linux-kernel - Re: [PATCH 2/2] sched/fair: Simple runqueue order on migrate

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Date:   Thu, 31 Mar 2022 13:47:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>, Ingo Molnar <mingo@...e.hu>,
        Juri Lelli <juri.lelli@...hat.com>,
        Yu Chen <yu.c.chen@...el.com>,
        Walter Mack <walter.mack@...el.com>,
        Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] sched/fair: Simple runqueue order on migrate


It's been 3 months since I wrote these patches, so memory is vague at
best :/

On Tue, Mar 29, 2022 at 11:03:44AM +0200, Vincent Guittot wrote:

> > +static void place_entity_migrate(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > +{
> > +       if (!sched_feat(PLACE_MIGRATE))
> > +               return;
> > +
> > +       if (cfs_rq->nr_running < se->migrated) {
> > +               /*
> > +                * Migrated to a shorter runqueue, go first because
> > +                * we were under-served on the old runqueue.
> > +                */
> > +               se->vruntime = cfs_rq->min_vruntime;
> > +               return;
> > +       }
> > +
> > +       /*
> > +        * Migrated to a longer runqueue, go last because
> > +        * we got over-served on the old runqueue.
> > +        */
> > +       se->vruntime = cfs_rq->min_vruntime + sched_vslice(cfs_rq, se);
> > +}
> > +
> >  static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
> >
> >  static inline bool cfs_bandwidth_used(void);
> > @@ -4296,6 +4317,8 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> >
> >         if (flags & ENQUEUE_WAKEUP)
> >                 place_entity(cfs_rq, se, 0);
> > +       else if (se->migrated)
> > +               place_entity_migrate(cfs_rq, se);
> >
> >         check_schedstat_required();
> >         update_stats_enqueue_fair(cfs_rq, se, flags);

> > @@ -6973,14 +6997,15 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> >                  * wakee task is less decayed, but giving the wakee more load
> >                  * sounds not bad.
> >                  */
> > -               remove_entity_load_avg(&p->se);
> > +               remove_entity_load_avg(se);
> >         }
> >
> >         /* Tell new CPU we are migrated */
> > -       p->se.avg.last_update_time = 0;
> > +       se->avg.last_update_time = 0;
> >
> >         /* We have migrated, no longer consider this task hot */
> > -       p->se.migrated = 1;
> > +       for_each_sched_entity(se)
> > +               se->migrated = READ_ONCE(cfs_rq_of(se)->nr_running) + !se->on_rq;
> 
> Why do we need to loop on se ? Isn't p->se enough ?

Yeah; I really don't recall why I did that. And looking at it now, it
doesn't really make much sense. I suppose it will trigger
place_entity_migrate() for the group entries, but on the old CPU.