linux-kernel - Re: [RFC] sched: Limit idle_balance() when it is being used too frequently

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Message-ID: <20130718123531.GO27075@twins.programming.kicks-ass.net>
Date:	Thu, 18 Jul 2013 14:35:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Rik van Riel <riel@...hat.com>, Jason Low <jason.low2@...com>,
	Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Mel Gorman <mgorman@...e.de>, aswin@...com,
	scott.norton@...com, chegu_vinod@...com
Subject: Re: [RFC] sched: Limit idle_balance() when it is being used too
 frequently

On Thu, Jul 18, 2013 at 05:45:46PM +0530, Srikar Dronamraju wrote:
> We take locks if and only if we see imbalance and want to pull the
> tasks.
> However if the newly idle balance is not finding an imbalance then this
> may not be an issue.
> 
> Probably /proc/schedstats will give a better picture.

Right, so we're interested in move_tasks() calls that fail to 'deliver'.
There's a few conditions in there that can cause us to not move a task,
most of them not counted.

The few that are; are from can_mirgrate_task():

  se.statistics.nr_failed_migrations_affine
  se.statistics.nr_failed_migrations_running
  se.statistics.nr_failed_migrations_hot

If we see significant increments on those we'll be taking locks. 

The only one I can see a good way around is the hot one, we could ignore
hotness in favour of newidle -- although I could see that being
detrimental, we'll just have to try or so ;-)

_running shouldn't be much of a problem since we don't bother if
nr_running <= 1. And _affine is out of our reach anyway.

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