From: James Bottomley <James.Bottomley@HansenPartnership.com>
Subject: Re: [PATCH 0/3] Reduce impact to overall system of SLUB using
 high-order allocations
Date: Thu, 12 May 2011 09:04:56 -0500
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Cc: David Rientjes <rientjes@google.com>, Mel Gorman <mgorman@suse.de>,
 Andrew Morton <akpm@linux-foundation.org>, Colin King
 <colin.king@canonical.com>, Raghavendra D Prabhu
 <raghu.prabhu13@gmail.com>, Jan Kara <jack@suse.cz>, Chris Mason
 <chris.mason@oracle.com>, Christoph Lameter <cl@linux.com>, Rik van Riel
 <riel@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>, linux-fsdevel
 <linux-fsdevel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
 linux-kernel <linux-kernel@vger.kernel.org>, linux-ext4
 <linux-ext4@vger.kernel.org>
To: Pekka Enberg <penberg@cs.helsinki.fi>
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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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