from __rcu_read_unlock() where the interrupt handlers call wake_up(). This situation can cause the instance of __rcu_read_unlock() invoked from interrupt to do some of the processing that would otherwise have been carried out by the task-level instance of __rcu_read_unlock(). When the interrupt-level instance of __rcu_read_unlock() is called with a scheduler lock held from interrupt-entry/exit situations where in_irq() returns false, deadlock can result. This commit resolves these deadlocks by using negative values of the per-task ->rcu_read_lock_nesting counter to indicate that an instance of __rcu_read_unlock() is in flight, which in turn prevents instances from interrupt handlers from doing any special processing. This patch is inspired by Steven Rostedt's earlier patch that similarly made __rcu_read_unlock() guard against interrupt-mediated recursion (see https://lkml.org/lkml/2011/7/15/326), but this commit refines Steven's approach to avoid the need for preemption disabling on the __rcu_read_unlock() fastpath and to also avoid the need for manipulating a separate per-CPU variable. This patch avoids need for preempt_disable() by instead using negative values of the per-task ->rcu_read_lock_nesting counter. Note that nested rcu_read_lock()/rcu_read_unlock() pairs are still permitted, but they will never see ->rcu_read_lock_nesting go to zero, and will therefore never invoke rcu_read_unlock_special(), thus preventing them from seeing the RCU_READ_UNLOCK_BLOCKED bit should it be set in ->rcu_read_unlock_special. This patch also adds a check for ->rcu_read_unlock_special being negative in rcu_check_callbacks(), thus preventing the RCU_READ_UNLOCK_NEED_QS bit from being set should a scheduling-clock interrupt occur while __rcu_read_unlock() is exiting from an outermost RCU read-side critical section. Of course, __rcu_read_unlock() can be preempted during the time that ->rcu_read_lock_nesting is negative. This could result in the setting of the RCU_READ_UNLOCK_BLOCKED bit after __rcu_read_unlock() checks it, and would also result it this task being queued on the corresponding rcu_node structure's blkd_tasks list. Therefore, some later RCU read-side critical section would enter rcu_read_unlock_special() to clean up -- which could result in deadlock if that critical section happened to be in the scheduler where the runqueue or priority-inheritance locks were held. This situation is dealt with by making rcu_preempt_note_context_switch() check for negative ->rcu_read_lock_nesting, thus refraining from queuing the task (and from setting RCU_READ_UNLOCK_BLOCKED) if we are already exiting from the outermost RCU read-side critical section (in other words, we really are no longer actually in that RCU read-side critical section). In addition, rcu_preempt_note_context_switch() invokes rcu_read_unlock_special() to carry out the cleanup in this case, which clears out the ->rcu_read_unlock_special bits and dequeues the task (if necessary), in turn avoiding needless delay of the current RCU grace period and needless RCU priority boosting. It is still illegal to call rcu_read_unlock() while holding a scheduler lock if the prior RCU read-side critical section has ever had either preemption or irqs enabled. However, the common use case is legal, namely where then entire RCU read-side critical section executes with irqs disabled, for example, when the scheduler lock is held across the entire lifetime of the RCU read-side critical section. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2011-07-18 04:14:35 +00:00
if (t->rcu_read_lock_nesting > 0 &&
per_cpu(rcu_preempt_data, cpu).qs_pending)
t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
}
#ifdef CONFIG_RCU_BOOST
rcu: Use softirq to address performance regression Commit a26ac2455ffcf3(rcu: move TREE_RCU from softirq to kthread) introduced performance regression. In an AIM7 test, this commit degraded performance by about 40%. The commit runs rcu callbacks in a kthread instead of softirq. We observed high rate of context switch which is caused by this. Out test system has 64 CPUs and HZ is 1000, so we saw more than 64k context switch per second which is caused by RCU's per-CPU kthread. A trace showed that most of the time the RCU per-CPU kthread doesn't actually handle any callbacks, but instead just does a very small amount of work handling grace periods. This means that RCU's per-CPU kthreads are making the scheduler do quite a bit of work in order to allow a very small amount of RCU-related processing to be done. Alex Shi's analysis determined that this slowdown is due to lock contention within the scheduler. Unfortunately, as Peter Zijlstra points out, the scheduler's real-time semantics require global action, which means that this contention is inherent in real-time scheduling. (Yes, perhaps someone will come up with a workaround -- otherwise, -rt is not going to do well on large SMP systems -- but this patch will work around this issue in the meantime. And "the meantime" might well be forever.) This patch therefore re-introduces softirq processing to RCU, but only for core RCU work. RCU callbacks are still executed in kthread context, so that only a small amount of RCU work runs in softirq context in the common case. This should minimize ksoftirqd execution, allowing us to skip boosting of ksoftirqd for CONFIG_RCU_BOOST=y kernels. Signed-off-by: Shaohua Li <shaohua.li@intel.com> Tested-by: "Alex,Shi" <alex.shi@intel.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2011-06-14 05:26:25 +00:00
static void rcu_preempt_do_callbacks(void)
{
rcu_do_batch(&rcu_preempt_state, &__get_cpu_var(rcu_preempt_data));
}
#endif /* #ifdef CONFIG_RCU_BOOST */
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
/*
* Queue a preemptible-RCU callback for invocation after a grace period.
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
*/
void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
__call_rcu(head, func, &rcu_preempt_state, -1, 0);
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
}
EXPORT_SYMBOL_GPL(call_rcu);
/*
* Queue an RCU callback for lazy invocation after a grace period.
* This will likely be later named something like "call_rcu_lazy()",
* but this change will require some way of tagging the lazy RCU
* callbacks in the list of pending callbacks. Until then, this
* function may only be called from __kfree_rcu().
*/
void kfree_call_rcu(struct rcu_head *head,
void (*func)(struct rcu_head *rcu))
{
__call_rcu(head, func, &rcu_preempt_state, -1, 1);
}
EXPORT_SYMBOL_GPL(kfree_call_rcu);
/**
* synchronize_rcu - wait until a grace period has elapsed.
*
* Control will return to the caller some time after a full grace
* period has elapsed, in other words after all currently executing RCU
* read-side critical sections have completed. Note, however, that
* upon return from synchronize_rcu(), the caller might well be executing
* concurrently with new RCU read-side critical sections that began while
* synchronize_rcu() was waiting. RCU read-side critical sections are
* delimited by rcu_read_lock() and rcu_read_unlock(), and may be nested.
*
* See the description of synchronize_sched() for more detailed information
* on memory ordering guarantees.
*/
void synchronize_rcu(void)
{
rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
!lock_is_held(&rcu_lock_map) &&
!lock_is_held(&rcu_sched_lock_map),
"Illegal synchronize_rcu() in RCU read-side critical section");
if (!rcu_scheduler_active)
return;
if (rcu_expedited)
synchronize_rcu_expedited();
else
wait_rcu_gp(call_rcu);
}
EXPORT_SYMBOL_GPL(synchronize_rcu);
static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
static unsigned long sync_rcu_preempt_exp_count;
static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
/*
* Return non-zero if there are any tasks in RCU read-side critical
* sections blocking the current preemptible-RCU expedited grace period.
* If there is no preemptible-RCU expedited grace period currently in
* progress, returns zero unconditionally.
*/
static int rcu_preempted_readers_exp(struct rcu_node *rnp)
{
return rnp->exp_tasks != NULL;
}
/*
* return non-zero if there is no RCU expedited grace period in progress
* for the specified rcu_node structure, in other words, if all CPUs and
* tasks covered by the specified rcu_node structure have done their bit
* for the current expedited grace period. Works only for preemptible
* RCU -- other RCU implementation use other means.
*
* Caller must hold sync_rcu_preempt_exp_mutex.
*/
static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
{
return !rcu_preempted_readers_exp(rnp) &&
ACCESS_ONCE(rnp->expmask) == 0;
}
/*
* Report the exit from RCU read-side critical section for the last task
* that queued itself during or before the current expedited preemptible-RCU
* grace period. This event is reported either to the rcu_node structure on
* which the task was queued or to one of that rcu_node structure's ancestors,
* recursively up the tree. (Calm down, calm down, we do the recursion
* iteratively!)
*
* Most callers will set the "wake" flag, but the task initiating the
* expedited grace period need not wake itself.
*
* Caller must hold sync_rcu_preempt_exp_mutex.
*/
static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
bool wake)
{
unsigned long flags;
unsigned long mask;
raw_spin_lock_irqsave(&rnp->lock, flags);
for (;;) {
if (!sync_rcu_preempt_exp_done(rnp)) {
raw_spin_unlock_irqrestore(&rnp->lock, flags);
break;
}
if (rnp->parent == NULL) {
raw_spin_unlock_irqrestore(&rnp->lock, flags);
if (wake)
wake_up(&sync_rcu_preempt_exp_wq);
break;
}
mask = rnp->grpmask;
raw_spin_unlock(&rnp->lock); /* irqs remain disabled */
rnp = rnp->parent;
raw_spin_lock(&rnp->lock); /* irqs already disabled */
rnp->expmask &= ~mask;
}
}
/*
* Snapshot the tasks blocking the newly started preemptible-RCU expedited
* grace period for the specified rcu_node structure. If there are no such
* tasks, report it up the rcu_node hierarchy.
*
* Caller must hold sync_rcu_preempt_exp_mutex and must exclude
* CPU hotplug operations.
*/
static void
sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
{
unsigned long flags;
int must_wait = 0;
raw_spin_lock_irqsave(&rnp->lock, flags);
if (list_empty(&rnp->blkd_tasks)) {
raw_spin_unlock_irqrestore(&rnp->lock, flags);
} else {
rnp->exp_tasks = rnp->blkd_tasks.next;
rcu_initiate_boost(rnp, flags); /* releases rnp->lock */
must_wait = 1;
}
if (!must_wait)
rcu_report_exp_rnp(rsp, rnp, false); /* Don't wake self. */
}
/**
* synchronize_rcu_expedited - Brute-force RCU grace period
*
* Wait for an RCU-preempt grace period, but expedite it. The basic
* idea is to invoke synchronize_sched_expedited() to push all the tasks to
* the ->blkd_tasks lists and wait for this list to drain. This consumes
* significant time on all CPUs and is unfriendly to real-time workloads,
* so is thus not recommended for any sort of common-case code.
* In fact, if you are using synchronize_rcu_expedited() in a loop,
* please restructure your code to batch your updates, and then Use a
* single synchronize_rcu() instead.
*
* Note that it is illegal to call this function while holding any lock
* that is acquired by a CPU-hotplug notifier. And yes, it is also illegal
* to call this function from a CPU-hotplug notifier. Failing to observe
* these restriction will result in deadlock.
*/
void synchronize_rcu_expedited(void)
{
unsigned long flags;
struct rcu_node *rnp;
struct rcu_state *rsp = &rcu_preempt_state;
unsigned long snap;
int trycount = 0;
smp_mb(); /* Caller's modifications seen first by other CPUs. */
snap = ACCESS_ONCE(sync_rcu_preempt_exp_count) + 1;
smp_mb(); /* Above access cannot bleed into critical section. */
/*
* Block CPU-hotplug operations. This means that any CPU-hotplug
* operation that finds an rcu_node structure with tasks in the
* process of being boosted will know that all tasks blocking
* this expedited grace period will already be in the process of
* being boosted. This simplifies the process of moving tasks
* from leaf to root rcu_node structures.
*/
get_online_cpus();
/*
* Acquire lock, falling back to synchronize_rcu() if too many
* lock-acquisition failures. Of course, if someone does the
* expedited grace period for us, just leave.
*/
while (!mutex_trylock(&sync_rcu_preempt_exp_mutex)) {
if (ULONG_CMP_LT(snap,
ACCESS_ONCE(sync_rcu_preempt_exp_count))) {
put_online_cpus();
goto mb_ret; /* Others did our work for us. */
}
if (trycount++ < 10) {
udelay(trycount * num_online_cpus());
} else {
put_online_cpus();
wait_rcu_gp(call_rcu);
return;
}
}
if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count))) {
put_online_cpus();
goto unlock_mb_ret; /* Others did our work for us. */
}
/* force all RCU readers onto ->blkd_tasks lists. */
synchronize_sched_expedited();
/* Initialize ->expmask for all non-leaf rcu_node structures. */
rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) {
raw_spin_lock_irqsave(&rnp->lock, flags);
rnp->expmask = rnp->qsmaskinit;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
/* Snapshot current state of ->blkd_tasks lists. */
rcu_for_each_leaf_node(rsp, rnp)
sync_rcu_preempt_exp_init(rsp, rnp);
if (NUM_RCU_NODES > 1)
sync_rcu_preempt_exp_init(rsp, rcu_get_root(rsp));
put_online_cpus();
/* Wait for snapshotted ->blkd_tasks lists to drain. */
rnp = rcu_get_root(rsp);
wait_event(sync_rcu_preempt_exp_wq,
sync_rcu_preempt_exp_done(rnp));
/* Clean up and exit. */
smp_mb(); /* ensure expedited GP seen before counter increment. */
ACCESS_ONCE(sync_rcu_preempt_exp_count)++;
unlock_mb_ret:
mutex_unlock(&sync_rcu_preempt_exp_mutex);
mb_ret:
smp_mb(); /* ensure subsequent action seen after grace period. */
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
/**
* rcu_barrier - Wait until all in-flight call_rcu() callbacks complete.
