Re: [PATCHv3 0/2] vhost: a kernel-level virtio server

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Aug 20, 2009 at 04:34:39PM +0300, Or Gerlitz wrote:
> Michael S. Tsirkin wrote:
>> Yes. master
> okay, will get testing this later next week. Any chance you can provide  
> some packet-per-second numbers (netperf udp stream with small packets)?
>
> Or.

If you do, maybe you should apply the following patch on top
(seems to save 2 atomics in about 50% of cases for me).

---

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@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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);