linux-kernel - Re: [PATCH v5 4/5] cpusets: Update tasks' cpus_allowed mask upon updates to root cpuset

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Message-ID: <4FBE288F.8060802@linux.vnet.ibm.com>
Date:	Thu, 24 May 2012 17:54:47 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	mingo@...nel.org, pjt@...gle.com, paul@...lmenage.org,
	akpm@...ux-foundation.org, rjw@...k.pl, nacc@...ibm.com,
	rientjes@...gle.com, paulmck@...ux.vnet.ibm.com,
	tglx@...utronix.de, seto.hidetoshi@...fujitsu.com, tj@...nel.org,
	mschmidt@...hat.com, berrange@...hat.com,
	nikunj@...ux.vnet.ibm.com, vatsa@...ux.vnet.ibm.com,
	liuj97@...il.com, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH v5 4/5] cpusets: Update tasks' cpus_allowed mask upon
 updates to root cpuset

On 05/24/2012 05:28 PM, Peter Zijlstra wrote:

> On Thu, 2012-05-24 at 15:14 +0530, Srivatsa S. Bhat wrote:
> 
>> Sorry, my bad, I hadn't considered that. Thanks for pointing it out!
>>
>> So, I am wondering how we ought to deal with CPU hotplug for tasks attached
>> to the root cpuset..
> 
> By not doing anything at all. If a task is in the root set it is
> supposed to behave as it cpusets don't exist.
> 
>> Considering tasks attached to the root cpuset, if a cpu present in a task's
>> cpus_allowed mask goes offline, it should be removed from that mask right?
> 
> Nope, that shouldn't happen. We only reset the mask if all cpus in the
> affinity mask go away. This is where task affinity and cpusets differ.
> 
>> And if that cpu comes back online, it should not be put back to the task's
>> cpus_allowed mask (just like we don't put back cpus in non-root cpusets).
> 
> Online shouldn't ever change anything.
> 
>> Is the above understanding correct?
> 
> Nope.
> 
>> In the current kernel, during cpu hotplug, we don't touch cpus_allowed mask
>> of the tasks attached to the root cpuset at all.. Whereas we update the
>> cpus_allowed mask of tasks belonging to non-root cpusets, during cpu offline.
>>
>> So, is this differentiation intended?
> 
> Yes, although arguably the cpuset case is 'weird' in that it came later
> and didn't mirror the cpu affinity semantics.
> 
> Tasks aren't attached to the root cpuset, they live there because
> there's no other place to be when you don't use cpusets. This very much
> means that tasks in the root set should behave as if cpusets didn't
> exist.
> 


Ok, got it.. that logic makes sense. Thanks a lot for the explanation!
I will resubmit this patchset without patch 4/5 then, if you don't have any
other objections.

 

Regards,
Srivatsa S. Bhat

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