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

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Date:	Tue, 14 Feb 2012 02:19:09 +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 02/14/2012 02:09 AM, Paul E. McKenney wrote:

> On Mon, Feb 13, 2012 at 11:17:53PM +0530, Srivatsa S. Bhat wrote:
>> 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.
> 
> Yep, it would only help for incremental cases.  Or if all cpusets had
> CPU 0 in them.  So preserving cpusets across suspend will require a
> bigger hammer.
> 


Hehe ;-)

Regards,
Srivatsa S. Bhat

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