From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH 2/2] mm: page allocator: Do not drain per-cpu lists via
 IPI from page allocator context
Date: Thu, 12 Jan 2012 16:52:31 +0100
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To: Mel Gorman <mgorman@suse.de>
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On Thu, 2012-01-12 at 15:37 +0000, Mel Gorman wrote:
> On Thu, Jan 12, 2012 at 04:18:12PM +0100, Peter Zijlstra wrote:
> > On Wed, 2012-01-11 at 10:11 +0000, Mel Gorman wrote:
> > > At least one bug report has
> > > been seen on ppc64 against a 3.0 era kernel that looked like a bug
> > > receiving interrupts on a CPU being offlined. 
> > 
> > Got details on that Mel? The preempt_disable() in on_each_cpu() should
> > serialize against the stop_machine() crap in unplug.
> 
> I might have added 2 and 2 together and got 5.
> 
> The stack trace clearly was while sending IPIs in on_each_cpu() and
> always when under memory pressure and stuck in direct reclaim. This was
> on !PREEMPT kernels where preempt_disable() is a no-op. That is why I
> thought get_online_cpu() would be necessary.

For non-preempt the required scheduling of stop_machine() will have to
wait even longer. Still there might be something funny, some of the
hotplug notifiers are ran before the stop_machine thing does its thing
so there might be some fun interaction.