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

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Date:	Fri, 17 Jun 2011 15:20:14 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Hugh Dickins <hughd@...gle.com>,
	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>,
	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, 17 Jun 2011, Linus Torvalds wrote:
> On Fri, Jun 17, 2011 at 11:32 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > something like so I guess, completely untested etc..
> 
> Having gone over it a bit more, I actually think I prefer to just
> special-case the allocation instead.
> 
> We already have to drop the anon_vma lock for the "out of memory"
> case, and a slight re-organization of clone_anon_vma() makes it easy
> to just first try a NOIO allocation with the lock still held, and then
> if that fails do the "drop lock, retry, and hard-fail" case.
> 
> IOW, something like the attached (on top of the patches already posted
> except for your memory reclaim thing)
> 
> Hugh, does this fix the lockdep issue?

Yes, that fixed the lockdep issue, and ran nicely under load for an hour.

I agree that it's better to do this GFP_NOWAIT and fallback,
than trylock the anon_vma.

And I'm happy that you've still got that WARN_ON_ONCE(root) in: I do not
have a fluid mental model of the anon_vma_chains, get lost there; and
though it's obvious that we must have the same anon_vma->root going
down the same_anon_vma list, I could not put my finger on a killer
demonstration for why the same has to be true of the same_vma list.

But I've not seen your WARN_ON_ONCE fire, and it's hard to imagine
how there could be more than one root in the whole bundle of lists.

Hugh
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