linux-kernel - Re: [RFC PATCH v2] sched: Limit idle

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Message-ID: <20130723120501.GY27075@twins.programming.kicks-ass.net>
Date:	Tue, 23 Jul 2013 14:05:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	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>, Rik van Riel <riel@...hat.com>,
	aswin@...com, scott.norton@...com, chegu_vinod@...com
Subject: Re: [RFC PATCH v2] sched: Limit idle_balance()

On Tue, Jul 23, 2013 at 04:36:46PM +0530, Srikar Dronamraju wrote:

> May be the current max value is a limiting factor, but I think there
> should be a limit to the maximum value. Peter and Ingo may help us
> understand why they limited to the 1ms. But I dont think we should
> introduce a new variable just for this.

/me blames it all on Mike.. I tried to remember why he did that, but
alas.

> If idle balance did succeed, then it means that the system was indeed
> imbalanced. So idle balance was the right thing to do. May be we chose
> the wrong task to pull. May be after numa balancing enhancements go in,
> we pick a better task to pull atleast across nodes. And there could be
> other opportunities/strategies to select a right task to pull.
> 
> Again, schedstats during the application run should give us hints here.

Not necessarily so, IIRC the newidle idx is 0 which means that its very
aggressive at pulling load, there might not actually be an imbalance
with higher idx averages.

> I was saying it the other way. 
> your suggestion is to run idle balance once in n runs .. where n is 10
> to 20. 
> My thinking was to not run idle balance once in n unsuccessful runs. 

I think you're talking past each other. Each having a different N :-)

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