*
* Note that this primitive does not necessarily wait for an RCU grace period
* to complete. For example, if there are no RCU callbacks queued anywhere
* in the system, then rcu_barrier() is within its rights to return
* immediately, without waiting for anything, much less an RCU grace period.
*/
void rcu_barrier(void)
{
_rcu_barrier(&rcu_preempt_state);
}
EXPORT_SYMBOL_GPL(rcu_barrier);
/*
* Initialize preemptible RCU's state structures.
*/
static void __init __rcu_init_preempt(void)
{
rcu_init_one(&rcu_preempt_state, &rcu_preempt_data);
}
/*
* Check for a task exiting while in a preemptible-RCU read-side
* critical section, clean up if so. No need to issue warnings,
* as debug_check_no_locks_held() already does this if lockdep
* is enabled.
*/
void exit_rcu(void)
{
struct task_struct *t = current;
if (likely(list_empty(&current->rcu_node_entry)))
return;
t->rcu_read_lock_nesting = 1;
barrier();
t->rcu_read_unlock_special = RCU_READ_UNLOCK_BLOCKED;
__rcu_read_unlock();
}
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
#else /* #ifdef CONFIG_TREE_PREEMPT_RCU */
static struct rcu_state *rcu_state = &rcu_sched_state;
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
/*
* Tell them what RCU they are running.
*/
static void __init rcu_bootup_announce(void)
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
{
pr_info("Hierarchical RCU implementation.\n");
rcu_bootup_announce_oddness();
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
}
/*
* Return the number of RCU batches processed thus far for debug & stats.
*/
long rcu_batches_completed(void)
{
return rcu_batches_completed_sched();
}
EXPORT_SYMBOL_GPL(rcu_batches_completed);
/*
* Force a quiescent state for RCU, which, because there is no preemptible
* RCU, becomes the same as rcu-sched.
*/
void rcu_force_quiescent_state(void)
{
rcu_sched_force_quiescent_state();
}
EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
/*
* Because preemptible RCU does not exist, we never have to check for
* CPUs being in quiescent states.
*/
static void rcu_preempt_note_context_switch(int cpu)
{
}
/*
* Because preemptible RCU does not exist, there are never any preempted
* RCU readers.
*/
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
{
return 0;
}
rcu: Fix grace-period-stall bug on large systems with CPU hotplug When the last CPU of a given leaf rcu_node structure goes offline, all of the tasks queued on that leaf rcu_node structure (due to having blocked in their current RCU read-side critical sections) are requeued onto the root rcu_node structure. This requeuing is carried out by rcu_preempt_offline_tasks(). However, it is possible that these queued tasks are the only thing preventing the leaf rcu_node structure from reporting a quiescent state up the rcu_node hierarchy. Unfortunately, the old code would fail to do this reporting, resulting in a grace-period stall given the following sequence of events: 1. Kernel built for more than 32 CPUs on 32-bit systems or for more than 64 CPUs on 64-bit systems, so that there is more than one rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set to a number smaller than CONFIG_NR_CPUS.) 2. The kernel is built with CONFIG_TREE_PREEMPT_RCU. 3. A task running on a CPU associated with a given leaf rcu_node structure blocks while in an RCU read-side critical section -and- that CPU has not yet passed through a quiescent state for the current RCU grace period. This will cause the task to be queued on the leaf rcu_node's blocked_tasks[] array, in particular, on the element of this array corresponding to the current grace period. 4. Each of the remaining CPUs corresponding to this same leaf rcu_node structure pass through a quiescent state. However, the task is still in its RCU read-side critical section, so these quiescent states cannot be reported further up the rcu_node hierarchy. Nevertheless, all bits in the leaf rcu_node structure's ->qsmask field are now zero. 5. Each of the remaining CPUs go offline. (The events in step #4 and #5 can happen in any order as long as each CPU passes through a quiescent state before going offline.) 6. When the last CPU goes offline, __rcu_offline_cpu() will invoke rcu_preempt_offline_tasks(), which will move the task to the root rcu_node structure, but without reporting a quiescent state up the rcu_node hierarchy (and this failure to report a quiescent state is the bug). But because this leaf rcu_node structure's ->qsmask field is already zero and its ->block_tasks[] entries are all empty, force_quiescent_state() will skip this rcu_node structure. Therefore, grace periods are now hung. This patch abstracts some code out of rcu_read_unlock_special(), calling the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet() from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking task_quiet() from __rcu_offline_cpu() reports the quiescent state up the rcu_node hierarchy, fixing the bug. This ends up requiring a separate lock_class_key per level of the rcu_node hierarchy, which this patch also provides. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12589088301770-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-11-22 16:53:48 +00:00
#ifdef CONFIG_HOTPLUG_CPU
/* Because preemptible RCU does not exist, no quieting of tasks. */
rcu: Rename "quiet" functions The number of "quiet" functions has grown recently, and the names are no longer very descriptive. The point of all of these functions is to do some portion of the task of reporting a quiescent state, so rename them accordingly: o cpu_quiet() becomes rcu_report_qs_rdp(), which reports a quiescent state to the per-CPU rcu_data structure. If this turns out to be a new quiescent state for this grace period, then rcu_report_qs_rnp() will be invoked to propagate the quiescent state up the rcu_node hierarchy. o cpu_quiet_msk() becomes rcu_report_qs_rnp(), which reports a quiescent state for a given CPU (or possibly a set of CPUs) up the rcu_node hierarchy. o cpu_quiet_msk_finish() becomes rcu_report_qs_rsp(), which reports a full set of quiescent states to the global rcu_state structure. o task_quiet() becomes rcu_report_unblock_qs_rnp(), which reports a quiescent state due to a task exiting an RCU read-side critical section that had previously blocked in that same critical section. As indicated by the new name, this type of quiescent state is reported up the rcu_node hierarchy (using rcu_report_qs_rnp() to do so). Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Josh Triplett <josh@joshtriplett.org> Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com> Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12597846163698-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-12-02 20:10:13 +00:00
static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp, unsigned long flags)
rcu: Fix grace-period-stall bug on large systems with CPU hotplug When the last CPU of a given leaf rcu_node structure goes offline, all of the tasks queued on that leaf rcu_node structure (due to having blocked in their current RCU read-side critical sections) are requeued onto the root rcu_node structure. This requeuing is carried out by rcu_preempt_offline_tasks(). However, it is possible that these queued tasks are the only thing preventing the leaf rcu_node structure from reporting a quiescent state up the rcu_node hierarchy. Unfortunately, the old code would fail to do this reporting, resulting in a grace-period stall given the following sequence of events: 1. Kernel built for more than 32 CPUs on 32-bit systems or for more than 64 CPUs on 64-bit systems, so that there is more than one rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set to a number smaller than CONFIG_NR_CPUS.) 2. The kernel is built with CONFIG_TREE_PREEMPT_RCU. 3. A task running on a CPU associated with a given leaf rcu_node structure blocks while in an RCU read-side critical section -and- that CPU has not yet passed through a quiescent state for the current RCU grace period. This will cause the task to be queued on the leaf rcu_node's blocked_tasks[] array, in particular, on the element of this array corresponding to the current grace period. 4. Each of the remaining CPUs corresponding to this same leaf rcu_node structure pass through a quiescent state. However, the task is still in its RCU read-side critical section, so these quiescent states cannot be reported further up the rcu_node hierarchy. Nevertheless, all bits in the leaf rcu_node structure's ->qsmask field are now zero. 5. Each of the remaining CPUs go offline. (The events in step #4 and #5 can happen in any order as long as each CPU passes through a quiescent state before going offline.) 6. When the last CPU goes offline, __rcu_offline_cpu() will invoke rcu_preempt_offline_tasks(), which will move the task to the root rcu_node structure, but without reporting a quiescent state up the rcu_node hierarchy (and this failure to report a quiescent state is the bug). But because this leaf rcu_node structure's ->qsmask field is already zero and its ->block_tasks[] entries are all empty, force_quiescent_state() will skip this rcu_node structure. Therefore, grace periods are now hung. This patch abstracts some code out of rcu_read_unlock_special(), calling the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet() from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking task_quiet() from __rcu_offline_cpu() reports the quiescent state up the rcu_node hierarchy, fixing the bug. This ends up requiring a separate lock_class_key per level of the rcu_node hierarchy, which this patch also provides. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12589088301770-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-11-22 16:53:48 +00:00
{
raw_spin_unlock_irqrestore(&rnp->lock, flags);
rcu: Fix grace-period-stall bug on large systems with CPU hotplug When the last CPU of a given leaf rcu_node structure goes offline, all of the tasks queued on that leaf rcu_node structure (due to having blocked in their current RCU read-side critical sections) are requeued onto the root rcu_node structure. This requeuing is carried out by rcu_preempt_offline_tasks(). However, it is possible that these queued tasks are the only thing preventing the leaf rcu_node structure from reporting a quiescent state up the rcu_node hierarchy. Unfortunately, the old code would fail to do this reporting, resulting in a grace-period stall given the following sequence of events: 1. Kernel built for more than 32 CPUs on 32-bit systems or for more than 64 CPUs on 64-bit systems, so that there is more than one rcu_node structure. (Or CONFIG_RCU_FANOUT is artificially set to a number smaller than CONFIG_NR_CPUS.) 2. The kernel is built with CONFIG_TREE_PREEMPT_RCU. 3. A task running on a CPU associated with a given leaf rcu_node structure blocks while in an RCU read-side critical section -and- that CPU has not yet passed through a quiescent state for the current RCU grace period. This will cause the task to be queued on the leaf rcu_node's blocked_tasks[] array, in particular, on the element of this array corresponding to the current grace period. 4. Each of the remaining CPUs corresponding to this same leaf rcu_node structure pass through a quiescent state. However, the task is still in its RCU read-side critical section, so these quiescent states cannot be reported further up the rcu_node hierarchy. Nevertheless, all bits in the leaf rcu_node structure's ->qsmask field are now zero. 5. Each of the remaining CPUs go offline. (The events in step #4 and #5 can happen in any order as long as each CPU passes through a quiescent state before going offline.) 6. When the last CPU goes offline, __rcu_offline_cpu() will invoke rcu_preempt_offline_tasks(), which will move the task to the root rcu_node structure, but without reporting a quiescent state up the rcu_node hierarchy (and this failure to report a quiescent state is the bug). But because this leaf rcu_node structure's ->qsmask field is already zero and its ->block_tasks[] entries are all empty, force_quiescent_state() will skip this rcu_node structure. Therefore, grace periods are now hung. This patch abstracts some code out of rcu_read_unlock_special(), calling the result task_quiet() by analogy with cpu_quiet(), and invokes task_quiet() from both rcu_read_lock_special() and __rcu_offline_cpu(). Invoking task_quiet() from __rcu_offline_cpu() reports the quiescent state up the rcu_node hierarchy, fixing the bug. This ends up requiring a separate lock_class_key per level of the rcu_node hierarchy, which this patch also provides. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <12589088301770-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-11-22 16:53:48 +00:00
}
#endif /* #ifdef CONFIG_HOTPLUG_CPU */
/*
* Because preemptible RCU does not exist, we never have to check for
* tasks blocked within RCU read-side critical sections.
*/
static void rcu_print_detail_task_stall(struct rcu_state *rsp)
{
}
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
/*
* Because preemptible RCU does not exist, we never have to check for
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
* tasks blocked within RCU read-side critical sections.
*/
static int rcu_print_task_stall(struct rcu_node *rnp)
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
{
return 0;
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
}
/*
* Because there is no preemptible RCU, there can be no readers blocked,
* so there is no need to check for blocked tasks. So check only for
* bogus qsmask values.
*/
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
WARN_ON_ONCE(rnp->qsmask);
}
#ifdef CONFIG_HOTPLUG_CPU
/*
* Because preemptible RCU does not exist, it never needs to migrate
rcu: Fix TREE_PREEMPT_RCU CPU_HOTPLUG bad-luck hang If the following sequence of events occurs, then TREE_PREEMPT_RCU will hang waiting for a grace period to complete, eventually OOMing the system: o A TREE_PREEMPT_RCU build of the kernel is booted on a system with more than 64 physical CPUs present (32 on a 32-bit system). Alternatively, a TREE_PREEMPT_RCU build of the kernel is booted with RCU_FANOUT set to a sufficiently small value that the physical CPUs populate two or more leaf rcu_node structures. o A task is preempted in an RCU read-side critical section while running on a CPU corresponding to a given leaf rcu_node structure. o All CPUs corresponding to this same leaf rcu_node structure record quiescent states for the current grace period. o All of these same CPUs go offline (hence the need for enough physical CPUs to populate more than one leaf rcu_node structure). This causes the preempted task to be moved to the root rcu_node structure. At this point, there is nothing left to cause the quiescent state to be propagated up the rcu_node tree, so the current grace period never completes. The simplest fix, especially after considering the deadlock possibilities, is to detect this situation when the last CPU is offlined, and to set that CPU's ->qsmask bit in its leaf rcu_node structure. This will cause the next invocation of force_quiescent_state() to end the grace period. Without this fix, this hang can be triggered in an hour or so on some machines with rcutorture and random CPU onlining/offlining. With this fix, these same machines pass a full 10 hours of this sort of abuse. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <20091015162614.GA19131@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-10-15 16:26:14 +00:00
* tasks that were blocked within RCU read-side critical sections, and
* such non-existent tasks cannot possibly have been blocking the current
* grace period.
