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

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Message-ID: <876236rdum.fsf@sejong.aot.lge.com>
Date:	Wed, 09 Jan 2013 16:54:09 +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

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.

Thanks,
Namhyung
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