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

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Date:	Fri, 17 Jun 2011 09:36:48 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andi Kleen <ak@...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 4:28 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>
> Something like so? Compiles and runs the benchmark in question.

Yup.

Except I really think that test for a NULL anon_vma should go away.

If an avc entry has a NULL anon_vma, something is seriously wrong. The
comment about anon_vma_fork failure is definitely just bogus: the
anon_vma is allocated before the avc entry, so there's no way a avc
can have a NULL anon_vma from there.

But yes, your patch is cleaner than the one I was playing around with
(your "remove if not list empty" is prettier than what I was toying
with - having a separate flag in the avc)

Tim, can you test Peter's (second - the cleaned up one) patch on top
of mine, and see if that helps things further?

The only thing I don't love about the batching is that we now do hold
the lock over some situations where we _could_ have allowed
concurrency (notably some avc allocations), but I think it's a good
trade-off. And walking the list twice at unlink_anon_vmas() should be
basically free.

                       Linus
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