*/
rcu: Fix TREE_PREEMPT_RCU CPU_HOTPLUG bad-luck hang If the following sequence of events occurs, then TREE_PREEMPT_RCU will hang waiting for a grace period to complete, eventually OOMing the system: o A TREE_PREEMPT_RCU build of the kernel is booted on a system with more than 64 physical CPUs present (32 on a 32-bit system). Alternatively, a TREE_PREEMPT_RCU build of the kernel is booted with RCU_FANOUT set to a sufficiently small value that the physical CPUs populate two or more leaf rcu_node structures. o A task is preempted in an RCU read-side critical section while running on a CPU corresponding to a given leaf rcu_node structure. o All CPUs corresponding to this same leaf rcu_node structure record quiescent states for the current grace period. o All of these same CPUs go offline (hence the need for enough physical CPUs to populate more than one leaf rcu_node structure). This causes the preempted task to be moved to the root rcu_node structure. At this point, there is nothing left to cause the quiescent state to be propagated up the rcu_node tree, so the current grace period never completes. The simplest fix, especially after considering the deadlock possibilities, is to detect this situation when the last CPU is offlined, and to set that CPU's ->qsmask bit in its leaf rcu_node structure. This will cause the next invocation of force_quiescent_state() to end the grace period. Without this fix, this hang can be triggered in an hour or so on some machines with rcutorture and random CPU onlining/offlining. With this fix, these same machines pass a full 10 hours of this sort of abuse. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <20091015162614.GA19131@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-10-15 16:26:14 +00:00
static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
struct rcu_node *rnp,
struct rcu_data *rdp)
{
rcu: Fix TREE_PREEMPT_RCU CPU_HOTPLUG bad-luck hang If the following sequence of events occurs, then TREE_PREEMPT_RCU will hang waiting for a grace period to complete, eventually OOMing the system: o A TREE_PREEMPT_RCU build of the kernel is booted on a system with more than 64 physical CPUs present (32 on a 32-bit system). Alternatively, a TREE_PREEMPT_RCU build of the kernel is booted with RCU_FANOUT set to a sufficiently small value that the physical CPUs populate two or more leaf rcu_node structures. o A task is preempted in an RCU read-side critical section while running on a CPU corresponding to a given leaf rcu_node structure. o All CPUs corresponding to this same leaf rcu_node structure record quiescent states for the current grace period. o All of these same CPUs go offline (hence the need for enough physical CPUs to populate more than one leaf rcu_node structure). This causes the preempted task to be moved to the root rcu_node structure. At this point, there is nothing left to cause the quiescent state to be propagated up the rcu_node tree, so the current grace period never completes. The simplest fix, especially after considering the deadlock possibilities, is to detect this situation when the last CPU is offlined, and to set that CPU's ->qsmask bit in its leaf rcu_node structure. This will cause the next invocation of force_quiescent_state() to end the grace period. Without this fix, this hang can be triggered in an hour or so on some machines with rcutorture and random CPU onlining/offlining. With this fix, these same machines pass a full 10 hours of this sort of abuse. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <20091015162614.GA19131@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-10-15 16:26:14 +00:00
return 0;
}
#endif /* #ifdef CONFIG_HOTPLUG_CPU */
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
/*
* Because preemptible RCU does not exist, it never has any callbacks
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
* to check.
*/
static void rcu_preempt_check_callbacks(int cpu)
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
{
}
/*
* Queue an RCU callback for lazy invocation after a grace period.
* This will likely be later named something like "call_rcu_lazy()",
* but this change will require some way of tagging the lazy RCU
* callbacks in the list of pending callbacks. Until then, this
* function may only be called from __kfree_rcu().
*
* Because there is no preemptible RCU, we use RCU-sched instead.
*/
void kfree_call_rcu(struct rcu_head *head,
void (*func)(struct rcu_head *rcu))
{
__call_rcu(head, func, &rcu_sched_state, -1, 1);
}
EXPORT_SYMBOL_GPL(kfree_call_rcu);
/*
* Wait for an rcu-preempt grace period, but make it happen quickly.
* But because preemptible RCU does not exist, map to rcu-sched.
*/
void synchronize_rcu_expedited(void)
{
synchronize_sched_expedited();
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
#ifdef CONFIG_HOTPLUG_CPU
/*
* Because preemptible RCU does not exist, there is never any need to
* report on tasks preempted in RCU read-side critical sections during
* expedited RCU grace periods.
*/
static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
bool wake)
{
}
#endif /* #ifdef CONFIG_HOTPLUG_CPU */
/*
* Because preemptible RCU does not exist, rcu_barrier() is just
* another name for rcu_barrier_sched().
*/
void rcu_barrier(void)
{
rcu_barrier_sched();
}
EXPORT_SYMBOL_GPL(rcu_barrier);
/*
* Because preemptible RCU does not exist, it need not be initialized.
*/
static void __init __rcu_init_preempt(void)
{
}
/*
* Because preemptible RCU does not exist, tasks cannot possibly exit
* while in preemptible RCU read-side critical sections.
*/
void exit_rcu(void)
{
}
rcu: Merge preemptable-RCU functionality into hierarchical RCU Create a kernel/rcutree_plugin.h file that contains definitions for preemptable RCU (or, under the #else branch of the #ifdef, empty definitions for the classic non-preemptable semantics). These definitions fit into plugins defined in kernel/rcutree.c for this purpose. This variant of preemptable RCU uses a new algorithm whose read-side expense is roughly that of classic hierarchical RCU under CONFIG_PREEMPT. This new algorithm's update-side expense is similar to that of classic hierarchical RCU, and, in absence of read-side preemption or blocking, is exactly that of classic hierarchical RCU. Perhaps more important, this new algorithm has a much simpler implementation, saving well over 1,000 lines of code compared to mainline's implementation of preemptable RCU, which will hopefully be retired in favor of this new algorithm. The simplifications are obtained by maintaining per-task nesting state for running tasks, and using a simple lock-protected algorithm to handle accounting when tasks block within RCU read-side critical sections, making use of lessons learned while creating numerous user-level RCU implementations over the past 18 months. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org LKML-Reference: <12509746134003-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2009-08-22 20:56:52 +00:00
#endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
rcu: Accelerate grace period if last non-dynticked CPU Currently, rcu_needs_cpu() simply checks whether the current CPU has an outstanding RCU callback, which means that the last CPU to go into dyntick-idle mode might wait a few ticks for the relevant grace periods to complete. However, if all the other CPUs are in dyntick-idle mode, and if this CPU is in a quiescent state (which it is for RCU-bh and RCU-sched any time that we are considering going into dyntick-idle mode), then the grace period is instantly complete. This patch therefore repeatedly invokes the RCU grace-period machinery in order to force any needed grace periods to complete quickly. It does so a limited number of times in order to prevent starvation by an RCU callback function that might pass itself to call_rcu(). However, if any CPU other than the current one is not in dyntick-idle mode, fall back to simply checking (with fix to bug noted by Lai Jiangshan). Also, take advantage of last grace-period forcing, the opportunity to do so noted by Steve Rostedt. And apply simplified #ifdef condition suggested by Frederic Weisbecker. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <1266887105-1528-15-git-send-email-paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2010-02-23 01:04:59 +00:00
#ifdef CONFIG_RCU_BOOST
#include "rtmutex_common.h"
#ifdef CONFIG_RCU_TRACE
static void rcu_initiate_boost_trace(struct rcu_node *rnp)
{
if (list_empty(&rnp->blkd_tasks))
rnp->n_balk_blkd_tasks++;
else if (rnp->exp_tasks == NULL && rnp->gp_tasks == NULL)
rnp->n_balk_exp_gp_tasks++;
else if (rnp->gp_tasks != NULL && rnp->boost_tasks != NULL)
rnp->n_balk_boost_tasks++;
else if (rnp->gp_tasks != NULL && rnp->qsmask != 0)
rnp->n_balk_notblocked++;
else if (rnp->gp_tasks != NULL &&
ULONG_CMP_LT(jiffies, rnp->boost_time))
rnp->n_balk_notyet++;
else
rnp->n_balk_nos++;
}
#else /* #ifdef CONFIG_RCU_TRACE */
static void rcu_initiate_boost_trace(struct rcu_node *rnp)
{
}
#endif /* #else #ifdef CONFIG_RCU_TRACE */
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
static void rcu_wake_cond(struct task_struct *t, int status)
{
/*
* If the thread is yielding, only wake it when this
* is invoked from idle
*/
if (status != RCU_KTHREAD_YIELDING || is_idle_task(current))
wake_up_process(t);
}
/*
* Carry out RCU priority boosting on the task indicated by ->exp_tasks
* or ->boost_tasks, advancing the pointer to the next task in the
* ->blkd_tasks list.
*
* Note that irqs must be enabled: boosting the task can block.
* Returns 1 if there are more tasks needing to be boosted.
*/
static int rcu_boost(struct rcu_node *rnp)
{
unsigned long flags;
struct rt_mutex mtx;
struct task_struct *t;
struct list_head *tb;
if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL)
return 0; /* Nothing left to boost. */
raw_spin_lock_irqsave(&rnp->lock, flags);
/*
* Recheck under the lock: all tasks in need of boosting
* might exit their RCU read-side critical sections on their own.
*/
if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {
raw_spin_unlock_irqrestore(&rnp->lock, flags);
return 0;
}
/*
* Preferentially boost tasks blocking expedited grace periods.
* This cannot starve the normal grace periods because a second
* expedited grace period must boost all blocked tasks, including
* those blocking the pre-existing normal grace period.
*/
if (rnp->exp_tasks != NULL) {
tb = rnp->exp_tasks;
rnp->n_exp_boosts++;
} else {
tb = rnp->boost_tasks;
rnp->n_normal_boosts++;
}
rnp->n_tasks_boosted++;
/*
* We boost task t by manufacturing an rt_mutex that appears to
* be held by task t. We leave a pointer to that rt_mutex where
* task t can find it, and task t will release the mutex when it
* exits its outermost RCU read-side critical section. Then
* simply acquiring this artificial rt_mutex will boost task
* t's priority. (Thanks to tglx for suggesting this approach!)
*
* Note that task t must acquire rnp->lock to remove itself from
* the ->blkd_tasks list, which it will do from exit() if from
* nowhere else. We therefore are guaranteed that task t will
* stay around at least until we drop rnp->lock. Note that
* rnp->lock also resolves races between our priority boosting
* and task t's exiting its outermost RCU read-side critical
* section.
*/
t = container_of(tb, struct task_struct, rcu_node_entry);
rt_mutex_init_proxy_locked(&mtx, t);
t->rcu_boost_mutex = &mtx;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
rt_mutex_lock(&mtx); /* Side effect: boosts task t's priority. */
rt_mutex_unlock(&mtx); /* Keep lockdep happy. */
return ACCESS_ONCE(rnp->exp_tasks) != NULL ||
ACCESS_ONCE(rnp->boost_tasks) != NULL;
}
/*
* Priority-boosting kthread. One per leaf rcu_node and one for the
* root rcu_node.
*/
static int rcu_boost_kthread(void *arg)
{
struct rcu_node *rnp = (struct rcu_node *)arg;
int spincnt = 0;
int more2boost;
trace_rcu_utilization("Start boost kthread@init");
for (;;) {
rnp->boost_kthread_status = RCU_KTHREAD_WAITING;
trace_rcu_utilization("End boost kthread@rcu_wait");
rcu_wait(rnp->boost_tasks || rnp->exp_tasks);
trace_rcu_utilization("Start boost kthread@rcu_wait");
rnp->boost_kthread_status = RCU_KTHREAD_RUNNING;
more2boost = rcu_boost(rnp);
if (more2boost)
spincnt++;
else
spincnt = 0;
if (spincnt > 10) {
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
rnp->boost_kthread_status = RCU_KTHREAD_YIELDING;
trace_rcu_utilization("End boost kthread@rcu_yield");
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
schedule_timeout_interruptible(2);
trace_rcu_utilization("Start boost kthread@rcu_yield");
spincnt = 0;
}
}
/* NOTREACHED */
trace_rcu_utilization("End boost kthread@notreached");
return 0;
}
/*
* Check to see if it is time to start boosting RCU readers that are
* blocking the current grace period, and, if so, tell the per-rcu_node
* kthread to start boosting them. If there is an expedited grace
* period in progress, it is always time to boost.
*
* The caller must hold rnp->lock, which this function releases.
* The ->boost_kthread_task is immortal, so we don't need to worry
* about it going away.
*/
static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
{
struct task_struct *t;
if (!rcu_preempt_blocked_readers_cgp(rnp) && rnp->exp_tasks == NULL) {
rnp->n_balk_exp_gp_tasks++;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
return;
}
if (rnp->exp_tasks != NULL ||
(rnp->gp_tasks != NULL &&
rnp->boost_tasks == NULL &&
rnp->qsmask == 0 &&
ULONG_CMP_GE(jiffies, rnp->boost_time))) {
if (rnp->exp_tasks == NULL)
rnp->boost_tasks = rnp->gp_tasks;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
t = rnp->boost_kthread_task;
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
if (t)
rcu_wake_cond(t, rnp->boost_kthread_status);
} else {
rcu_initiate_boost_trace(rnp);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
}
/*
* Wake up the per-CPU kthread to invoke RCU callbacks.
