Linux Virtualization - Aug 2009 - [PATCHv5 2/3] 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. 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 9989c2f..0777654 100644
--- a/mm/mmu_context.c
+++ b/mm/mmu_context.c
@@ -27,13 +27,16 @@ 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);
 }
 EXPORT_SYMBOL_GPL(use_mm);
 
-- 
1.6.2.5