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Date: Thu, 9 Feb 2012 16:11:58 +0100
From: Ingo Molnar <mingo@elte.hu>
To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, paul@paulmenage.org, rjw@sisk.pl,
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Subject: Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling
 related to cpusets
Message-ID: <20120209151158.GA22489@elte.hu>
References: <20120207185411.7482.43576.stgit@srivatsabhat.in.ibm.com>
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* Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> wrote:

> Hi Ingo,
> 
> On 02/09/2012 01:27 PM, Ingo Molnar wrote:
> 
> > 
> > * Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> wrote:
> > 
> >> IOW, consider:
> >>
> >> cpuset A has 0-10
> >>
> >> - Take CPU 10 offline
> >>   [We are forced to remove CPU 10 from cpuset A, which becomes 0-9 now]
> >>
> >>
> >>    <Userspace didn't request any change to cpuset A>
> >>
> >>
> >> - Bring back CPU 10 online
> >>
> >> Now cpuset A is still 0-9! IMO, it should have been 0-10.
> > 
> > Why is CPU 10 taken out of the cpuset to begin with?
> > 
> > The cpuset code should be fixed to work with offline CPUs as 
> > well - it can obviously not schedule to them, but otherwise 
> > it should be fine to have a wider cpuset than the hw can 
> > support.
> 
> My understanding of the code is that when a CPU is taken 
> offline, it is removed from all the cpusets and then the 
> scan_for_empty_cpusets() function is run to move tasks from 
> empty cpusets to their parent cpusets.

Why is that done that way? offlining a CPU should be an 
invariant as far as cpusets are concerned.

Not touching the cpuset would avoid the hot-replug complications 
as well.

Thanks,

	Ingo