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 15:32:04 -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:

> >  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.
> > 
> 
> I think there is a valid reason behind doing that.
> 
> Cpusets translates to sched domains in scheduler terms. So whenever you update
> cpusets, the sched domains are updated. IOW, if you don't touch cpusets during
> hotplug (suspend/resume case), you allow them to have offline cpus, meaning,
> you allow sched domains to have offline cpus. Hence sched domains are rendered
> stale.
> 

It's not possible to update the sched domains for s/r to be a subset of 
cpuset.cpus?  It would be the same situation for a thread using 
sched_setaffinity() while bound to a cpuset with a superset of allowed 
nodes.  If you do that, there's no reason to alter cpuset.cpus at all and 
you don't need to carry another cpumask around for each cpuset.
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