linux-kernel - Re: [PATCH 01/15] sched/fair: Add avg

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Date:   Fri, 2 Jun 2023 16:27:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, corbet@....net, qyousef@...alina.io,
        chris.hyser@...cle.com, patrick.bellasi@...bug.net, pjt@...gle.com,
        pavel@....cz, qperret@...gle.com, tim.c.chen@...ux.intel.com,
        joshdon@...gle.com, timj@....org, kprateek.nayak@....com,
        yu.c.chen@...el.com, youssefesmat@...omium.org,
        joel@...lfernandes.org, efault@....de, tglx@...utronix.de
Subject: Re: [PATCH 01/15] sched/fair: Add avg_vruntime

On Fri, Jun 02, 2023 at 03:51:53PM +0200, Vincent Guittot wrote:
> On Wed, 31 May 2023 at 14:47, Peter Zijlstra <peterz@...radead.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.