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Commit 3ccfebed authored by Mathieu Desnoyers's avatar Mathieu Desnoyers Committed by Ingo Molnar
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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: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarPeter 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: default avatarIngo Molnar <mingo@kernel.org>
parent 667ca1ec

MAINTAINERS

+1 −0
Original line number Diff line number Diff line
@@ -8944,6 +8944,7 @@ L: linux-kernel@vger.kernel.org 
S:	Supported

F:	kernel/sched/membarrier.c

F:	include/uapi/linux/membarrier.h

F:	arch/powerpc/include/asm/membarrier.h



MEMORY MANAGEMENT

L:	linux-mm@kvack.org

arch/powerpc/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -140,6 +140,7 @@ config PPC 
	select ARCH_HAS_FORTIFY_SOURCE

	select ARCH_HAS_GCOV_PROFILE_ALL

	select ARCH_HAS_PMEM_API                if PPC64

	select ARCH_HAS_MEMBARRIER_CALLBACKS

	select ARCH_HAS_SCALED_CPUTIME		if VIRT_CPU_ACCOUNTING_NATIVE

	select ARCH_HAS_SG_CHAIN

	select ARCH_HAS_TICK_BROADCAST		if GENERIC_CLOCKEVENTS_BROADCAST

arch/powerpc/include/asm/membarrier.h

0 → 100644
+26 −0
Original line number Diff line number Diff line 
#ifndef _ASM_POWERPC_MEMBARRIER_H

#define _ASM_POWERPC_MEMBARRIER_H



static inline void membarrier_arch_switch_mm(struct mm_struct *prev,

					     struct mm_struct *next,

					     struct task_struct *tsk)

{

	/*

	 * Only need the full barrier when switching between processes.

	 * Barrier when switching from kernel to userspace is not

	 * required here, given that it is implied by mmdrop(). Barrier

	 * when switching from userspace to kernel is not needed after

	 * store to rq->curr.

	 */

	if (likely(!(atomic_read(&next->membarrier_state) &

		     MEMBARRIER_STATE_PRIVATE_EXPEDITED) || !prev))

		return;



	/*

	 * The membarrier system call requires a full memory barrier

	 * after storing to rq->curr, before going back to user-space.

	 */

	smp_mb();

}



#endif /* _ASM_POWERPC_MEMBARRIER_H */

arch/powerpc/mm/mmu_context.c

+7 −0
Original line number Diff line number Diff line
@@ -12,6 +12,7 @@ 


#include <linux/mm.h>

#include <linux/cpu.h>

#include <linux/sched/mm.h>



#include <asm/mmu_context.h>
@@ -58,6 +59,10 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, 
		 *

		 * 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();
@@ -80,6 +85,8 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, 


	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

include/linux/sched/mm.h

+12 −1
Original line number Diff line number Diff line
@@ -215,14 +215,25 @@ static inline void memalloc_noreclaim_restore(unsigned int flags) 
#ifdef CONFIG_MEMBARRIER

enum {

	MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY	= (1U << 0),

	MEMBARRIER_STATE_SWITCH_MM			= (1U << 1),

	MEMBARRIER_STATE_PRIVATE_EXPEDITED		= (1U << 1),

};



#ifdef CONFIG_ARCH_HAS_MEMBARRIER_CALLBACKS

#include <asm/membarrier.h>

#endif



static inline void membarrier_execve(struct task_struct *t)

{

	atomic_set(&t->mm->membarrier_state, 0);

}

#else

#ifdef CONFIG_ARCH_HAS_MEMBARRIER_CALLBACKS

static inline void membarrier_arch_switch_mm(struct mm_struct *prev,

					     struct mm_struct *next,

					     struct task_struct *tsk)

{

}

#endif

static inline void membarrier_execve(struct task_struct *t)

{

}