mm: reduce atomic use on use_mm fast path

When the mm being switched to matches the active mm, we don't need to
increment and then drop the mm count.  In a simple benchmark this happens
in about 50% of time.  Making that conditional reduces contention on that
cacheline on SMP systems.

Acked-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/mm/mmu_context.c b/mm/mmu_context.c
index fd473b51c903..ded9081f4021 100644
--- a/mm/mmu_context.c
+++ b/mm/mmu_context.c
@@ -26,13 +26,16 @@ void use_mm(struct mm_struct *mm)
 
         task_lock(tsk);
         active_mm = tsk->active_mm;
-        atomic_inc(&mm->mm_count);
+        if (active_mm != mm) {
+                atomic_inc(&mm->mm_count);
+                tsk->active_mm = mm;
+        }
         tsk->mm = mm;
-        tsk->active_mm = mm;
         switch_mm(active_mm, mm, tsk);
         task_unlock(tsk);
 
-        mmdrop(active_mm);
+        if (active_mm != mm)
+                mmdrop(active_mm);
 }
 
 /*