Re: [BUG] fatal hang untarring 90GB file, possibly writebackrelated.

[James Bottomley]

On Thu, 2011-04-28 at 16:08 +0100, Mel Gorman wrote:
> On Thu, Apr 28, 2011 at 09:25:14AM -0500, James Bottomley wrote:
> > On Thu, 2011-04-28 at 15:07 +0100, Mel Gorman wrote:
> > > On Thu, Apr 28, 2011 at 03:52:28PM +0200, Jan Kara wrote:
> > > > On Thu 28-04-11 12:36:30, Colin Ian King wrote:
> > > > > One more data point to add, I've been looking at an identical issue when
> > > > > copying large amounts of data.  I bisected this - and the lockups occur
> > > > > with commit 
> > > > > 3e7d344970673c5334cf7b5bb27c8c0942b06126 - before that I don't see the
> > > > > issue. With this commit, my file copy test locks up after ~8-10
> > > > > iterations, before this commit I can copy > 100 times and don't see the
> > > > > lockup.
> > > >   Adding Mel to CC, I guess he'll be interested. Mel, it seems this commit
> > > > of yours causes kswapd on non-preempt kernels spin for a *long* time...
> > > > 
> > > 
> > > I'm still thinking about the traces which do not point the finger
> > > directly at compaction per-se but it's possible that the change means
> > > kswapd is not reclaiming like it should be.
> > > 
> > > To test this theory, does applying
> > > [d527caf2: mm: compaction: prevent kswapd compacting memory to reduce
> > > CPU usage] help?
> > 
> > I can answer definitively no to this.  The upstream kernel I reproduced
> > this on has that patch included.
> > 
> 
> So it is.
> 
> Another consequence of this patch is that when high order allocations
> are in progress (is the test case fork heavy in any way for
> example?

It's a simple huge untar, so it shouldn't fork.

>  alternatively, it might be something in the storage stack
> that requires high-order allocs)

I've tried switching from CFQ to deadline with no apparent changes

>  we are no longer necessarily going
> to sleep because of should_reclaim_continue() check. This could
> explain kswapd-at-99% but would only apply if CONFIG_COMPACTION is
> set (does unsetting CONFIG_COMPACTION help). If the bug only triggers
> for CONFIG_COMPACTION, does the following *untested* patch help any?

Turning off COMPACTION and HUGEPAGES doesn't help ... kswapd still goes
to 99% on the PREEMPT kernel, so it doesn't seem to be related

James


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