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

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Message-ID: <4F394CC9.6010103@linux.vnet.ibm.com>
Date:	Mon, 13 Feb 2012 23:17:53 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, 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, 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 02/10/2012 10:23 PM, Paul E. McKenney wrote:

> On Fri, Feb 10, 2012 at 04:52:07PM +0100, Peter Zijlstra 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.
> 
> OK, I will ask the stupid question...  Hey, somebody has to!  ;-)
> 
> Would it make sense for offlining the last CPU in a cpuset to be
> destructive, but to allow offlining of a non-last CPU to be reversible?
> 
> For example, assume that cpuset A has CPUs 0 and 1, and cpuset B has
> 1, 2, and 3.  Then offlining any single CPU and then onlining it would
> restore the cpusets to their original state.  Offlining both CPUs 0 and 1
> would be destructive to cpuset A, so that onlining those two CPUs would
> leave any tasks in cpuset A in some ancestor of cpuset A, and would
> leave cpuset A with no assigned CPUs.  However, that same operation
> (offlining both CPUs 0 and 1, then onlining them) would restore cpuset
> B to its original state, covering CPUs 1, 2, and 3.
> 


But how would this scheme help us? During suspend, all non-boot CPUs are
taken offline. Which means, it would be destructive to any cpuset that
didn't originally contain CPU0 (even when using the above scheme). So, upon
resume, it is still not the same as how it was before suspend.

Regards,
Srivatsa S. Bhat

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