linux-kernel - Re: [PATCH 2/2] sched/fair: Always propagate runnable_load

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Message-ID: <20170502220154.GD5335@htj.duckdns.org>
Date:   Tue, 2 May 2017 18:01:54 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>, Paul Turner <pjt@...gle.com>,
        Chris Mason <clm@...com>, kernel-team@...com
Subject: Re: [PATCH 2/2] sched/fair: Always propagate runnable_load_avg

Hello,

On Mon, May 01, 2017 at 05:56:13PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 28, 2017 at 04:33:47PM -0400, Tejun Heo wrote:
> > I'm attaching the debug patch.  With your change (avg instead of
> > runnable_avg), the following trace shows why it's wrong.
> 
> Ah, OK. So you really want runnable_avg (and I understand why), which is
> rather unfortunate, since we have everything on load_avg.
> 
> So for shares, load_avg gives a more stable number. This is important
> since tg->load_avg is a global number, so computing it is expensive (and
> slow). Therefore more stable numbers are good.
> 
> > The thing with cfs_rq se's load_avg is that, it isn't really used
> > anywhere else AFAICS, so overriding it to the cfs_rq's
> > runnable_load_avg isn't prettiest but doesn't really change anything.
> 
> You mean, consistently expressing a group's se->load.weight in terms of
> runnable_load_avg? See the above.

I think you got this on the other thread but for clarity:

cfs_rq->avg.load_avg is used for share calculation and we want to keep
it that way as the calcluation is expensive and rather decoupled
across CPUs (the deviation can be quite a bit without the stability).
But the group *se*->avg.load_avg is a separate thing which isn't
really used anywhere except for as a propagation channel from group
cfs_rq to its parent cfs_rq.

Thanks.

-- 
tejun