*/
static void invoke_rcu_callbacks_kthread(void)
{
unsigned long flags;
local_irq_save(flags);
__this_cpu_write(rcu_cpu_has_work, 1);
if (__this_cpu_read(rcu_cpu_kthread_task) != NULL &&
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
current != __this_cpu_read(rcu_cpu_kthread_task)) {
rcu_wake_cond(__this_cpu_read(rcu_cpu_kthread_task),
__this_cpu_read(rcu_cpu_kthread_status));
}
local_irq_restore(flags);
}
/*
* Is the current CPU running the RCU-callbacks kthread?
* Caller must have preemption disabled.
*/
static bool rcu_is_callbacks_kthread(void)
{
return __get_cpu_var(rcu_cpu_kthread_task) == current;
}
#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)
/*
* Do priority-boost accounting for the start of a new grace period.
*/
static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
{
rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
}
/*
* Create an RCU-boost kthread for the specified node if one does not
* already exist. We only create this kthread for preemptible RCU.
* Returns zero if all is well, a negated errno otherwise.
*/
static int rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
struct rcu_node *rnp)
{
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
int rnp_index = rnp - &rsp->node[0];
unsigned long flags;
struct sched_param sp;
struct task_struct *t;
if (&rcu_preempt_state != rsp)
return 0;
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
if (!rcu_scheduler_fully_active || rnp->qsmaskinit == 0)
return 0;
rsp->boost = 1;
if (rnp->boost_kthread_task != NULL)
return 0;
t = kthread_create(rcu_boost_kthread, (void *)rnp,
"rcub/%d", rnp_index);
if (IS_ERR(t))
return PTR_ERR(t);
raw_spin_lock_irqsave(&rnp->lock, flags);
rnp->boost_kthread_task = t;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
sp.sched_priority = RCU_BOOST_PRIO;
sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
return 0;
}
static void rcu_kthread_do_work(void)
{
rcu_do_batch(&rcu_sched_state, &__get_cpu_var(rcu_sched_data));
rcu_do_batch(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
rcu_preempt_do_callbacks();
}
static void rcu_cpu_kthread_setup(unsigned int cpu)
{
struct sched_param sp;
sp.sched_priority = RCU_KTHREAD_PRIO;
sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
}
static void rcu_cpu_kthread_park(unsigned int cpu)
{
per_cpu(rcu_cpu_kthread_status, cpu) = RCU_KTHREAD_OFFCPU;
}
static int rcu_cpu_kthread_should_run(unsigned int cpu)
{
return __get_cpu_var(rcu_cpu_has_work);
}
/*
* Per-CPU kernel thread that invokes RCU callbacks. This replaces the
* RCU softirq used in flavors and configurations of RCU that do not
* support RCU priority boosting.
*/
static void rcu_cpu_kthread(unsigned int cpu)
{
unsigned int *statusp = &__get_cpu_var(rcu_cpu_kthread_status);
char work, *workp = &__get_cpu_var(rcu_cpu_has_work);
int spincnt;
for (spincnt = 0; spincnt < 10; spincnt++) {
trace_rcu_utilization("Start CPU kthread@rcu_wait");
local_bh_disable();
*statusp = RCU_KTHREAD_RUNNING;
this_cpu_inc(rcu_cpu_kthread_loops);
local_irq_disable();
work = *workp;
*workp = 0;
local_irq_enable();
if (work)
rcu_kthread_do_work();
local_bh_enable();
if (*workp == 0) {
trace_rcu_utilization("End CPU kthread@rcu_wait");
*statusp = RCU_KTHREAD_WAITING;
return;
}
}
*statusp = RCU_KTHREAD_YIELDING;
trace_rcu_utilization("Start CPU kthread@rcu_yield");
schedule_timeout_interruptible(2);
trace_rcu_utilization("End CPU kthread@rcu_yield");
*statusp = RCU_KTHREAD_WAITING;
}
/*
* Set the per-rcu_node kthread's affinity to cover all CPUs that are
* served by the rcu_node in question. The CPU hotplug lock is still
* held, so the value of rnp->qsmaskinit will be stable.
*
* We don't include outgoingcpu in the affinity set, use -1 if there is
* no outgoing CPU. If there are no CPUs left in the affinity set,
* this function allows the kthread to execute on any CPU.
*/
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
{
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
struct task_struct *t = rnp->boost_kthread_task;
unsigned long mask = rnp->qsmaskinit;
cpumask_var_t cm;
int cpu;
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
if (!t)
return;
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
return;
for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask >>= 1)
if ((mask & 0x1) && cpu != outgoingcpu)
cpumask_set_cpu(cpu, cm);
if (cpumask_weight(cm) == 0) {
cpumask_setall(cm);
for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++)
cpumask_clear_cpu(cpu, cm);
WARN_ON_ONCE(cpumask_weight(cm) == 0);
}
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
set_cpus_allowed_ptr(t, cm);
free_cpumask_var(cm);
}
static struct smp_hotplug_thread rcu_cpu_thread_spec = {
.store = &rcu_cpu_kthread_task,
.thread_should_run = rcu_cpu_kthread_should_run,
.thread_fn = rcu_cpu_kthread,
.thread_comm = "rcuc/%u",
.setup = rcu_cpu_kthread_setup,
.park = rcu_cpu_kthread_park,
};
/*
* Spawn all kthreads -- called as soon as the scheduler is running.
*/
static int __init rcu_spawn_kthreads(void)
{
struct rcu_node *rnp;
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
int cpu;
rcu_scheduler_fully_active = 1;
for_each_possible_cpu(cpu)
per_cpu(rcu_cpu_has_work, cpu) = 0;
BUG_ON(smpboot_register_percpu_thread(&rcu_cpu_thread_spec));
rnp = rcu_get_root(rcu_state);
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
(void)rcu_spawn_one_boost_kthread(rcu_state, rnp);
if (NUM_RCU_NODES > 1) {
rcu_for_each_leaf_node(rcu_state, rnp)
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
(void)rcu_spawn_one_boost_kthread(rcu_state, rnp);
}
return 0;
}
early_initcall(rcu_spawn_kthreads);
static void rcu_prepare_kthreads(int cpu)
{
struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
struct rcu_node *rnp = rdp->mynode;
/* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
if (rcu_scheduler_fully_active)
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
(void)rcu_spawn_one_boost_kthread(rcu_state, rnp);
}
#else /* #ifdef CONFIG_RCU_BOOST */
static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
{
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
static void invoke_rcu_callbacks_kthread(void)
{
WARN_ON_ONCE(1);
}
static bool rcu_is_callbacks_kthread(void)
{
return false;
}
static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
{
}
rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Cc: Rusty Russell <rusty@rustcorp.com.au> Cc: Namhyung Kim <namhyung@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
2012-07-16 10:42:35 +00:00
static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
{
}
static int __init rcu_scheduler_really_started(void)
{
rcu_scheduler_fully_active = 1;
return 0;
}
early_initcall(rcu_scheduler_really_started);
static void rcu_prepare_kthreads(int cpu)
{
}
#endif /* #else #ifdef CONFIG_RCU_BOOST */
rcu: Accelerate grace period if last non-dynticked CPU Currently, rcu_needs_cpu() simply checks whether the current CPU has an outstanding RCU callback, which means that the last CPU to go into dyntick-idle mode might wait a few ticks for the relevant grace periods to complete. However, if all the other CPUs are in dyntick-idle mode, and if this CPU is in a quiescent state (which it is for RCU-bh and RCU-sched any time that we are considering going into dyntick-idle mode), then the grace period is instantly complete. This patch therefore repeatedly invokes the RCU grace-period machinery in order to force any needed grace periods to complete quickly. It does so a limited number of times in order to prevent starvation by an RCU callback function that might pass itself to call_rcu(). However, if any CPU other than the current one is not in dyntick-idle mode, fall back to simply checking (with fix to bug noted by Lai Jiangshan). Also, take advantage of last grace-period forcing, the opportunity to do so noted by Steve Rostedt. And apply simplified #ifdef condition suggested by Frederic Weisbecker. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <1266887105-1528-15-git-send-email-paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2010-02-23 01:04:59 +00:00
#if !defined(CONFIG_RCU_FAST_NO_HZ)
/*
* Check to see if any future RCU-related work will need to be done
* by the current CPU, even if none need be done immediately, returning
* 1 if so. This function is part of the RCU implementation; it is -not-
* an exported member of the RCU API.
*
* Because we not have RCU_FAST_NO_HZ, just check whether this CPU needs
* any flavor of RCU.
rcu: Accelerate grace period if last non-dynticked CPU Currently, rcu_needs_cpu() simply checks whether the current CPU has an outstanding RCU callback, which means that the last CPU to go into dyntick-idle mode might wait a few ticks for the relevant grace periods to complete. However, if all the other CPUs are in dyntick-idle mode, and if this CPU is in a quiescent state (which it is for RCU-bh and RCU-sched any time that we are considering going into dyntick-idle mode), then the grace period is instantly complete. This patch therefore repeatedly invokes the RCU grace-period machinery in order to force any needed grace periods to complete quickly. It does so a limited number of times in order to prevent starvation by an RCU callback function that might pass itself to call_rcu(). However, if any CPU other than the current one is not in dyntick-idle mode, fall back to simply checking (with fix to bug noted by Lai Jiangshan). Also, take advantage of last grace-period forcing, the opportunity to do so noted by Steve Rostedt. And apply simplified #ifdef condition suggested by Frederic Weisbecker. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <1266887105-1528-15-git-send-email-paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2010-02-23 01:04:59 +00:00
*/
rcu: Precompute RCU_FAST_NO_HZ timer offsets When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick() calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the next wakeup time based on the timer wheels. Only later, when actually entering the idle loop, rcu_prepare_for_idle() will be invoked. In some cases, rcu_prepare_for_idle() will post timers to wake the CPU back up. But all for naught: The next wakeup time for the CPU has already been computed, and posting a timer afterwards does not force that wakeup time to be recomputed. This means that rcu_prepare_for_idle()'s have no effect. This is not a problem on a busy system because something else will wake up the CPU soon enough. However, on lightly loaded systems, the CPU might stay asleep for a considerable length of time. If that CPU has a callback that the rest of the system is waiting on, the system might run very slowly or (in theory) even hang. This commit avoids this problem by having rcu_needs_cpu() give tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU to wake back up, which tick_nohz_stop_sched_tick() takes into account when programming the CPU's wakeup time. An alternative approach is for rcu_prepare_for_idle() to use hrtimers instead of normal timers, but timers are much more efficient than are hrtimers for frequently and repeatedly posting and cancelling a given timer, which is exactly what RCU_FAST_NO_HZ does. Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com> Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
2012-05-10 23:41:44 +00:00
int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
rcu: Accelerate grace period if last non-dynticked CPU Currently, rcu_needs_cpu() simply checks whether the current CPU has an outstanding RCU callback, which means that the last CPU to go into dyntick-idle mode might wait a few ticks for the relevant grace periods to complete. However, if all the other CPUs are in dyntick-idle mode, and if this CPU is in a quiescent state (which it is for RCU-bh and RCU-sched any time that we are considering going into dyntick-idle mode), then the grace period is instantly complete. This patch therefore repeatedly invokes the RCU grace-period machinery in order to force any needed grace periods to complete quickly. It does so a limited number of times in order to prevent starvation by an RCU callback function that might pass itself to call_rcu(). However, if any CPU other than the current one is not in dyntick-idle mode, fall back to simply checking (with fix to bug noted by Lai Jiangshan). Also, take advantage of last grace-period forcing, the opportunity to do so noted by Steve Rostedt. And apply simplified #ifdef condition suggested by Frederic Weisbecker. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <1266887105-1528-15-git-send-email-paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2010-02-23 01:04:59 +00:00
{
rcu: Precompute RCU_FAST_NO_HZ timer offsets When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick() calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the next wakeup time based on the timer wheels. Only later, when actually entering the idle loop, rcu_prepare_for_idle() will be invoked. In some cases, rcu_prepare_for_idle() will post timers to wake the CPU back up. But all for naught: The next wakeup time for the CPU has already been computed, and posting a timer afterwards does not force that wakeup time to be recomputed. This means that rcu_prepare_for_idle()'s have no effect. This is not a problem on a busy system because something else will wake up the CPU soon enough. However, on lightly loaded systems, the CPU might stay asleep for a considerable length of time. If that CPU has a callback that the rest of the system is waiting on, the system might run very slowly or (in theory) even hang. This commit avoids this problem by having rcu_needs_cpu() give tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU to wake back up, which tick_nohz_stop_sched_tick() takes into account when programming the CPU's wakeup time. An alternative approach is for rcu_prepare_for_idle() to use hrtimers instead of normal timers, but timers are much more efficient than are hrtimers for frequently and repeatedly posting and cancelling a given timer, which is exactly what RCU_FAST_NO_HZ does. Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com> Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
2012-05-10 23:41:44 +00:00
*delta_jiffies = ULONG_MAX;
return rcu_cpu_has_callbacks(cpu, NULL);
}
/*
* Because we do not have RCU_FAST_NO_HZ, don't bother cleaning up
* after it.
*/
static void rcu_cleanup_after_idle(int cpu)
{
}
/*
* Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=n,
* is nothing.
*/
static void rcu_prepare_for_idle(int cpu)
{
}
/*
* Don't bother keeping a running count of the number of RCU callbacks
* posted because CONFIG_RCU_FAST_NO_HZ=n.
