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

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Date:	Thu, 9 Feb 2012 16:11:58 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, 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, paulmck@...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


* Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com> wrote:

> Hi Ingo,
> 
> On 02/09/2012 01:27 PM, Ingo Molnar wrote:
> 
> > 
> > * Srivatsa S. Bhat <srivatsa.bhat@...ux.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
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