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

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Date:	Thu, 12 May 2011 10:53:44 -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 09:04 -0500, James Bottomley wrote:
> 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.

OK, here's the trace from a PREEMPT kernel (2.6.38.6) when kswapd hits
99% and stays there.  I've only enabled the vmscan tracepoints to try
and get a longer run.  It mosly looks like kswapd waking itself, but
there might be more in there that mm trained eyes can see.

James


Download attachment "tmp.trace.gz" of type "application/x-gzip" (175858 bytes)