Date: Tue, 15 May 2012 15:32:04 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Nishanth Aravamudan <nacc@linux.vnet.ibm.com>, mingo@kernel.org,
        pjt@google.com, paul@paulmenage.org, akpm@linux-foundation.org,
        rjw@sisk.pl, nacc@us.ibm.com, paulmck@linux.vnet.ibm.com,
        tglx@linutronix.de, seto.hidetoshi@jp.fujitsu.com, tj@kernel.org,
        mschmidt@redhat.com, berrange@redhat.com, nikunj@linux.vnet.ibm.com,
        vatsa@linux.vnet.ibm.com, liuj97@gmail.com,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH v3 5/5] cpusets, suspend: Save and restore cpusets during
 suspend/resume
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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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