*/
static void rcu_idle_count_callbacks_posted(void)
{
}
rcu: Accelerate grace period if last non-dynticked CPU Currently, rcu_needs_cpu() simply checks whether the current CPU has an outstanding RCU callback, which means that the last CPU to go into dyntick-idle mode might wait a few ticks for the relevant grace periods to complete. However, if all the other CPUs are in dyntick-idle mode, and if this CPU is in a quiescent state (which it is for RCU-bh and RCU-sched any time that we are considering going into dyntick-idle mode), then the grace period is instantly complete. This patch therefore repeatedly invokes the RCU grace-period machinery in order to force any needed grace periods to complete quickly. It does so a limited number of times in order to prevent starvation by an RCU callback function that might pass itself to call_rcu(). However, if any CPU other than the current one is not in dyntick-idle mode, fall back to simply checking (with fix to bug noted by Lai Jiangshan). Also, take advantage of last grace-period forcing, the opportunity to do so noted by Steve Rostedt. And apply simplified #ifdef condition suggested by Frederic Weisbecker. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <1266887105-1528-15-git-send-email-paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2010-02-23 01:04:59 +00:00
#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
/*
* This code is invoked when a CPU goes idle, at which point we want
* to have the CPU do everything required for RCU so that it can enter
* the energy-efficient dyntick-idle mode. This is handled by a
* state machine implemented by rcu_prepare_for_idle() below.
*
* The following three proprocessor symbols control this state machine:
*
* RCU_IDLE_GP_DELAY gives the number of jiffies that a CPU is permitted
* to sleep in dyntick-idle mode with RCU callbacks pending. This
* is sized to be roughly one RCU grace period. Those energy-efficiency
* benchmarkers who might otherwise be tempted to set this to a large
* number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
* system. And if you are -that- concerned about energy efficiency,
* just power the system down and be done with it!
* RCU_IDLE_LAZY_GP_DELAY gives the number of jiffies that a CPU is
* permitted to sleep in dyntick-idle mode with only lazy RCU
* callbacks pending. Setting this too high can OOM your system.
*
* The values below work well in practice. If future workloads require
* adjustment, they can be converted into kernel config parameters, though
* making the state machine smarter might be a better option.
*/
#define RCU_IDLE_GP_DELAY 4 /* Roughly one grace period. */
#define RCU_IDLE_LAZY_GP_DELAY (6 * HZ) /* Roughly six seconds. */
static int rcu_idle_gp_delay = RCU_IDLE_GP_DELAY;
module_param(rcu_idle_gp_delay, int, 0644);
static int rcu_idle_lazy_gp_delay = RCU_IDLE_LAZY_GP_DELAY;
module_param(rcu_idle_lazy_gp_delay, int, 0644);
extern int tick_nohz_enabled;
/*
* Try to advance callbacks for all flavors of RCU on the current CPU.
* Afterwards, if there are any callbacks ready for immediate invocation,
* return true.
*/
static bool rcu_try_advance_all_cbs(void)
{
bool cbs_ready = false;
struct rcu_data *rdp;
struct rcu_node *rnp;
struct rcu_state *rsp;
for_each_rcu_flavor(rsp) {
rdp = this_cpu_ptr(rsp->rda);
rnp = rdp->mynode;
/*
* Don't bother checking unless a grace period has
* completed since we last checked and there are
* callbacks not yet ready to invoke.
*/
if (rdp->completed != rnp->completed &&
rdp->nxttail[RCU_DONE_TAIL] != rdp->nxttail[RCU_NEXT_TAIL])
note_gp_changes(rsp, rdp);
if (cpu_has_callbacks_ready_to_invoke(rdp))
cbs_ready = true;
}
return cbs_ready;
}
rcu: Precompute RCU_FAST_NO_HZ timer offsets When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick() calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the next wakeup time based on the timer wheels. Only later, when actually entering the idle loop, rcu_prepare_for_idle() will be invoked. In some cases, rcu_prepare_for_idle() will post timers to wake the CPU back up. But all for naught: The next wakeup time for the CPU has already been computed, and posting a timer afterwards does not force that wakeup time to be recomputed. This means that rcu_prepare_for_idle()'s have no effect. This is not a problem on a busy system because something else will wake up the CPU soon enough. However, on lightly loaded systems, the CPU might stay asleep for a considerable length of time. If that CPU has a callback that the rest of the system is waiting on, the system might run very slowly or (in theory) even hang. This commit avoids this problem by having rcu_needs_cpu() give tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU to wake back up, which tick_nohz_stop_sched_tick() takes into account when programming the CPU's wakeup time. An alternative approach is for rcu_prepare_for_idle() to use hrtimers instead of normal timers, but timers are much more efficient than are hrtimers for frequently and repeatedly posting and cancelling a given timer, which is exactly what RCU_FAST_NO_HZ does. Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com> Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
2012-05-10 23:41:44 +00:00
/*
* Allow the CPU to enter dyntick-idle mode unless it has callbacks ready
* to invoke. If the CPU has callbacks, try to advance them. Tell the
* caller to set the timeout based on whether or not there are non-lazy
* callbacks.
rcu: Precompute RCU_FAST_NO_HZ timer offsets When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick() calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the next wakeup time based on the timer wheels. Only later, when actually entering the idle loop, rcu_prepare_for_idle() will be invoked. In some cases, rcu_prepare_for_idle() will post timers to wake the CPU back up. But all for naught: The next wakeup time for the CPU has already been computed, and posting a timer afterwards does not force that wakeup time to be recomputed. This means that rcu_prepare_for_idle()'s have no effect. This is not a problem on a busy system because something else will wake up the CPU soon enough. However, on lightly loaded systems, the CPU might stay asleep for a considerable length of time. If that CPU has a callback that the rest of the system is waiting on, the system might run very slowly or (in theory) even hang. This commit avoids this problem by having rcu_needs_cpu() give tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU to wake back up, which tick_nohz_stop_sched_tick() takes into account when programming the CPU's wakeup time. An alternative approach is for rcu_prepare_for_idle() to use hrtimers instead of normal timers, but timers are much more efficient than are hrtimers for frequently and repeatedly posting and cancelling a given timer, which is exactly what RCU_FAST_NO_HZ does. Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com> Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
2012-05-10 23:41:44 +00:00
*
* The caller must have disabled interrupts.
rcu: Precompute RCU_FAST_NO_HZ timer offsets When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick() calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the next wakeup time based on the timer wheels. Only later, when actually entering the idle loop, rcu_prepare_for_idle() will be invoked. In some cases, rcu_prepare_for_idle() will post timers to wake the CPU back up. But all for naught: The next wakeup time for the CPU has already been computed, and posting a timer afterwards does not force that wakeup time to be recomputed. This means that rcu_prepare_for_idle()'s have no effect. This is not a problem on a busy system because something else will wake up the CPU soon enough. However, on lightly loaded systems, the CPU might stay asleep for a considerable length of time. If that CPU has a callback that the rest of the system is waiting on, the system might run very slowly or (in theory) even hang. This commit avoids this problem by having rcu_needs_cpu() give tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU to wake back up, which tick_nohz_stop_sched_tick() takes into account when programming the CPU's wakeup time. An alternative approach is for rcu_prepare_for_idle() to use hrtimers instead of normal timers, but timers are much more efficient than are hrtimers for frequently and repeatedly posting and cancelling a given timer, which is exactly what RCU_FAST_NO_HZ does. Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com> Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
2012-05-10 23:41:44 +00:00
*/
int rcu_needs_cpu(int cpu, unsigned long *dj)
rcu: Precompute RCU_FAST_NO_HZ timer offsets When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick() calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the next wakeup time based on the timer wheels. Only later, when actually entering the idle loop, rcu_prepare_for_idle() will be invoked. In some cases, rcu_prepare_for_idle() will post timers to wake the CPU back up. But all for naught: The next wakeup time for the CPU has already been computed, and posting a timer afterwards does not force that wakeup time to be recomputed. This means that rcu_prepare_for_idle()'s have no effect. This is not a problem on a busy system because something else will wake up the CPU soon enough. However, on lightly loaded systems, the CPU might stay asleep for a considerable length of time. If that CPU has a callback that the rest of the system is waiting on, the system might run very slowly or (in theory) even hang. This commit avoids this problem by having rcu_needs_cpu() give tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU to wake back up, which tick_nohz_stop_sched_tick() takes into account when programming the CPU's wakeup time. An alternative approach is for rcu_prepare_for_idle() to use hrtimers instead of normal timers, but timers are much more efficient than are hrtimers for frequently and repeatedly posting and cancelling a given timer, which is exactly what RCU_FAST_NO_HZ does. Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com> Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
2012-05-10 23:41:44 +00:00
{
struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
/* Snapshot to detect later posting of non-lazy callback. */
rdtp->nonlazy_posted_snap = rdtp->nonlazy_posted;
rcu: Precompute RCU_FAST_NO_HZ timer offsets When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick() calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the next wakeup time based on the timer wheels. Only later, when actually entering the idle loop, rcu_prepare_for_idle() will be invoked. In some cases, rcu_prepare_for_idle() will post timers to wake the CPU back up. But all for naught: The next wakeup time for the CPU has already been computed, and posting a timer afterwards does not force that wakeup time to be recomputed. This means that rcu_prepare_for_idle()'s have no effect. This is not a problem on a busy system because something else will wake up the CPU soon enough. However, on lightly loaded systems, the CPU might stay asleep for a considerable length of time. If that CPU has a callback that the rest of the system is waiting on, the system might run very slowly or (in theory) even hang. This commit avoids this problem by having rcu_needs_cpu() give tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU to wake back up, which tick_nohz_stop_sched_tick() takes into account when programming the CPU's wakeup time. An alternative approach is for rcu_prepare_for_idle() to use hrtimers instead of normal timers, but timers are much more efficient than are hrtimers for frequently and repeatedly posting and cancelling a given timer, which is exactly what RCU_FAST_NO_HZ does. Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com> Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
2012-05-10 23:41:44 +00:00
/* If no callbacks, RCU doesn't need the CPU. */
if (!rcu_cpu_has_callbacks(cpu, &rdtp->all_lazy)) {
*dj = ULONG_MAX;
rcu: Precompute RCU_FAST_NO_HZ timer offsets When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick() calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the next wakeup time based on the timer wheels. Only later, when actually entering the idle loop, rcu_prepare_for_idle() will be invoked. In some cases, rcu_prepare_for_idle() will post timers to wake the CPU back up. But all for naught: The next wakeup time for the CPU has already been computed, and posting a timer afterwards does not force that wakeup time to be recomputed. This means that rcu_prepare_for_idle()'s have no effect. This is not a problem on a busy system because something else will wake up the CPU soon enough. However, on lightly loaded systems, the CPU might stay asleep for a considerable length of time. If that CPU has a callback that the rest of the system is waiting on, the system might run very slowly or (in theory) even hang. This commit avoids this problem by having rcu_needs_cpu() give tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU to wake back up, which tick_nohz_stop_sched_tick() takes into account when programming the CPU's wakeup time. An alternative approach is for rcu_prepare_for_idle() to use hrtimers instead of normal timers, but timers are much more efficient than are hrtimers for frequently and repeatedly posting and cancelling a given timer, which is exactly what RCU_FAST_NO_HZ does. Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com> Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
2012-05-10 23:41:44 +00:00
return 0;
}
/* Attempt to advance callbacks. */
if (rcu_try_advance_all_cbs()) {
/* Some ready to invoke, so initiate later invocation. */
invoke_rcu_core();
rcu: Precompute RCU_FAST_NO_HZ timer offsets When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick() calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the next wakeup time based on the timer wheels. Only later, when actually entering the idle loop, rcu_prepare_for_idle() will be invoked. In some cases, rcu_prepare_for_idle() will post timers to wake the CPU back up. But all for naught: The next wakeup time for the CPU has already been computed, and posting a timer afterwards does not force that wakeup time to be recomputed. This means that rcu_prepare_for_idle()'s have no effect. This is not a problem on a busy system because something else will wake up the CPU soon enough. However, on lightly loaded systems, the CPU might stay asleep for a considerable length of time. If that CPU has a callback that the rest of the system is waiting on, the system might run very slowly or (in theory) even hang. This commit avoids this problem by having rcu_needs_cpu() give tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU to wake back up, which tick_nohz_stop_sched_tick() takes into account when programming the CPU's wakeup time. An alternative approach is for rcu_prepare_for_idle() to use hrtimers instead of normal timers, but timers are much more efficient than are hrtimers for frequently and repeatedly posting and cancelling a given timer, which is exactly what RCU_FAST_NO_HZ does. Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com> Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
2012-05-10 23:41:44 +00:00
return 1;
}
rdtp->last_accelerate = jiffies;
/* Request timer delay depending on laziness, and round. */
if (!rdtp->all_lazy) {
*dj = round_up(rcu_idle_gp_delay + jiffies,
rcu_idle_gp_delay) - jiffies;
} else {
*dj = round_jiffies(rcu_idle_lazy_gp_delay + jiffies) - jiffies;
}
rcu: Precompute RCU_FAST_NO_HZ timer offsets When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick() calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the next wakeup time based on the timer wheels. Only later, when actually entering the idle loop, rcu_prepare_for_idle() will be invoked. In some cases, rcu_prepare_for_idle() will post timers to wake the CPU back up. But all for naught: The next wakeup time for the CPU has already been computed, and posting a timer afterwards does not force that wakeup time to be recomputed. This means that rcu_prepare_for_idle()'s have no effect. This is not a problem on a busy system because something else will wake up the CPU soon enough. However, on lightly loaded systems, the CPU might stay asleep for a considerable length of time. If that CPU has a callback that the rest of the system is waiting on, the system might run very slowly or (in theory) even hang. This commit avoids this problem by having rcu_needs_cpu() give tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU to wake back up, which tick_nohz_stop_sched_tick() takes into account when programming the CPU's wakeup time. An alternative approach is for rcu_prepare_for_idle() to use hrtimers instead of normal timers, but timers are much more efficient than are hrtimers for frequently and repeatedly posting and cancelling a given timer, which is exactly what RCU_FAST_NO_HZ does. Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com> Tested-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr>
2012-05-10 23:41:44 +00:00
return 0;
}
rcu: Make RCU_FAST_NO_HZ handle timer migration The current RCU_FAST_NO_HZ assumes that timers do not migrate unless a CPU goes offline, in which case it assumes that the CPU will have to come out of dyntick-idle mode (cancelling the timer) in order to go offline. This is important because when RCU_FAST_NO_HZ permits a CPU to enter dyntick-idle mode despite having RCU callbacks pending, it posts a timer on that CPU to force a wakeup on that CPU. This wakeup ensures that the CPU will eventually handle the end of the grace period, including invoking its RCU callbacks. However, Pascal Chapperon's test setup shows that the timer handler rcu_idle_gp_timer_func() really does get invoked in some cases. This is problematic because this can cause the CPU that entered dyntick-idle mode despite still having RCU callbacks pending to remain in dyntick-idle mode indefinitely, which means that its RCU callbacks might never be invoked. This situation can result in grace-period delays or even system hangs, which matches Pascal's observations of slow boot-up and shutdown (https://lkml.org/lkml/2012/4/5/142). See also the bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=806548 This commit therefore causes the "should never be invoked" timer handler rcu_idle_gp_timer_func() to use smp_call_function_single() to wake up the CPU for which the timer was intended, allowing that CPU to invoke its RCU callbacks in a timely manner. Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2012-04-30 21:16:19 +00:00
/*
* Prepare a CPU for idle from an RCU perspective. The first major task
* is to sense whether nohz mode has been enabled or disabled via sysfs.
