linux-kernel - Re: [PATCH 4/7 v3] sched: propagate load during synchronous attach/detach

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Message-ID: <CAKfTPtBnDaEzv1foHZuanPUO6S_hNRytEtrW3hkK=Ko3F1R4aQ@mail.gmail.com>
Date:   Thu, 15 Sep 2016 16:51:55 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Yuyang Du <yuyang.du@...el.com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Paul Turner <pjt@...gle.com>,
        Benjamin Segall <bsegall@...gle.com>
Subject: Re: [PATCH 4/7 v3] sched: propagate load during synchronous attach/detach

On 15 September 2016 at 16:43, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Sep 12, 2016 at 09:47:49AM +0200, Vincent Guittot wrote:
>> +static inline void
>> +update_tg_cfs_load(struct cfs_rq *cfs_rq, struct sched_entity *se)
>> +{
>> +     struct cfs_rq *gcfs_rq = group_cfs_rq(se);
>> +     long delta, load = gcfs_rq->avg.load_avg;
>> +
>> +     /* If the load of group cfs_rq is null, the load of the
>> +      * sched_entity will also be null so we can skip the formula
>> +      */
>> +     if (load) {
>> +             long tg_load;
>> +
>> +             /* Get tg's load and ensure tg_load > 0 */
>> +             tg_load = atomic_long_read(&gcfs_rq->tg->load_avg) + 1;
>> +
>> +             /* Ensure tg_load >= load and updated with current load*/
>> +             tg_load -= gcfs_rq->tg_load_avg_contrib;
>> +             tg_load += load;
>> +
>> +             /* scale gcfs_rq's load into tg's shares*/
>> +             load *= scale_load_down(gcfs_rq->tg->shares);
>> +             load /= tg_load;
>> +
>> +             /*
>> +              * we need to compute a correction term in the case that the
>> +              * task group is consuming <1 cpu so that we would contribute
>> +              * the same load as a task of equal weight.
>> +             */
>> +             if (tg_load < scale_load_down(gcfs_rq->tg->shares)) {
>> +                     load *= tg_load;
>> +                     load /= scale_load_down(gcfs_rq->tg->shares);
>> +             }
>
> Note that you're reversing the exact scaling you just applied.

Yes, Indeed

>
> That is:
>                shares    tg_load
>         load * ------- * ------- == load
>                tg_load   shares
>
>> +     }
>
> So something like:
>
>         shares = scale_load_down(gcfs_rq->tg->shares);
>
>         if (tg_load >= shares) {
>                 load *= shares;
>                 load /= tg_load;
>         }
>
> should be the same as the above and saves a bunch of math, no?

Yes