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

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Date:   Thu, 27 Apr 2017 10:59:12 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Tejun Heo <tj@...nel.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

On 27 April 2017 at 00:27, Tejun Heo <tj@...nel.org> wrote:
> 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?

Yes the end result is the same, it was mainly to point out the range
difference and explain why we need scale_load_down(shares) for the 2nd
argument of min.
This should also explain the warning issue you mentioned earlier

>
>> 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