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:

quoted

On Wed, 2012-01-11 at 10:11 +0000, Mel Gorman wrote:

quoted

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.

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