linux/arch/powerpc/mm/mmu_context.c
Mathieu Desnoyers 3ccfebedd8 powerpc, membarrier: Skip memory barrier in switch_mm()
Allow PowerPC to skip the full memory barrier in switch_mm(), and
only issue the barrier when scheduling into a task belonging to a
process that has registered to use expedited private.

Threads targeting the same VM but which belong to different thread
groups is a tricky case. It has a few consequences:

It turns out that we cannot rely on get_nr_threads(p) to count the
number of threads using a VM. We can use
(atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)
instead to skip the synchronize_sched() for cases where the VM only has
a single user, and that user only has a single thread.

It also turns out that we cannot use for_each_thread() to set
thread flags in all threads using a VM, as it only iterates on the
thread group.

Therefore, test the membarrier state variable directly rather than
relying on thread flags. This means
membarrier_register_private_expedited() needs to set the
MEMBARRIER_STATE_PRIVATE_EXPEDITED flag, issue synchronize_sched(), and
only then set MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY which allows
private expedited membarrier commands to succeed.
membarrier_arch_switch_mm() now tests for the
MEMBARRIER_STATE_PRIVATE_EXPEDITED flag.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Andrew Hunter <ahh@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Avi Kivity <avi@scylladb.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Dave Watson <davejwatson@fb.com>
Cc: David Sehr <sehr@google.com>
Cc: Greg Hackmann <ghackmann@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maged Michael <maged.michael@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-api@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lkml.kernel.org/r/20180129202020.8515-3-mathieu.desnoyers@efficios.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2018-02-05 21:34:02 +01:00

98 lines
2.6 KiB
C

/*
* Common implementation of switch_mm_irqs_off
*
* Copyright IBM Corp. 2017
*
* 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.
*
*/
#include <linux/mm.h>
#include <linux/cpu.h>
#include <linux/sched/mm.h>
#include <asm/mmu_context.h>
#if defined(CONFIG_PPC32)
static inline void switch_mm_pgdir(struct task_struct *tsk,
struct mm_struct *mm)
{
/* 32-bit keeps track of the current PGDIR in the thread struct */
tsk->thread.pgdir = mm->pgd;
}
#elif defined(CONFIG_PPC_BOOK3E_64)
static inline void switch_mm_pgdir(struct task_struct *tsk,
struct mm_struct *mm)
{
/* 64-bit Book3E keeps track of current PGD in the PACA */
get_paca()->pgd = mm->pgd;
}
#else
static inline void switch_mm_pgdir(struct task_struct *tsk,
struct mm_struct *mm) { }
#endif
void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
struct task_struct *tsk)
{
bool new_on_cpu = false;
/* Mark this context has been used on the new CPU */
if (!cpumask_test_cpu(smp_processor_id(), mm_cpumask(next))) {
cpumask_set_cpu(smp_processor_id(), mm_cpumask(next));
inc_mm_active_cpus(next);
/*
* This full barrier orders the store to the cpumask above vs
* a subsequent operation which allows this CPU to begin loading
* translations for next.
*
* When using the radix MMU that operation is the load of the
* MMU context id, which is then moved to SPRN_PID.
*
* For the hash MMU it is either the first load from slb_cache
* in switch_slb(), and/or the store of paca->mm_ctx_id in
* copy_mm_to_paca().
*
* On the read side the barrier is in pte_xchg(), which orders
* the store to the PTE vs the load of mm_cpumask.
*
* This full barrier is needed by membarrier when switching
* between processes after store to rq->curr, before user-space
* memory accesses.
*/
smp_mb();
new_on_cpu = true;
}
/* Some subarchs need to track the PGD elsewhere */
switch_mm_pgdir(tsk, next);
/* Nothing else to do if we aren't actually switching */
if (prev == next)
return;
/*
* We must stop all altivec streams before changing the HW
* context
*/
if (cpu_has_feature(CPU_FTR_ALTIVEC))
asm volatile ("dssall");
if (new_on_cpu)
radix_kvm_prefetch_workaround(next);
else
membarrier_arch_switch_mm(prev, next, tsk);
/*
* The actual HW switching method differs between the various
* sub architectures. Out of line for now
*/
switch_mmu_context(prev, next, tsk);
}