linux-kernel - Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets

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Message-ID: <Pine.LNX.4.44L0.1202101647440.1179-100000@iolanthe.rowland.org>
Date:	Fri, 10 Feb 2012 16:51:17 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...e.hu>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	<paul@...lmenage.org>, <rjw@...k.pl>, <tj@...nel.org>,
	<frank.rowand@...sony.com>, <pjt@...gle.com>, <tglx@...utronix.de>,
	<lizf@...fujitsu.com>, <prashanth@...ux.vnet.ibm.com>,
	<vatsa@...ux.vnet.ibm.com>, <linux-kernel@...r.kernel.org>,
	<linux-pm@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related
 to cpusets

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

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