linux-kernel - Re: REGRESSION: Performance regressions from switching anon

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Date:	Fri, 17 Jun 2011 12:40:29 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Shaohua Li <shaohua.li@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Russell King <rmk@....linux.org.uk>,
	Paul Mundt <lethal@...ux-sh.org>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	"Luck, Tony" <tony.luck@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Namhyung Kim <namhyung@...il.com>,
	"Shi, Alex" <alex.shi@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: REGRESSION: Performance regressions from switching
 anon_vma->lock to mutex

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@...llo.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@...ux.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@...ux.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@...ux.intel.com -- Speaking for myself only
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