linux-kernel - Re: [PATCH v3 5/5] cpusets, suspend: Save and restore cpusets during suspend/resume

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Date:	Tue, 15 May 2012 14:49:47 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>,
	mingo@...nel.org, pjt@...gle.com, paul@...lmenage.org,
	akpm@...ux-foundation.org, rjw@...k.pl, nacc@...ibm.com,
	paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
	seto.hidetoshi@...fujitsu.com, tj@...nel.org, mschmidt@...hat.com,
	berrange@...hat.com, nikunj@...ux.vnet.ibm.com,
	vatsa@...ux.vnet.ibm.com, liuj97@...il.com,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 5/5] cpusets, suspend: Save and restore cpusets during
 suspend/resume

On Wed, 16 May 2012, Srivatsa S. Bhat wrote:

> What you are suggesting was precisely the v1 of this patchset, which went
> upstream as commit 8f2f748b06562 (CPU hotplug, cpusets, suspend: Don't touch
> cpusets during suspend/resume).
> 
> It got reverted due to a nasty suspend hang in some corner case, where the
> sched domains not being up-to-date got the scheduler confused.
> Here is the thread with that discussion:
> http://thread.gmane.org/gmane.linux.kernel/1262802/focus=1286289
> 
> As Peter suggested, I'll try to fix the issues at the 2 places that I found
> where the scheduler gets confused despite the cpu_active mask being up-to-date.
> 
> But, I really want to avoid that scheduler fix and this cpuset fix from
> being tied together, for the fear that until we root-cause and fix all
> scheduler bugs related to cpu_active mask, we can never get cpusets fixed
> once and for all for suspend/resume. So, this patchset does an explicit
> save and restore to be sure, and so that we don't depend on some other/unknown
> factors to make this work reliably.
> 

Ok, so it seems like this is papering over an existing cpusets issue or an 
interaction with the scheduler that is buggy.  There's no reason why a 
cpuset.cpus that is a superset of cpu_active_mask should cause an issue 
since that's exactly what the root cpuset has.  I know root is special 
cased all over the cpuset code, but I think the real fix here is to figure 
out why it can't be left as a superset and then we end up doing nothing 
for s/r.

I don't have a preference for cpu hotplug and whether cpuset.cpus = 1-3 
remains 1-3 when cpu 2 is offlined or not, I think it could be argued both 
ways, but I disagree with saving the cpumask, removing all suspended cpus, 
and then reinstating it for no reason.
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