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

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Date:	Thu, 10 Jan 2013 14:49:01 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.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>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Alex Shi <alex.shi@...el.com>
Subject: Re: [PATCH] sched: Get rid of unnecessary checks from select_idle_sibling

Hi Preeti,

On Wed, 09 Jan 2013 13:51:00 +0530, Preeti U. Murthy wrote:
> On 01/09/2013 12:20 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]
> If NUMA_TTWU_BIAS or NUMA_TTWU_TO is true(it is false by

I can't find those bits in the code.  I've checked v3.8-rc2,
next-20130110, tip/master and tip/numa/core but there's nothing like
above.  Which tree are you saying?


> default),cpu/prev_cpu can be changed to be a random node_cpu(the node
> that 'this_cpu' is on). In which case even if the node cpu is idle,it
> would not be a viable target,looks like.Maybe that is why
> select_idle_sibling() makes the check if the target is prev_cpu/this cpu.

Looking into tip/numa/core, I can see that there's a code added for
CONFIG_NUMA_BALANCING.  But still, it seems nothing changed on a path
from select_task_rq_fair() to select_idle_sibling() - i.e. if the
select_idle_sibling called, the target would be either prev_cpu or this
cpu.  Am I missing something?

Thanks,
Namhyung
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