linux-kernel - Re: [RFC PATCH] sched: Reduce overestimating avg

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Date:	Fri, 02 Aug 2013 01:20:05 -0700
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>, KML <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,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH] sched: Reduce overestimating avg_idle

On Wed, 2013-07-31 at 11:53 +0200, Peter Zijlstra wrote:

> No they're quite unrelated. I think you can measure the max time we've
> ever spend in newidle balance and use that to clip the values.

So I tried using the rq's max newidle balance cost to compare with the
average and used sysctl_migration_cost as the initial/default max. One
thing I noticed when running this on 8 socket machine was that the max
idle balance cost was a lot higher during around boot time compared to
after boot time. Not sure if IRQ/NMI/SMI was the cause of this. A
temporary "fix" I made was to reset the max idle balance costs every 2
minutes.

> Similarly, I've thought about how we updated the sd->avg_cost in the
> previous patches and wondered if we should not track max_cost.
> 
> The 'only' down-side I could come up with is that its all ran from
> SoftIRQ context which means IRQ/NMI/SMI can all stretch/warp the time it
> takes to actually do the idle balance.

Another thing that I thought of was that max idle balance cost may also
vary based on the workload that is running. So running a workload in
which there are shorter idle balances after running a workload that has
longer idle balances may sometimes cause it to make use of a higher idle
balance cost. But I guess it is okay if we're trying to reduce
overrunning the average.

Jason

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