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Date: Fri, 10 Feb 2012 16:51:17 -0500 (EST)
From: Alan Stern <stern@rowland.harvard.edu>
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To: Peter Zijlstra <a.p.zijlstra@chello.nl>
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Subject: Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related
 to cpusets
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On Fri, 10 Feb 2012, Peter Zijlstra wrote:

> Now the whole problem here seems to be that suspend uses cpu-hotplug to
> reduce the machine to UP -- I've no clue why it does that but I can
> imagine its because the BIOS calls only work on CPU0 and/or the resume
> only wakes CPU0 so you have to bootstrap the SMP thing again..
> 
> Some suspend person wanna clarify? Rafael?

If I understand correctly, ACPI requires that only CPU0 be running when
the system is suspended (i.e., your guess is right).  Of course, this
doesn't apply to non-ACPI systems, but it's easiest to do the same
thing everywhere.

> Anyway, the whole suspend case is magic anyway since all tasks will have
> been frozen, so we could simply leave all of cpuset alone and ignore the
> hotplug notifier on CPU_TASKS_FROZEN callbacks, hmm?

I don't see why not.  Presumably no CPUs will be added or removed while 
the system is asleep.

> Do we unfreeze after we bring up the machine again?

Yes.

Alan Stern