linux-kernel - Re: [PATCH 3/3] vmscan: Force kswapd to take notice faster when high-order watermarks are being hit

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Date:	Tue, 3 Nov 2009 22:08:09 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Frans Pop <elendil@...net.nl>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, stable@...nel.org,
	linux-kernel@...r.kernel.org,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Sven Geggus <lists@...hsschwanzdomain.de>,
	Karol Lewandowski <karol.k.lewandowski@...il.com>,
	Tobias Oetiker <tobi@...iker.ch>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Stephan von Krawczynski <skraw@...net.com>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: [PATCH 3/3] vmscan: Force kswapd to take notice faster when
	high-order watermarks are being hit

On Tue, Nov 03, 2009 at 11:01:50PM +0100, Frans Pop wrote:
> On Monday 02 November 2009, Mel Gorman wrote:
> > On Mon, Nov 02, 2009 at 06:32:54PM +0100, Frans Pop wrote:
> > > On Monday 02 November 2009, Mel Gorman wrote:
> > > > vmscan: Help debug kswapd issues by counting number of rewakeups and
> > > > premature sleeps
> > > >
> > > > There is a growing amount of anedotal evidence that high-order
> > > > atomic allocation failures have been increasing since 2.6.31-rc1.
> > > > The two strongest possibilities are a marked increase in the number
> > > > of GFP_ATOMIC allocations and alterations in timing. Debugging
> > > > printk patches have shown for example that kswapd is sleeping for
> > > > shorter intervals and going to sleep when watermarks are still not
> > > > being met.
> > > >
> > > > This patch adds two kswapd counters to help identify if timing is an
> > > > issue. The first counter kswapd_highorder_rewakeup counts the number
> > > > of times that kswapd stops reclaiming at one order and restarts at a
> > > > higher order. The second counter kswapd_slept_prematurely counts the
> > > > number of times kswapd went to sleep when the high watermark was not
> > > > met.
> > >
> > > What testing would you like done with this patch?
> >
> > Same reproduction as before except post what the contents of
> > /proc/vmstat were after the problem was triggered.
> 
> With a representative test I get 0 for kswapd_slept_prematurely.
> Tested with .32-rc6 + patches 1-3 + this patch.
> 

Assuming the problem actually reproduced, can you still retest with the
patch I posted as a follow-up and see if fast or slow premature sleeps
are happening and if the problem still occurs please? It's still
possible with the patch as-is could be timing related. After I posted
this patch, I continued testing and found I could get counts fairly
reliably if kswapd was calling printk() before making the premature
check so the window appears to be very small.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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