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

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Message-ID: <BANLkTinEhVY4aZ+M6H=380zd0Osr_6VFCA@mail.gmail.com>
Date:	Tue, 14 Jun 2011 18:21:14 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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

On Tue, Jun 14, 2011 at 5:29 PM, Tim Chen <tim.c.chen@...ux.intel.com> wrote:
>
> On 2.6.39, the contention of anon_vma->lock occupies 3.25% of cpu.
> However, after the switch of the lock to mutex on 3.0-rc2, the mutex
> acquisition jumps to 18.6% of cpu.  This seems to be the main cause of
> the 52% throughput regression.

Argh. That's nasty.

Even the 3.25% is horrible. We scale so well in other situations that
it's really sad how the anon_vma lock is now one of our worst issues.

Anyway, please check me if I'm wrong, but won't the "anon_vma->root"
be the same for all the anon_vma's that are associated with one
particular vma?

The reason I ask is because when I look at anon_vma_clone(), we do that

   list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
      ...
      anon_vma_chain_link(dst, avc, pavc->anon_vma);
   }

an dthen we do that anon_vma_lock()/unlock() dance on each of those
pavc->anon_vma's. But if the anon_vma->root is always the same, then
that would mean that we could do the lock just once, and hold it over
the loop.

Because I think the real problem with that anon_vma locking is that it
gets called so _much_. We'd be better off holding the lock for a
longer time, and just not do the lock/unlock thing so often. The
contention would go down simply because we wouldn't waste our time
with those atomic lock/unlock instructions as much.

Gaah. I knew exactly how the anon_vma locking worked a few months ago,
but it's complicated enough that I've swapped out all the details. So
I'm not at all sure that the anon_vma->root will be the same for every
anon_vma on the same_vma list.

Somebody hit me over the head with a clue-bat. Anybody?

                   Linus
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