Subject: Re: REGRESSION: Performance regressions from switching
 anon_vma->lock to mutex
From: Tim Chen <tim.c.chen@linux.intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
        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>
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Date: Wed, 15 Jun 2011 13:13:04 -0700
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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.

Tim

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