Re: REGRESSION: Performance regressions from switching anon_vma->lock to mutex

Andi Kleen
2011-06-17T19:41:55Z

On Fri, Jun 17, 2011 at 09:46:00AM -0700, Linus Torvalds wrote:
> On Fri, Jun 17, 2011 at 4:28 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> >
> > Something like so? Compiles and runs the benchmark in question.
>
> Oh, and can you do this with a commit log and sign-off, and I'll put
> it in my "anon_vma-locking" branch that I have. I'm not going to
> actually merge that branch into mainline until I've seen a few more
> acks or more testing by Tim.
>
> But if Tim's numbers hold up (-32% to +15% performance by just the
> first one, and +15% isn't actually an improvement since tmpfs
> read-ahead should have gotten us to +66%), I think we have to do this
> just to avoid the performance regression.

You could also add the mutex "optimize caching protocol"
patch I posted earlier to that branch.

It didn't actually improve Tim's throughput number, but it made the CPU
consumption of the mutex go down.

-Andi

---
>From 34d4c1e579b3dfbc9a01967185835f5829bd52f0 Mon Sep 17 00:00:00 2001
From: Andi Kleen <ak@linux.intel.com>
Date: Tue, 14 Jun 2011 16:27:54 -0700
Subject: [PATCH] mutex: while spinning read count before attempting cmpxchg

Under heavy contention it's better to read first before trying
to do an atomic operation on the interconnect.

This gives a few percent improvement for the mutex CPU time
under heavy contention and likely saves some power too.

Signed-off-by: Andi Kleen <ak@linux.intel.com>

diff --git a/kernel/mutex.c b/kernel/mutex.c
index d607ed5..1abffa9 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -170,7 +170,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
if (owner && !mutex_spin_on_owner(lock, owner))
break;

- if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
+ if (atomic_read(&lock->count) == 1 &&
+ atomic_cmpxchg(&lock->count, 1, 0) == 1) {
lock_acquired(&lock->dep_map, ip);
mutex_set_owner(lock);
preempt_enable();

--
ak@linux.intel.com -- Speaking for myself only