* The second major task is to check to see if a non-lazy callback has
* arrived at a CPU that previously had only lazy callbacks. The third
* major task is to accelerate (that is, assign grace-period numbers to)
* any recently arrived callbacks.
*
* The caller must have disabled interrupts.
rcu: Accelerate grace period if last non-dynticked CPU Currently, rcu_needs_cpu() simply checks whether the current CPU has an outstanding RCU callback, which means that the last CPU to go into dyntick-idle mode might wait a few ticks for the relevant grace periods to complete. However, if all the other CPUs are in dyntick-idle mode, and if this CPU is in a quiescent state (which it is for RCU-bh and RCU-sched any time that we are considering going into dyntick-idle mode), then the grace period is instantly complete. This patch therefore repeatedly invokes the RCU grace-period machinery in order to force any needed grace periods to complete quickly. It does so a limited number of times in order to prevent starvation by an RCU callback function that might pass itself to call_rcu(). However, if any CPU other than the current one is not in dyntick-idle mode, fall back to simply checking (with fix to bug noted by Lai Jiangshan). Also, take advantage of last grace-period forcing, the opportunity to do so noted by Steve Rostedt. And apply simplified #ifdef condition suggested by Frederic Weisbecker. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <1266887105-1528-15-git-send-email-paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2010-02-23 01:04:59 +00:00
*/
static void rcu_prepare_for_idle(int cpu)
rcu: Accelerate grace period if last non-dynticked CPU Currently, rcu_needs_cpu() simply checks whether the current CPU has an outstanding RCU callback, which means that the last CPU to go into dyntick-idle mode might wait a few ticks for the relevant grace periods to complete. However, if all the other CPUs are in dyntick-idle mode, and if this CPU is in a quiescent state (which it is for RCU-bh and RCU-sched any time that we are considering going into dyntick-idle mode), then the grace period is instantly complete. This patch therefore repeatedly invokes the RCU grace-period machinery in order to force any needed grace periods to complete quickly. It does so a limited number of times in order to prevent starvation by an RCU callback function that might pass itself to call_rcu(). However, if any CPU other than the current one is not in dyntick-idle mode, fall back to simply checking (with fix to bug noted by Lai Jiangshan). Also, take advantage of last grace-period forcing, the opportunity to do so noted by Steve Rostedt. And apply simplified #ifdef condition suggested by Frederic Weisbecker. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <1266887105-1528-15-git-send-email-paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2010-02-23 01:04:59 +00:00
{
struct rcu_data *rdp;
struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
struct rcu_node *rnp;
struct rcu_state *rsp;
int tne;
/* Handle nohz enablement switches conservatively. */
tne = ACCESS_ONCE(tick_nohz_enabled);
if (tne != rdtp->tick_nohz_enabled_snap) {
if (rcu_cpu_has_callbacks(cpu, NULL))
invoke_rcu_core(); /* force nohz to see update. */
rdtp->tick_nohz_enabled_snap = tne;
return;
}
if (!tne)
return;
/* If this is a no-CBs CPU, no callbacks, just return. */
Merge branch 'timers-nohz-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull 'full dynticks' support from Ingo Molnar: "This tree from Frederic Weisbecker adds a new, (exciting! :-) core kernel feature to the timer and scheduler subsystems: 'full dynticks', or CONFIG_NO_HZ_FULL=y. This feature extends the nohz variable-size timer tick feature from idle to busy CPUs (running at most one task) as well, potentially reducing the number of timer interrupts significantly. This feature got motivated by real-time folks and the -rt tree, but the general utility and motivation of full-dynticks runs wider than that: - HPC workloads get faster: CPUs running a single task should be able to utilize a maximum amount of CPU power. A periodic timer tick at HZ=1000 can cause a constant overhead of up to 1.0%. This feature removes that overhead - and speeds up the system by 0.5%-1.0% on typical distro configs even on modern systems. - Real-time workload latency reduction: CPUs running critical tasks should experience as little jitter as possible. The last remaining source of kernel-related jitter was the periodic timer tick. - A single task executing on a CPU is a pretty common situation, especially with an increasing number of cores/CPUs, so this feature helps desktop and mobile workloads as well. The cost of the feature is mainly related to increased timer reprogramming overhead when a CPU switches its tick period, and thus slightly longer to-idle and from-idle latency. Configuration-wise a third mode of operation is added to the existing two NOHZ kconfig modes: - CONFIG_HZ_PERIODIC: [formerly !CONFIG_NO_HZ], now explicitly named as a config option. This is the traditional Linux periodic tick design: there's a HZ tick going on all the time, regardless of whether a CPU is idle or not. - CONFIG_NO_HZ_IDLE: [formerly CONFIG_NO_HZ=y], this turns off the periodic tick when a CPU enters idle mode. - CONFIG_NO_HZ_FULL: this new mode, in addition to turning off the tick when a CPU is idle, also slows the tick down to 1 Hz (one timer interrupt per second) when only a single task is running on a CPU. The .config behavior is compatible: existing !CONFIG_NO_HZ and CONFIG_NO_HZ=y settings get translated to the new values, without the user having to configure anything. CONFIG_NO_HZ_FULL is turned off by default. This feature is based on a lot of infrastructure work that has been steadily going upstream in the last 2-3 cycles: related RCU support and non-periodic cputime support in particular is upstream already. This tree adds the final pieces and activates the feature. The pull request is marked RFC because: - it's marked 64-bit only at the moment - the 32-bit support patch is small but did not get ready in time. - it has a number of fresh commits that came in after the merge window. The overwhelming majority of commits are from before the merge window, but still some aspects of the tree are fresh and so I marked it RFC. - it's a pretty wide-reaching feature with lots of effects - and while the components have been in testing for some time, the full combination is still not very widely used. That it's default-off should reduce its regression abilities and obviously there are no known regressions with CONFIG_NO_HZ_FULL=y enabled either. - the feature is not completely idempotent: there is no 100% equivalent replacement for a periodic scheduler/timer tick. In particular there's ongoing work to map out and reduce its effects on scheduler load-balancing and statistics. This should not impact correctness though, there are no known regressions related to this feature at this point. - it's a pretty ambitious feature that with time will likely be enabled by most Linux distros, and we'd like you to make input on its design/implementation, if you dislike some aspect we missed. Without flaming us to crisp! :-) Future plans: - there's ongoing work to reduce 1Hz to 0Hz, to essentially shut off the periodic tick altogether when there's a single busy task on a CPU. We'd first like 1 Hz to be exposed more widely before we go for the 0 Hz target though. - once we reach 0 Hz we can remove the periodic tick assumption from nr_running>=2 as well, by essentially interrupting busy tasks only as frequently as the sched_latency constraints require us to do - once every 4-40 msecs, depending on nr_running. I am personally leaning towards biting the bullet and doing this in v3.10, like the -rt tree this effort has been going on for too long - but the final word is up to you as usual. More technical details can be found in Documentation/timers/NO_HZ.txt" * 'timers-nohz-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (39 commits) sched: Keep at least 1 tick per second for active dynticks tasks rcu: Fix full dynticks' dependency on wide RCU nocb mode nohz: Protect smp_processor_id() in tick_nohz_task_switch() nohz_full: Add documentation. cputime_nsecs: use math64.h for nsec resolution conversion helpers nohz: Select VIRT_CPU_ACCOUNTING_GEN from full dynticks config nohz: Reduce overhead under high-freq idling patterns nohz: Remove full dynticks' superfluous dependency on RCU tree nohz: Fix unavailable tick_stop tracepoint in dynticks idle nohz: Add basic tracing nohz: Select wide RCU nocb for full dynticks nohz: Disable the tick when irq resume in full dynticks CPU nohz: Re-evaluate the tick for the new task after a context switch nohz: Prepare to stop the tick on irq exit nohz: Implement full dynticks kick nohz: Re-evaluate the tick from the scheduler IPI sched: New helper to prevent from stopping the tick in full dynticks sched: Kick full dynticks CPU that have more than one task enqueued. perf: New helper to prevent full dynticks CPUs from stopping tick perf: Kick full dynticks CPU if events rotation is needed ...
2013-05-05 20:23:27 +00:00
if (rcu_is_nocb_cpu(cpu))
return;
/*
* If a non-lazy callback arrived at a CPU having only lazy
* callbacks, invoke RCU core for the side-effect of recalculating
* idle duration on re-entry to idle.
*/
if (rdtp->all_lazy &&
rdtp->nonlazy_posted != rdtp->nonlazy_posted_snap) {
invoke_rcu_core();
return;
}
/*
* If we have not yet accelerated this jiffy, accelerate all
* callbacks on this CPU.
*/
if (rdtp->last_accelerate == jiffies)
return;
rdtp->last_accelerate = jiffies;
for_each_rcu_flavor(rsp) {
rdp = per_cpu_ptr(rsp->rda, cpu);
if (!*rdp->nxttail[RCU_DONE_TAIL])
continue;
rnp = rdp->mynode;
raw_spin_lock(&rnp->lock); /* irqs already disabled. */
rcu_accelerate_cbs(rsp, rnp, rdp);
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
}
}
/*
* Clean up for exit from idle. Attempt to advance callbacks based on
* any grace periods that elapsed while the CPU was idle, and if any
* callbacks are now ready to invoke, initiate invocation.
