linux-kernel - Re: [PATCH] sched: select 'idle' cfs_rq per task-group to prevent tg-internal imbalance

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Date:	Thu, 03 Jul 2014 05:51:02 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Michael wang <wangyun@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Alex Shi <alex.shi@...aro.org>,
	Paul Turner <pjt@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: select 'idle' cfs_rq per task-group to prevent
 tg-internal imbalance

On Wed, 2014-07-02 at 10:47 -0400, Rik van Riel wrote: 
> On 07/01/2014 04:38 AM, Michael wang wrote:
> > On 07/01/2014 04:20 PM, Peter Zijlstra wrote:
> > [snip]
> >>>
> >>> Just wondering could we make this another scheduler feature?
> >>
> >> No; sched_feat() is for debugging, BIG CLUE: its guarded by
> >> CONFIG_SCHED_DEBUG, anybody using it in production or anywhere else is
> >> broken.
> >>
> >> If people are using it, I should remove or at least randomize the
> >> interface.
> > 
> > Fair enough... but is there any suggestions on how to handle this issue?
> > 
> > Currently when dbench running with stress, it could only gain one CPU,
> > and cpu-cgroup cpu.shares is meaningless, is there any good methods to
> > address that?
> 
> select_idle_sibling will iterate over all of the CPUs
> in an LLC domain if there is no idle cpu in the domain.
> 
> I suspect it would not take much extra code to track
> down the idlest CPU in the LLC domain, and make sure to
> schedule tasks there, in case no completely idle CPU
> was found.
> 
> Are there any major problems with that thinking?

That's full wake balance.. if that was cheap, select_idle_sibling()
would not exist.

-Mike

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