Linux Virtualization - Sep 2009 - [PATCHv3 2/2] mm: reduce atomic use on use_mm fast path

When mm switched to matches that of 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 cache line on SMP systems.

Acked-by: Andrea Arcangeli <aarcange at redhat.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---
 mm/mmu_context.c |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

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 @@ 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);
 }
 
 /*
-- 
1.6.2.5