linux-ext4 - Re: [PATCH 0/3] Reduce impact to overall system of SLUB using high-order allocations

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Message-ID: <1305209096.2575.14.camel@mulgrave.site>
Date:	Thu, 12 May 2011 09:04:56 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	David Rientjes <rientjes@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Colin King <colin.king@...onical.com>,
	Raghavendra D Prabhu <raghu.prabhu13@...il.com>,
	Jan Kara <jack@...e.cz>, Chris Mason <chris.mason@...cle.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 0/3] Reduce impact to overall system of SLUB using
 high-order allocations

On Thu, 2011-05-12 at 14:13 +0300, Pekka Enberg wrote:
> On 5/12/11 1:34 AM, James Bottomley wrote:
> > On Wed, 2011-05-11 at 15:28 -0700, David Rientjes wrote:
> >> On Wed, 11 May 2011, James Bottomley wrote:
> >>
> >>> OK, I confirm that I can't seem to break this one.  No hangs visible,
> >>> even when loading up the system with firefox, evolution, the usual
> >>> massive untar, X and even a distribution upgrade.
> >>>
> >>> You can add my tested-by
> >>>
> >> Your system still hangs with patches 1 and 2 only?
> > Yes, but only once in all the testing.  With patches 1 and 2 the hang is
> > much harder to reproduce, but it still seems to be present if I hit it
> > hard enough.
> 
> Patches 1-2 look reasonable to me. I'm not completely convinced of patch 
> 3, though. Why are we seeing these problems now? This has been in 
> mainline for a long time already. Shouldn't we fix kswapd?

So I'm open to this.  The hang occurs when kswapd races around in
shrink_slab and never exits.  It looks like there's a massive number of
wakeups triggering this, but we haven't been able to diagnose it
further.  turning on PREEMPT gets rid of the hang, so I could try to
reproduce with PREEMPT and turn on tracing.  The problem so far has been
that the number of events is so huge that the trace buffer only captures
a few microseconds of output.

James


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