linux-kernel - Re: [PATCH] sched: Get rid of unnecessary checks from select_idle

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Date:	Wed, 09 Jan 2013 16:34:39 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Namhyung Kim <namhyung@...nel.org>
CC:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	Namhyung Kim <namhyung.kim@....com>,
	Mike Galbraith <efault@....de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] sched: Get rid of unnecessary checks from select_idle_sibling

On 01/09/2013 03:54 PM, Namhyung Kim wrote:
> Hi Alex,
> 
> On Wed, 09 Jan 2013 15:33:40 +0800, Alex Shi wrote:
>> On 01/09/2013 02:50 PM, Namhyung Kim wrote:
>>> From: Namhyung Kim <namhyung.kim@....com>
>>>
>>> AFAICS @target cpu of select_idle_sibling() is always either prev_cpu
>>> or this_cpu.  So no need to check it again and the conditionals can be
>>> consolidated.
> [snip]
>> Uh, we don't know if the target is this_cpu or previous cpu, If we just
>> check the target idle status, we may miss another idle cpu. So this
>> patch change the logical in this function.
> 
> select_idle_sibling() is called only in select_task_rq_fair() if it
> found a suitable affine_sd.  The default target is the 'prev_cpu' of the
> task but if wake_affine() returns true it'd be (this) 'cpu'.
> 
> I cannot see where the prev_cpu or the cpu is set to another one before
> calling select_idle_sibling.

The old logical will return directly whenever prev_cpu or this cpu idle,
but your new logical just has one chance.


> 
> Thanks,
> Namhyung
> 


-- 
Thanks Alex
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