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

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Message-ID: <20110615201713.GC4762@elte.hu>
Date:	Wed, 15 Jun 2011 22:17:13 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.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>,
	Tony Luck <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>, ak@...ux.intel.com,
	shaohua.li@...el.com, alex.shi@...el.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: REGRESSION: Performance regressions from switching
 anon_vma->lock to mutex


* Tim Chen <tim.c.chen@...ux.intel.com> wrote:

> On Wed, 2011-06-15 at 12:11 -0700, Linus Torvalds wrote:
> 
> > 
> > And it results in real problems. For example, if you use "perf record"
> > to see what the hell is up, the use of kernel threads for RCU
> > callbacks means that the RCU cost is never even seen. I don't know how
> > Tim did his profiling to figure out the costs, and I don't know how he
> > decided that the spinlock to semaphore conversion was the culprit, but
> > it is entirely possible that Tim didn't actually bisect the problem,
> > but instead used "perf record" on the exim task, saw that the
> > semaphore costs had gone up, and decided that it must be the
> > conversion.
> > 
> 
> Yes, I was using perf to do the profiling. I thought that the mutex 
> conversion was the most likely culprit based on the change in 
> profile.

have you used callgraph profiling (perf record -g) or flat profiling? 
Flat profiling can be misleading when there's proxy work done.

Thanks,

	Ingo
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