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>
diff --git a/mm/mmu_context.c b/mm/mmu_context.c
index fd473b5..ded9081 100644
--- a/mm/mmu_context.c
+++ b/mm/mmu_context.c
@@ -26,13 +26,16 @@
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);
}
/*