linux-kernel - [RFC PATCH for 5.4 6/7] sched/membarrier: Skip IPIs when mm->mm

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Date:   Thu, 19 Sep 2019 13:37:04 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: [RFC PATCH for 5.4 6/7] sched/membarrier: Skip IPIs when mm->mm_users == 1

If there is only a single mm_user for the mm, the private expedited
membarrier command can skip the IPIs, because only a single thread
is using the mm.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc: Chris Metcalf <cmetcalf@...hip.com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Kirill Tkhai <tkhai@...dex.ru>
Cc: Mike Galbraith <efault@....de>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/membarrier.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index fce06a2e1d89..8afbdf92be0a 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -145,20 +145,21 @@ static int membarrier_private_expedited(int flags)
 	int cpu;
 	bool fallback = false;
 	cpumask_var_t tmpmask;
+	struct mm_struct *mm = current->mm;
 
 	if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
 		if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
 			return -EINVAL;
-		if (!(atomic_read(&current->mm->membarrier_state) &
+		if (!(atomic_read(&mm->membarrier_state) &
 		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY))
 			return -EPERM;
 	} else {
-		if (!(atomic_read(&current->mm->membarrier_state) &
+		if (!(atomic_read(&mm->membarrier_state) &
 		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY))
 			return -EPERM;
 	}
 
-	if (num_online_cpus() == 1)
+	if (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1)
 		return 0;
 
 	/*
@@ -193,7 +194,7 @@ static int membarrier_private_expedited(int flags)
 		if (cpu == raw_smp_processor_id())
 			continue;
 		p = task_rcu_dereference(&cpu_rq(cpu)->curr);
-		if (p && p->mm == current->mm) {
+		if (p && p->mm == mm) {
 			if (!fallback)
 				__cpumask_set_cpu(cpu, tmpmask);
 			else
-- 
2.17.1