*/
static void rcu_cleanup_after_idle(int cpu)
{
struct rcu_data *rdp;
struct rcu_state *rsp;
Merge branch 'timers-nohz-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull 'full dynticks' support from Ingo Molnar: "This tree from Frederic Weisbecker adds a new, (exciting! :-) core kernel feature to the timer and scheduler subsystems: 'full dynticks', or CONFIG_NO_HZ_FULL=y. This feature extends the nohz variable-size timer tick feature from idle to busy CPUs (running at most one task) as well, potentially reducing the number of timer interrupts significantly. This feature got motivated by real-time folks and the -rt tree, but the general utility and motivation of full-dynticks runs wider than that: - HPC workloads get faster: CPUs running a single task should be able to utilize a maximum amount of CPU power. A periodic timer tick at HZ=1000 can cause a constant overhead of up to 1.0%. This feature removes that overhead - and speeds up the system by 0.5%-1.0% on typical distro configs even on modern systems. - Real-time workload latency reduction: CPUs running critical tasks should experience as little jitter as possible. The last remaining source of kernel-related jitter was the periodic timer tick. - A single task executing on a CPU is a pretty common situation, especially with an increasing number of cores/CPUs, so this feature helps desktop and mobile workloads as well. The cost of the feature is mainly related to increased timer reprogramming overhead when a CPU switches its tick period, and thus slightly longer to-idle and from-idle latency. Configuration-wise a third mode of operation is added to the existing two NOHZ kconfig modes: - CONFIG_HZ_PERIODIC: [formerly !CONFIG_NO_HZ], now explicitly named as a config option. This is the traditional Linux periodic tick design: there's a HZ tick going on all the time, regardless of whether a CPU is idle or not. - CONFIG_NO_HZ_IDLE: [formerly CONFIG_NO_HZ=y], this turns off the periodic tick when a CPU enters idle mode. - CONFIG_NO_HZ_FULL: this new mode, in addition to turning off the tick when a CPU is idle, also slows the tick down to 1 Hz (one timer interrupt per second) when only a single task is running on a CPU. The .config behavior is compatible: existing !CONFIG_NO_HZ and CONFIG_NO_HZ=y settings get translated to the new values, without the user having to configure anything. CONFIG_NO_HZ_FULL is turned off by default. This feature is based on a lot of infrastructure work that has been steadily going upstream in the last 2-3 cycles: related RCU support and non-periodic cputime support in particular is upstream already. This tree adds the final pieces and activates the feature. The pull request is marked RFC because: - it's marked 64-bit only at the moment - the 32-bit support patch is small but did not get ready in time. - it has a number of fresh commits that came in after the merge window. The overwhelming majority of commits are from before the merge window, but still some aspects of the tree are fresh and so I marked it RFC. - it's a pretty wide-reaching feature with lots of effects - and while the components have been in testing for some time, the full combination is still not very widely used. That it's default-off should reduce its regression abilities and obviously there are no known regressions with CONFIG_NO_HZ_FULL=y enabled either. - the feature is not completely idempotent: there is no 100% equivalent replacement for a periodic scheduler/timer tick. In particular there's ongoing work to map out and reduce its effects on scheduler load-balancing and statistics. This should not impact correctness though, there are no known regressions related to this feature at this point. - it's a pretty ambitious feature that with time will likely be enabled by most Linux distros, and we'd like you to make input on its design/implementation, if you dislike some aspect we missed. Without flaming us to crisp! :-) Future plans: - there's ongoing work to reduce 1Hz to 0Hz, to essentially shut off the periodic tick altogether when there's a single busy task on a CPU. We'd first like 1 Hz to be exposed more widely before we go for the 0 Hz target though. - once we reach 0 Hz we can remove the periodic tick assumption from nr_running>=2 as well, by essentially interrupting busy tasks only as frequently as the sched_latency constraints require us to do - once every 4-40 msecs, depending on nr_running. I am personally leaning towards biting the bullet and doing this in v3.10, like the -rt tree this effort has been going on for too long - but the final word is up to you as usual. More technical details can be found in Documentation/timers/NO_HZ.txt" * 'timers-nohz-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (39 commits) sched: Keep at least 1 tick per second for active dynticks tasks rcu: Fix full dynticks' dependency on wide RCU nocb mode nohz: Protect smp_processor_id() in tick_nohz_task_switch() nohz_full: Add documentation. cputime_nsecs: use math64.h for nsec resolution conversion helpers nohz: Select VIRT_CPU_ACCOUNTING_GEN from full dynticks config nohz: Reduce overhead under high-freq idling patterns nohz: Remove full dynticks' superfluous dependency on RCU tree nohz: Fix unavailable tick_stop tracepoint in dynticks idle nohz: Add basic tracing nohz: Select wide RCU nocb for full dynticks nohz: Disable the tick when irq resume in full dynticks CPU nohz: Re-evaluate the tick for the new task after a context switch nohz: Prepare to stop the tick on irq exit nohz: Implement full dynticks kick nohz: Re-evaluate the tick from the scheduler IPI sched: New helper to prevent from stopping the tick in full dynticks sched: Kick full dynticks CPU that have more than one task enqueued. perf: New helper to prevent full dynticks CPUs from stopping tick perf: Kick full dynticks CPU if events rotation is needed ...
2013-05-05 20:23:27 +00:00
if (rcu_is_nocb_cpu(cpu))
return;
rcu_try_advance_all_cbs();
for_each_rcu_flavor(rsp) {
rdp = per_cpu_ptr(rsp->rda, cpu);
if (cpu_has_callbacks_ready_to_invoke(rdp))
invoke_rcu_core();
}
rcu: Accelerate grace period if last non-dynticked CPU Currently, rcu_needs_cpu() simply checks whether the current CPU has an outstanding RCU callback, which means that the last CPU to go into dyntick-idle mode might wait a few ticks for the relevant grace periods to complete. However, if all the other CPUs are in dyntick-idle mode, and if this CPU is in a quiescent state (which it is for RCU-bh and RCU-sched any time that we are considering going into dyntick-idle mode), then the grace period is instantly complete. This patch therefore repeatedly invokes the RCU grace-period machinery in order to force any needed grace periods to complete quickly. It does so a limited number of times in order to prevent starvation by an RCU callback function that might pass itself to call_rcu(). However, if any CPU other than the current one is not in dyntick-idle mode, fall back to simply checking (with fix to bug noted by Lai Jiangshan). Also, take advantage of last grace-period forcing, the opportunity to do so noted by Steve Rostedt. And apply simplified #ifdef condition suggested by Frederic Weisbecker. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <1266887105-1528-15-git-send-email-paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2010-02-23 01:04:59 +00:00
}
/*
* Keep a running count of the number of non-lazy callbacks posted
* on this CPU. This running counter (which is never decremented) allows
* rcu_prepare_for_idle() to detect when something out of the idle loop
* posts a callback, even if an equal number of callbacks are invoked.
* Of course, callbacks should only be posted from within a trace event
* designed to be called from idle or from within RCU_NONIDLE().
*/
static void rcu_idle_count_callbacks_posted(void)
{
__this_cpu_add(rcu_dynticks.nonlazy_posted, 1);
}
/*
* Data for flushing lazy RCU callbacks at OOM time.
*/
static atomic_t oom_callback_count;
static DECLARE_WAIT_QUEUE_HEAD(oom_callback_wq);
/*
* RCU OOM callback -- decrement the outstanding count and deliver the
* wake-up if we are the last one.
*/
static void rcu_oom_callback(struct rcu_head *rhp)
{
if (atomic_dec_and_test(&oom_callback_count))
wake_up(&oom_callback_wq);
}
/*
* Post an rcu_oom_notify callback on the current CPU if it has at
* least one lazy callback. This will unnecessarily post callbacks
* to CPUs that already have a non-lazy callback at the end of their
* callback list, but this is an infrequent operation, so accept some
* extra overhead to keep things simple.
*/
static void rcu_oom_notify_cpu(void *unused)
{
struct rcu_state *rsp;
struct rcu_data *rdp;
for_each_rcu_flavor(rsp) {
rdp = __this_cpu_ptr(rsp->rda);
if (rdp->qlen_lazy != 0) {
atomic_inc(&oom_callback_count);
rsp->call(&rdp->oom_head, rcu_oom_callback);
}
}
}
/*
* If low on memory, ensure that each CPU has a non-lazy callback.
* This will wake up CPUs that have only lazy callbacks, in turn
* ensuring that they free up the corresponding memory in a timely manner.
* Because an uncertain amount of memory will be freed in some uncertain
* timeframe, we do not claim to have freed anything.
*/
static int rcu_oom_notify(struct notifier_block *self,
unsigned long notused, void *nfreed)
{
int cpu;
/* Wait for callbacks from earlier instance to complete. */
wait_event(oom_callback_wq, atomic_read(&oom_callback_count) == 0);
/*
* Prevent premature wakeup: ensure that all increments happen
* before there is a chance of the counter reaching zero.
*/
atomic_set(&oom_callback_count, 1);
get_online_cpus();
for_each_online_cpu(cpu) {
smp_call_function_single(cpu, rcu_oom_notify_cpu, NULL, 1);
cond_resched();
}
put_online_cpus();
/* Unconditionally decrement: no need to wake ourselves up. */
atomic_dec(&oom_callback_count);
return NOTIFY_OK;
}
static struct notifier_block rcu_oom_nb = {
.notifier_call = rcu_oom_notify
};
static int __init rcu_register_oom_notifier(void)
{
register_oom_notifier(&rcu_oom_nb);
return 0;
}
early_initcall(rcu_register_oom_notifier);
rcu: Accelerate grace period if last non-dynticked CPU Currently, rcu_needs_cpu() simply checks whether the current CPU has an outstanding RCU callback, which means that the last CPU to go into dyntick-idle mode might wait a few ticks for the relevant grace periods to complete. However, if all the other CPUs are in dyntick-idle mode, and if this CPU is in a quiescent state (which it is for RCU-bh and RCU-sched any time that we are considering going into dyntick-idle mode), then the grace period is instantly complete. This patch therefore repeatedly invokes the RCU grace-period machinery in order to force any needed grace periods to complete quickly. It does so a limited number of times in order to prevent starvation by an RCU callback function that might pass itself to call_rcu(). However, if any CPU other than the current one is not in dyntick-idle mode, fall back to simply checking (with fix to bug noted by Lai Jiangshan). Also, take advantage of last grace-period forcing, the opportunity to do so noted by Steve Rostedt. And apply simplified #ifdef condition suggested by Frederic Weisbecker. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <1266887105-1528-15-git-send-email-paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
2010-02-23 01:04:59 +00:00
#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
#ifdef CONFIG_RCU_CPU_STALL_INFO
#ifdef CONFIG_RCU_FAST_NO_HZ
static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
{
struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
unsigned long nlpd = rdtp->nonlazy_posted - rdtp->nonlazy_posted_snap;
sprintf(cp, "last_accelerate: %04lx/%04lx, nonlazy_posted: %ld, %c%c",
rdtp->last_accelerate & 0xffff, jiffies & 0xffff,
ulong2long(nlpd),
rdtp->all_lazy ? 'L' : '.',
rdtp->tick_nohz_enabled_snap ? '.' : 'D');
}
#else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
{
*cp = '\0';
}
#endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */
/* Initiate the stall-info list. */
static void print_cpu_stall_info_begin(void)
{
pr_cont("\n");
}
/*
* Print out diagnostic information for the specified stalled CPU.
*
* If the specified CPU is aware of the current RCU grace period
* (flavor specified by rsp), then print the number of scheduling
* clock interrupts the CPU has taken during the time that it has
* been aware. Otherwise, print the number of RCU grace periods
* that this CPU is ignorant of, for example, "1" if the CPU was
* aware of the previous grace period.
*
* Also print out idle and (if CONFIG_RCU_FAST_NO_HZ) idle-entry info.
*/
static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
{
char fast_no_hz[72];
struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
struct rcu_dynticks *rdtp = rdp->dynticks;
char *ticks_title;
unsigned long ticks_value;
if (rsp->gpnum == rdp->gpnum) {
ticks_title = "ticks this GP";
ticks_value = rdp->ticks_this_gp;
} else {
ticks_title = "GPs behind";
ticks_value = rsp->gpnum - rdp->gpnum;
}
print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
pr_err("\t%d: (%lu %s) idle=%03x/%llx/%d softirq=%u/%u %s\n",
cpu, ticks_value, ticks_title,
atomic_read(&rdtp->dynticks) & 0xfff,
rdtp->dynticks_nesting, rdtp->dynticks_nmi_nesting,
rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
fast_no_hz);
}
/* Terminate the stall-info list. */
static void print_cpu_stall_info_end(void)
{
pr_err("\t");
}
/* Zero ->ticks_this_gp for all flavors of RCU. */
static void zero_cpu_stall_ticks(struct rcu_data *rdp)
{
rdp->ticks_this_gp = 0;
rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
}
/* Increment ->ticks_this_gp for all flavors of RCU. */
static void increment_cpu_stall_ticks(void)
{
struct rcu_state *rsp;
for_each_rcu_flavor(rsp)
__this_cpu_ptr(rsp->rda)->ticks_this_gp++;
}
#else /* #ifdef CONFIG_RCU_CPU_STALL_INFO */
static void print_cpu_stall_info_begin(void)
{
pr_cont(" {");
}
static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
{
pr_cont(" %d", cpu);
}
static void print_cpu_stall_info_end(void)
{
pr_cont("} ");
}
static void zero_cpu_stall_ticks(struct rcu_data *rdp)
{
}
static void increment_cpu_stall_ticks(void)
{
}
#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_INFO */
#ifdef CONFIG_RCU_NOCB_CPU
/*
* Offload callback processing from the boot-time-specified set of CPUs
* specified by rcu_nocb_mask. For each CPU in the set, there is a
* kthread created that pulls the callbacks from the corresponding CPU,
* waits for a grace period to elapse, and invokes the callbacks.
* The no-CBs CPUs do a wake_up() on their kthread when they insert
* a callback into any empty list, unless the rcu_nocb_poll boot parameter
* has been specified, in which case each kthread actively polls its
* CPU. (Which isn't so great for energy efficiency, but which does
* reduce RCU's overhead on that CPU.)
*
* This is intended to be used in conjunction with Frederic Weisbecker's
* adaptive-idle work, which would seriously reduce OS jitter on CPUs
* running CPU-bound user-mode computations.
*
* Offloading of callback processing could also in theory be used as
* an energy-efficiency measure because CPUs with no RCU callbacks
* queued are more aggressive about entering dyntick-idle mode.
