nohz: Basic full dynticks interface

For extreme usecases such as Real Time or HPC, having
the ability to shutdown the tick when a single task runs
on a CPU is a desired feature:

* Reducing the amount of interrupts improves throughput
for CPU-bound tasks. The CPU is less distracted from its
real job, from an execution time and from the cache point
of views.

* This also improve latency response as we have less critical
sections.

Start with introducing a very simple interface to define
full dynticks CPU: use a boot time option defined cpumask
through the "nohz_extended=" kernel parameter. CPUs that
are part of this range will have their tick shutdown
whenever possible: provided they run a single task and
they don't do kernel activity that require the periodic
tick. These details will be later documented in
Documentation/*

An online CPU must be kept outside this range to handle the
timekeeping.

Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
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: Namhyung Kim <namhyung.kim@lge.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>
This commit is contained in:
Frederic Weisbecker 2012-12-18 17:32:19 +01:00
parent a8d7ad52a7
commit a831881be2
4 changed files with 94 additions and 0 deletions

View file

@ -1913,6 +1913,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
Valid arguments: on, off
Default: on
nohz_extended= [KNL,BOOT]
In kernels built with CONFIG_NO_HZ_EXTENDED=y, set
the specified list of CPUs whose tick will be stopped
whenever possible. You need to keep at least one online
CPU outside the range to maintain the timekeeping.
noiotrap [SH] Disables trapped I/O port accesses.
noirqdebug [X86-32] Disables the code which attempts to detect and

View file

@ -157,6 +157,13 @@ static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return -1; }
static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
# endif /* !NO_HZ */
#ifdef CONFIG_NO_HZ_EXTENDED
extern int tick_nohz_extended_cpu(int cpu);
#else
static inline int tick_nohz_extended_cpu(int cpu) { return 0; }
#endif
# ifdef CONFIG_CPU_IDLE_GOV_MENU
extern void menu_hrtimer_cancel(void);
# else

View file

@ -79,6 +79,25 @@ config NO_HZ
only trigger on an as-needed basis both when the system is
busy and when the system is idle.
config NO_HZ_EXTENDED
bool "Full dynticks system"
depends on NO_HZ && RCU_USER_QS && VIRT_CPU_ACCOUNTING_GEN && RCU_NOCB_CPU && SMP
select CONTEXT_TRACKING_FORCE
help
Adaptively try to shutdown the tick whenever possible, even when
the CPU is running tasks. Typically this requires running a single
task on the CPU. Chances for running tickless are maximized when
the task mostly runs in userspace and has few kernel activity.
You need to fill up the nohz_extended boot parameter with the
desired range of dynticks CPUs.
This is implemented at the expense of some overhead in user <-> kernel
transitions: syscalls, exceptions and interrupts. Even when it's
dynamically off.
Say N.
config HIGH_RES_TIMERS
bool "High Resolution Timer Support"
depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS

View file

@ -142,6 +142,68 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
profile_tick(CPU_PROFILING);
}
#ifdef CONFIG_NO_HZ_EXTENDED
static cpumask_var_t nohz_extended_mask;
bool have_nohz_extended_mask;
int tick_nohz_extended_cpu(int cpu)
{
if (!have_nohz_extended_mask)
return 0;
return cpumask_test_cpu(cpu, nohz_extended_mask);
}
/* Parse the boot-time nohz CPU list from the kernel parameters. */
static int __init tick_nohz_extended_setup(char *str)
{
alloc_bootmem_cpumask_var(&nohz_extended_mask);
if (cpulist_parse(str, nohz_extended_mask) < 0)
pr_warning("NOHZ: Incorrect nohz_extended cpumask\n");
else
have_nohz_extended_mask = true;
return 1;
}
__setup("nohz_extended=", tick_nohz_extended_setup);
static int __init init_tick_nohz_extended(void)
{
cpumask_var_t online_nohz;
int cpu;
if (!have_nohz_extended_mask)
return 0;
if (!zalloc_cpumask_var(&online_nohz, GFP_KERNEL)) {
pr_warning("NO_HZ: Not enough memory to check extended nohz mask\n");
return -ENOMEM;
}
/*
* CPUs can probably not be concurrently offlined on initcall time.
* But we are paranoid, aren't we?
*/
get_online_cpus();
/* Ensure we keep a CPU outside the dynticks range for timekeeping */
cpumask_and(online_nohz, cpu_online_mask, nohz_extended_mask);
if (cpumask_equal(online_nohz, cpu_online_mask)) {
cpu = cpumask_any(cpu_online_mask);
pr_warning("NO_HZ: Must keep at least one online CPU "
"out of nohz_extended range\n");
pr_warning("NO_HZ: Clearing %d from nohz_extended range\n", cpu);
cpumask_clear_cpu(cpu, nohz_extended_mask);
}
put_online_cpus();
free_cpumask_var(online_nohz);
return 0;
}
core_initcall(init_tick_nohz_extended);
#else
#define have_nohz_extended_mask (0)
#endif
/*
* NOHZ - aka dynamic tick functionality
*/