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

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Date:	Fri, 10 Feb 2012 16:52:07 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
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

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.



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