*/
/* Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters. */
static int __init rcu_nocb_setup(char *str)
{
alloc_bootmem_cpumask_var(&rcu_nocb_mask);
have_rcu_nocb_mask = true;
cpulist_parse(str, rcu_nocb_mask);
return 1;
}
__setup("rcu_nocbs=", rcu_nocb_setup);
rcu: Make rcu_nocb_poll an early_param instead of module_param The as-documented rcu_nocb_poll will fail to enable this feature for two reasons. (1) there is an extra "s" in the documented name which is not in the code, and (2) since it uses module_param, it really is expecting a prefix, akin to "rcutree.fanout_leaf" and the prefix isn't documented. However, there are several reasons why we might not want to simply fix the typo and add the prefix: 1) we'd end up with rcutree.rcu_nocb_poll, and rather probably make a change to rcutree.nocb_poll 2) if we did #1, then the prefix wouldn't be consistent with the rcu_nocbs=<cpumap> parameter (i.e. one with, one without prefix) 3) the use of module_param in a header file is less than desired, since it isn't immediately obvious that it will get processed via rcutree.c and get the prefix from that (although use of module_param_named() could clarify that.) 4) the implied export of /sys/module/rcutree/parameters/rcu_nocb_poll data to userspace via module_param() doesn't really buy us anything, as it is read-only and we can tell if it is enabled already without it, since there is a printk at early boot telling us so. In light of all that, just change it from a module_param() to an early_setup() call, and worry about adding it to /sys later on if we decide to allow a dynamic setting of it. Also change the variable to be tagged as read_mostly, since it will only ever be fiddled with at most, once at boot. Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
2012-12-20 21:19:22 +00:00
static int __init parse_rcu_nocb_poll(char *arg)
{
rcu_nocb_poll = 1;
return 0;
}
early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
/*
* Do any no-CBs CPUs need another grace period?
*
* Interrupts must be disabled. If the caller does not hold the root
* rnp_node structure's ->lock, the results are advisory only.
*/
static int rcu_nocb_needs_gp(struct rcu_state *rsp)
{
struct rcu_node *rnp = rcu_get_root(rsp);
return rnp->need_future_gp[(ACCESS_ONCE(rnp->completed) + 1) & 0x1];
}
/*
* Wake up any no-CBs CPUs' kthreads that were waiting on the just-ended
* grace period.
*/
static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
{
wake_up_all(&rnp->nocb_gp_wq[rnp->completed & 0x1]);
}
/*
* Set the root rcu_node structure's ->need_future_gp field
* based on the sum of those of all rcu_node structures. This does
* double-count the root rcu_node structure's requests, but this
* is necessary to handle the possibility of a rcu_nocb_kthread()
* having awakened during the time that the rcu_node structures
* were being updated for the end of the previous grace period.
*/
static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
{
rnp->need_future_gp[(rnp->completed + 1) & 0x1] += nrq;
}
static void rcu_init_one_nocb(struct rcu_node *rnp)
{
init_waitqueue_head(&rnp->nocb_gp_wq[0]);
init_waitqueue_head(&rnp->nocb_gp_wq[1]);
}
/* Is the specified CPU a no-CPUs CPU? */
bool rcu_is_nocb_cpu(int cpu)
{
if (have_rcu_nocb_mask)
return cpumask_test_cpu(cpu, rcu_nocb_mask);
return false;
}
/*
* Enqueue the specified string of rcu_head structures onto the specified
* CPU's no-CBs lists. The CPU is specified by rdp, the head of the
* string by rhp, and the tail of the string by rhtp. The non-lazy/lazy
* counts are supplied by rhcount and rhcount_lazy.
*
* If warranted, also wake up the kthread servicing this CPUs queues.
*/
static void __call_rcu_nocb_enqueue(struct rcu_data *rdp,
struct rcu_head *rhp,
struct rcu_head **rhtp,
int rhcount, int rhcount_lazy)
{
int len;
struct rcu_head **old_rhpp;
struct task_struct *t;
/* Enqueue the callback on the nocb list and update counts. */
old_rhpp = xchg(&rdp->nocb_tail, rhtp);
ACCESS_ONCE(*old_rhpp) = rhp;
atomic_long_add(rhcount, &rdp->nocb_q_count);
atomic_long_add(rhcount_lazy, &rdp->nocb_q_count_lazy);
/* If we are not being polled and there is a kthread, awaken it ... */
t = ACCESS_ONCE(rdp->nocb_kthread);
if (rcu_nocb_poll | !t)
return;
len = atomic_long_read(&rdp->nocb_q_count);
if (old_rhpp == &rdp->nocb_head) {
wake_up(&rdp->nocb_wq); /* ... only if queue was empty ... */
rdp->qlen_last_fqs_check = 0;
} else if (len > rdp->qlen_last_fqs_check + qhimark) {
wake_up_process(t); /* ... or if many callbacks queued. */
rdp->qlen_last_fqs_check = LONG_MAX / 2;
}
return;
}
/*
* This is a helper for __call_rcu(), which invokes this when the normal
* callback queue is inoperable. If this is not a no-CBs CPU, this
* function returns failure back to __call_rcu(), which can complain
* appropriately.
*
* Otherwise, this function queues the callback where the corresponding
* "rcuo" kthread can find it.
*/
static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
bool lazy)
{
if (!rcu_is_nocb_cpu(rdp->cpu))
return 0;
__call_rcu_nocb_enqueue(rdp, rhp, &rhp->next, 1, lazy);
if (__is_kfree_rcu_offset((unsigned long)rhp->func))
trace_rcu_kfree_callback(rdp->rsp->name, rhp,
(unsigned long)rhp->func,
rdp->qlen_lazy, rdp->qlen);
else
trace_rcu_callback(rdp->rsp->name, rhp,
rdp->qlen_lazy, rdp->qlen);
return 1;
}
/*
* Adopt orphaned callbacks on a no-CBs CPU, or return 0 if this is
* not a no-CBs CPU.
*/
static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
struct rcu_data *rdp)
{
long ql = rsp->qlen;
long qll = rsp->qlen_lazy;
/* If this is not a no-CBs CPU, tell the caller to do it the old way. */
if (!rcu_is_nocb_cpu(smp_processor_id()))
return 0;
rsp->qlen = 0;
rsp->qlen_lazy = 0;
/* First, enqueue the donelist, if any. This preserves CB ordering. */
if (rsp->orphan_donelist != NULL) {
__call_rcu_nocb_enqueue(rdp, rsp->orphan_donelist,
rsp->orphan_donetail, ql, qll);
ql = qll = 0;
rsp->orphan_donelist = NULL;
rsp->orphan_donetail = &rsp->orphan_donelist;
}
if (rsp->orphan_nxtlist != NULL) {
__call_rcu_nocb_enqueue(rdp, rsp->orphan_nxtlist,
rsp->orphan_nxttail, ql, qll);
ql = qll = 0;
rsp->orphan_nxtlist = NULL;
rsp->orphan_nxttail = &rsp->orphan_nxtlist;
}
return 1;
}
/*
* If necessary, kick off a new grace period, and either way wait
* for a subsequent grace period to complete.
*/
static void rcu_nocb_wait_gp(struct rcu_data *rdp)
{
unsigned long c;
bool d;
unsigned long flags;
struct rcu_node *rnp = rdp->mynode;
raw_spin_lock_irqsave(&rnp->lock, flags);
c = rcu_start_future_gp(rnp, rdp);
raw_spin_unlock_irqrestore(&rnp->lock, flags);
/*
* Wait for the grace period. Do so interruptibly to avoid messing
* up the load average.
*/
trace_rcu_future_gp(rnp, rdp, c, "StartWait");
for (;;) {
wait_event_interruptible(
rnp->nocb_gp_wq[c & 0x1],
(d = ULONG_CMP_GE(ACCESS_ONCE(rnp->completed), c)));
if (likely(d))
break;
flush_signals(current);
trace_rcu_future_gp(rnp, rdp, c, "ResumeWait");
}
trace_rcu_future_gp(rnp, rdp, c, "EndWait");
smp_mb(); /* Ensure that CB invocation happens after GP end. */
}
/*
* Per-rcu_data kthread, but only for no-CBs CPUs. Each kthread invokes
* callbacks queued by the corresponding no-CBs CPU.
*/
static int rcu_nocb_kthread(void *arg)
{
int c, cl;
struct rcu_head *list;
struct rcu_head *next;
struct rcu_head **tail;
struct rcu_data *rdp = arg;
/* Each pass through this loop invokes one batch of callbacks */
for (;;) {
/* If not polling, wait for next batch of callbacks. */
if (!rcu_nocb_poll)
wait_event_interruptible(rdp->nocb_wq, rdp->nocb_head);
list = ACCESS_ONCE(rdp->nocb_head);
if (!list) {
schedule_timeout_interruptible(1);
flush_signals(current);
continue;
}
/*
* Extract queued callbacks, update counts, and wait
* for a grace period to elapse.
*/
ACCESS_ONCE(rdp->nocb_head) = NULL;
tail = xchg(&rdp->nocb_tail, &rdp->nocb_head);
c = atomic_long_xchg(&rdp->nocb_q_count, 0);
cl = atomic_long_xchg(&rdp->nocb_q_count_lazy, 0);
ACCESS_ONCE(rdp->nocb_p_count) += c;
ACCESS_ONCE(rdp->nocb_p_count_lazy) += cl;
rcu_nocb_wait_gp(rdp);
/* Each pass through the following loop invokes a callback. */
trace_rcu_batch_start(rdp->rsp->name, cl, c, -1);
c = cl = 0;
while (list) {
next = list->next;
/* Wait for enqueuing to complete, if needed. */
while (next == NULL && &list->next != tail) {
schedule_timeout_interruptible(1);
next = list->next;
}
debug_rcu_head_unqueue(list);
local_bh_disable();
if (__rcu_reclaim(rdp->rsp->name, list))
cl++;
c++;
local_bh_enable();
list = next;
}
trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1);
ACCESS_ONCE(rdp->nocb_p_count) -= c;
ACCESS_ONCE(rdp->nocb_p_count_lazy) -= cl;
rdp->n_nocbs_invoked += c;
}
return 0;
}
/* Initialize per-rcu_data variables for no-CBs CPUs. */
static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
{
rdp->nocb_tail = &rdp->nocb_head;
init_waitqueue_head(&rdp->nocb_wq);
}
/* Create a kthread for each RCU flavor for each no-CBs CPU. */
static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
{
int cpu;
struct rcu_data *rdp;
struct task_struct *t;
if (rcu_nocb_mask == NULL)
return;
for_each_cpu(cpu, rcu_nocb_mask) {
rdp = per_cpu_ptr(rsp->rda, cpu);
t = kthread_run(rcu_nocb_kthread, rdp,
"rcuo%c/%d", rsp->abbr, cpu);
BUG_ON(IS_ERR(t));
ACCESS_ONCE(rdp->nocb_kthread) = t;
}
}
/* Prevent __call_rcu() from enqueuing callbacks on no-CBs CPUs */
static bool init_nocb_callback_list(struct rcu_data *rdp)
{
if (rcu_nocb_mask == NULL ||
!cpumask_test_cpu(rdp->cpu, rcu_nocb_mask))
return false;
rdp->nxttail[RCU_NEXT_TAIL] = NULL;
return true;
}
#else /* #ifdef CONFIG_RCU_NOCB_CPU */
static int rcu_nocb_needs_gp(struct rcu_state *rsp)
{
return 0;
}
static void rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
{
}
static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
{
}
static void rcu_init_one_nocb(struct rcu_node *rnp)
{
}
static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
bool lazy)
{
return 0;
}
static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
struct rcu_data *rdp)
{
return 0;
}
static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
{
}
static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
{
}
static bool init_nocb_callback_list(struct rcu_data *rdp)
{
return false;
}
#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
rcu: Kick adaptive-ticks CPUs that are holding up RCU grace periods Adaptive-ticks CPUs inform RCU when they enter kernel mode, but they do not necessarily turn the scheduler-clock tick back on. This state of affairs could result in RCU waiting on an adaptive-ticks CPU running for an extended period in kernel mode. Such a CPU will never run the RCU state machine, and could therefore indefinitely extend the RCU state machine, sooner or later resulting in an OOM condition. This patch, inspired by an earlier patch by Frederic Weisbecker, therefore causes RCU's force-quiescent-state processing to check for this condition and to send an IPI to CPUs that remain in that state for too long. "Too long" currently means about three jiffies by default, which is quite some time for a CPU to remain in the kernel without blocking. The rcu_tree.jiffies_till_first_fqs and rcutree.jiffies_till_next_fqs sysfs variables may be used to tune "too long" if needed. Reported-by: Frederic Weisbecker <fweisbec@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Josh Triplett <josh@joshtriplett.org> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> Cc: Chris Metcalf <cmetcalf@tilera.com> Cc: Christoph Lameter <cl@linux.com> Cc: Geoff Levand <geoff@infradead.org> Cc: Gilad Ben Yossef <gilad@benyossef.com> Cc: Hakan Akkan <hakanakkan@gmail.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Kevin Hilman <khilman@linaro.org> Cc: Li Zhong <zhong@linux.vnet.ibm.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Thomas Gleixner <tglx@linutronix.de>
2013-04-12 23:19:10 +00:00
/*
* An adaptive-ticks CPU can potentially execute in kernel mode for an
* arbitrarily long period of time with the scheduling-clock tick turned
* off. RCU will be paying attention to this CPU because it is in the
* kernel, but the CPU cannot be guaranteed to be executing the RCU state
* machine because the scheduling-clock tick has been disabled. Therefore,
* if an adaptive-ticks CPU is failing to respond to the current grace
* period and has not be idle from an RCU perspective, kick it.
*/
static void rcu_kick_nohz_cpu(int cpu)
{
#ifdef CONFIG_NO_HZ_FULL
if (tick_nohz_full_cpu(cpu))
smp_send_reschedule(cpu);
#endif /* #ifdef CONFIG_NO_HZ_FULL */
}