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

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Date:	Fri, 11 Jan 2013 10:36:15 +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 01/10/2013 01:52 PM, Namhyung Kim wrote:
> On Wed, 09 Jan 2013 16:34:39 +0800, Alex Shi wrote:
>> 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.
> 
> Sorry, I can't get your point.  Could you elaborate on it a bit more?

Sorry. I misunderstand this.

Acked-by: Alex Shi

> 
> Thanks,
> Namhyung
> 


-- 
Thanks Alex
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