linux-kernel - Re: [patch 2/2] sched: fix select_idle_sibling() logic in select_task_rq

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Date:	Sat, 06 Mar 2010 09:21:55 +0100
From:	Mike Galbraith <efault@....de>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...ux.jf.intel.com>,
	linux-kernel@...r.kernel.org,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Yanmin Zhang <yanmin_zhang@...ux.jf.intel.com>,
	Gautham R Shenoy <ego@...ibm.com>
Subject: Re: [patch 2/2] sched: fix select_idle_sibling() logic in
 select_task_rq_fair()

On Fri, 2010-03-05 at 21:25 +0100, Mike Galbraith wrote: 
> On Fri, 2010-03-05 at 10:39 -0800, Suresh Siddha wrote:
>  
> > c) Also, selelct_idle_sibling() should also treat the current cpu as an idle
> >    cpu if it is a sync wakeup and we have only one task running.
> 
> I'm going to have to crawl over and test the above, but this bit sounds
> like a decidedly un-good thing to do.  Maybe I'm misunderstanding.

Nope, no misunderstanding.  This patch does kill throughput gains.  Once
awakened affine, always awaken affine is a bad idea.

I dug up my old P4 though.  With it's wimpy siblings, the cost of
running two schedulers doesn't appear to be generally worth it at a
glance.  You need considerable overlap to break even.

	-Mike

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