linux-kernel - Re: [patch v4 02/18] sched: select_task_rq

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Date:	Wed, 13 Feb 2013 22:44:12 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	torvalds@...ux-foundation.org, mingo@...hat.com,
	tglx@...utronix.de, akpm@...ux-foundation.org,
	arjan@...ux.intel.com, bp@...en8.de, pjt@...gle.com,
	namhyung@...nel.org, efault@....de, vincent.guittot@...aro.org,
	gregkh@...uxfoundation.org, preeti@...ux.vnet.ibm.com,
	viresh.kumar@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [patch v4 02/18] sched: select_task_rq_fair clean up

On 02/12/2013 06:14 PM, Peter Zijlstra wrote:
> On Thu, 2013-01-24 at 11:06 +0800, Alex Shi wrote:
>> It is impossible to miss a task allowed cpu in a eligible group.
> 
> I suppose your reasoning goes like: tsk->cpus_allowed is protected by
> ->pi_lock, we hold this, therefore it cannot change and
> find_idlest_group() dtrt?

yes.
> 
> We can then state that this is due to adding proper serialization to
> tsk->cpus_allowed.
> 
>> And since find_idlest_group only return a different group which
>> excludes old cpu, it's also impossible to find a new cpu same as old
>> cpu.
> 
> Sounds plausible, but I'm not convinced, do we have hard serialization
> against hotplug?

Any caller of select_task_rq will check if returned dst_cpu is still
working. So there is nothing need worry about.
> 
> 
> 


-- 
Thanks
    Alex
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