linux-kernel - Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq

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Message-ID: <20130222095416.GA4450@gmail.com>
Date:	Fri, 22 Feb 2013 10:54:16 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Mike Galbraith <efault@....de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>, alex.shi@...el.com,
	Ram Pai <linuxram@...ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair()


* Mike Galbraith <efault@....de> wrote:

> On Fri, 2013-02-22 at 09:36 +0100, Peter Zijlstra wrote: 
> > On Fri, 2013-02-22 at 10:37 +0800, Michael Wang wrote:
> > > But that's really some benefit hardly to be estimate, especially when
> > > the workload is heavy, the cost of wake_affine() is very high to
> > > calculated se one by one, is that worth for some benefit we could not
> > > promise?
> > 
> > Look at something like pipe-test.. wake_affine() used to 
> > ensure both client/server ran on the same cpu, but then I 
> > think we added select_idle_sibling() and wrecked it again :/
> 
> Yeah, that's the absolute worst case for 
> select_idle_sibling(), 100% synchronous, absolutely nothing to 
> be gained by cross cpu scheduling. Fortunately, most tasks do 
> more than that, but nonetheless, select_idle_sibling() 
> definitely is a two faced little b*tch.  I'd like to see the 
> evil b*tch die, but something needs to replace it's pretty 
> face.  One thing that you can do is simply don't call it when 
> the context switch rate is incredible.. its job is to recover 
> overlap, if you're scheduling near your max, there's no win 
> worth the cost.

Couldn't we make the cutoff dependent on sched_migration_cost? 
If the wakeup comes in faster than that then don't spread.

Thanks,

	Ingo
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