From: Michael S. Tsirkin <mst@redhat.com>
Date: Tue, 22 Sep 2009 00:03:52 +0000 (-0700)
Subject: mm: reduce atomic use on use_mm fast path
X-Git-Tag: v2.6.32-rc1~472
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=f68e14805085972b4e0b0ab684af37f713b9c262;p=profile%2Fcommon%2Fkernel-common.git

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 @@ 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);
 }
 
 /*