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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 17 Jun 2011 09:36:48 -0700
Message-ID: <BANLkTim2bmPfeRT1tS7hx2Z85QHjPHwU3Q@mail.gmail.com>
Subject: Re: REGRESSION: Performance regressions from switching anon_vma->lock
 to mutex
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tim Chen <tim.c.chen@linux.intel.com>, Andi Kleen <ak@linux.intel.com>,
        Shaohua Li <shaohua.li@intel.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>,
        "Luck, Tony" <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>, "Shi, Alex" <alex.shi@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "Rafael J. Wysocki" <rjw@sisk.pl>
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On Fri, Jun 17, 2011 at 4:28 AM, Peter Zijlstra <a.p.zijlstra@chello.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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