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

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Date:	Sat, 11 Feb 2012 14:39:29 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	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


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Thu, 2012-02-09 at 16:11 +0100, Ingo Molnar wrote:
> 
> > > 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.
> 
> Can't, tasks need to run someplace. There's two choices, add a 
> still online cpu to the now empty cpuset or move the tasks to 
> a parent that still has online cpus.
> 
> Both are destructive.

You aren't thinking hard enough ;-) There's several solutions 
off the top of my mind:

1) refuse the "impossible" offlining of the CPU, with a clear 
   enough error to make it actionable

2) offer a 'forced' offlinign of a CPU that will SIGTERM all 
   tasks that are on the now offline CPU and can only be there.

3) offer a 'nice' offlining variant that moves all orphan tasks
   to their or any other well-defined fallback CPU.

4) *allow* 'impossible' cpusets and just run them on CPU#0 or 
   any other natural approximation. Don't touch the cpuset!

All of these would be exception mechanisms with no need to do 
anything at hot-replug time.

Thanks,

	Ingo
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