[tip: sched/urgent] sched/membarrier: Skip IPIs when mm->mm_users == 1

[tip-bot2 for Mathieu Desnoyers]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     c6d68c1c4a4d6611fc0f8145d764226571d737ca
Gitweb:        
https://git.kernel.org/tip/c6d68c1c4a4d6611fc0f8145d764226571d737ca
Author:        Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
AuthorDate:    Thu, 19 Sep 2019 13:37:04 -04:00
Committer:     Ingo Molnar <mi...@kernel.org>
CommitterDate: Wed, 25 Sep 2019 17:42:31 +02:00

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.desnoy...@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: Chris Metcalf <cmetc...@ezchip.com>
Cc: Christoph Lameter <c...@linux.com>
Cc: Eric W. Biederman <ebied...@xmission.com>
Cc: Kirill Tkhai <tk...@yandex.ru>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Mike Galbraith <efa...@gmx.de>
Cc: Oleg Nesterov <o...@redhat.com>
Cc: Paul E. McKenney <paul...@linux.ibm.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Russell King - ARM Linux admin <li...@armlinux.org.uk>
Cc: Thomas Gleixner <t...@linutronix.de>
Link: 
https://lkml.kernel.org/r/20190919173705.2181-7-mathieu.desnoy...@efficios.com
Signed-off-by: Ingo Molnar <mi...@kernel.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 070cf43..fced54a 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;
 
        /*
@@ -194,7 +195,7 @@ static int membarrier_private_expedited(int flags)
                        continue;
                rcu_read_lock();
                p = 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