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

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Date:	Thu, 10 Jan 2013 14:52:02 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Alex Shi <alex.shi@...el.com>
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 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?

Thanks,
Namhyung
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