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Date: Wed, 15 Jun 2011 22:17:13 +0200
From: Ingo Molnar <mingo@elte.hu>
Subject: Re: REGRESSION: Performance regressions from switching
 anon_vma->lock to mutex
Message-ID: <20110615201713.GC4762@elte.hu>
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To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Peter Zijlstra <peterz@infradead.org>, Paul McKenney <paulmck@linux.vnet.ibm.com>, Andrew Morton <akpm@linux-foundation.org>, Hugh Dickins <hughd@google.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, David Miller <davem@davemloft.net>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Russell King <rmk@arm.linux.org.uk>, Paul Mundt <lethal@linux-sh.org>, Jeff Dike <jdike@addtoit.com>, Richard Weinberger <richard@nod.at>, Tony Luck <tony.luck@intel.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Mel Gorman <mel@csn.ul.ie>, Nick Piggin <npiggin@kernel.dk>, Namhyung Kim <namhyung@gmail.com>, ak@linux.intel.com, shaohua.li@intel.com, alex.shi@intel.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, "Rafael J. Wysocki" <rjw@sisk.pl>


* Tim Chen <tim.c.chen@linux.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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