linux-kernel - Re: [PATCH 1/2] sched/fair: Fix how load gets propagated from cfs_rq to its sched

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Message-ID: <20170426222744.GA9616@wtj.duckdns.org>
Date:   Wed, 26 Apr 2017 15:27:44 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 1/2] sched/fair: Fix how load gets propagated from cfs_rq
 to its sched_entity

Hello, Vincent.

On Wed, Apr 26, 2017 at 06:14:17PM +0200, Vincent Guittot wrote:
> > +       if (gcfs_rq->load.weight) {
> > +               long shares = calc_cfs_shares(gcfs_rq, gcfs_rq->tg);
> >
> > +               load = min(gcfs_rq->runnable_load_avg *
> > +                          shares / gcfs_rq->load.weight, shares);
> 
> There is a unit problem above:
> runnable_load_avg and shares are not in the same range but
> runnable_load_avg and  scale_load_down(gcfs_rq->load.weight) are so
> you should use
> gcfs_rq->runnable_load_avg * scale_load_down(shares) /
> scale_load_down(gcfs_rq->load.weight).

But the only difference there is that we lose accuracy in calculation;
otherwise, the end results are the same, no?

> Hopefully both  scale_load_down cancel between them
> But the min should be then tested with scale_load_down(shares) and not
> only shares

Ah, that's right.  The min should be against scaled down shares.

Thanks.

-- 
tejun