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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Mon, 5 Jun 2023 09:18:05 +0200
Message-ID: <CAKfTPtCZ2Thf0=jfhjGe9VHdqBX6=tU7OOzHwv2bDKUVZq+faw@mail.gmail.com>
Subject: Re: [PATCH 01/15] sched/fair: Add avg_vruntime
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     mingo@kernel.org, linux-kernel@vger.kernel.org,
        juri.lelli@redhat.com, dietmar.eggemann@arm.com,
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On Fri, 2 Jun 2023 at 16:27, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Jun 02, 2023 at 03:51:53PM +0200, Vincent Guittot wrote:
> > On Wed, 31 May 2023 at 14:47, Peter Zijlstra <peterz@infradead.org> wrote:
> > > +static void
> > > +avg_vruntime_add(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > > +{
> > > +       unsigned long weight = scale_load_down(se->load.weight);
> > > +       s64 key = entity_key(cfs_rq, se);
> > > +
> > > +       cfs_rq->avg_vruntime += key * weight;
> > > +       cfs_rq->avg_load += weight;
> >
> > isn't cfs_rq->avg_load similar to scale_load_down(cfs_rq->load.weight)  ?
> >
> > > +}
>
> Similar, yes, but not quite the same in two ways:
>
>  - it's sometimes off by one entry due to ordering of operations -- this
>    is probably fixable.
>
>  - it does the scale down after addition, whereas this does the scale
>    down before addition, esp for multiple low weight entries this makes
>    a significant difference.

Ah yes, we are still using the the scaled down value for computation

>