Re: [PATCH 02/18] sched/eevdf: Reset lag when waking up on idle cpu

[Vincent Guittot]

On Sun, 4 Oct 2026 at 19:30, Kayra Cizmeci <kayracizmeci@xxxxxxxxx> wrote:
>
> > When several tasks wake up simultaneously on an idle CPU, their final vlag
> > will depend of the ordering as the first one will lose its lag but not
> > the next ones.
> > Reset the lag when the enqueue happens while no fair task has already been
> > picked et set as the running task.
>
> There is a typo. ('et', also 'depend' and 'loose' Not sure that's all :>)

will fix the typo

>
> > As a typical example:
> > CPU0 is idle
> > TA with vlag 0ms and TB with vlag 5ms wake up on CPU0 simultaneously.
> > Depending which grab the lock 1st the behavior will be different:
> > If TA is enqueued 1st, TB will be enqueued with a positive lag and will
> > be picked 1st.
> > But if TB is enqueued 1st, it will loose its positive vlag and both TA and
> > TB will have 0 vlag when fair will pick a task.
>
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index d7cc77181ef9..32d7077ef148 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -592,6 +592,7 @@ struct sched_entity {
> >       u64                             vruntime;
> >       /* Approximated virtual lag: */
> >       s64                             vlag;
> > +     u32                             vlag_seq;
> >       /* 'Protected' deadline, to give out minimum quantums: */
> >       u64                             vprot;
> >       u64                             slice;
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 8cda1d39b037..4c8f12fc8869 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -893,6 +893,7 @@ bool update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >                       vlag = min(vlag, 0);
> >       }
> >       se->vlag = vlag;
> > +     se->vlag_seq = cfs_rq->idle_seq;
> >
> >       return avruntime - vlag != se->vruntime;
> >  }
> > @@ -914,9 +915,22 @@ void decay_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> >
> >       rq = rq_of(cfs_rq);
> >
> > +     /* You can't claim any lag when waking on idle CPU */
> > +     if (rq->curr == rq->idle) {
> > +             se->vlag = 0;
> > +             return;
> > +     }
> > +
> > +     /* Accessing remote rq task clock is a cost */
> >       if (flags & ENQUEUE_MIGRATED)
> >               return;
> >
> > +     /* CPU has been idle in between so the lag has been removed */
> > +     if (se->vlag_seq != cfs_rq->idle_seq) {
> > +             se->vlag = 0;
> > +             return;
> > +     }
> > +
> >       /* Compute sleep time */
> >       delta_exec = rq_clock_task(rq) - se->exec_start;
> >       if (unlikely(delta_exec <= 0))
> > @@ -8190,6 +8204,9 @@ static bool __dequeue_task(struct rq *rq, struct task_struct *p, int flags)
> >
> >       dequeue_hierarchy(p, flags);
> >
> > +     if (!cfs_rq->h_nr_queued)
> > +             cfs_rq->idle_seq++;
>
> Shouldn't this skip DEQUEUE_SAVE?

Yes


>
> > +
> >       if (sched_feat(PLACE_REL_DEADLINE) && !task_sleep) {
> >               se->deadline -= se->vruntime;
> >               se->rel_deadline = 1;
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index b98084e1f5b0..69a2a749e188 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -689,6 +689,7 @@ struct cfs_rq {
> >       u64                     sum_weight;
> >       u64                     zero_vruntime;
> >       unsigned int            sum_shift;
> > +     u32                     idle_seq;
> >
> >  #ifdef CONFIG_SCHED_CORE
> >       unsigned int            forceidle_seq;
>
> Yeah, well AFAICT other things seems OK.
>
> Also, while trying to get this patch series on my tip
> branch I had a hard time. I was at the caves
> of git and b4 figthing with... Everything.
> Like there were no v2 tags on some of the patches
> so b4 didn't tracked them. I had to do some
> things by hand and finally it worked. (Not too greatly tho.)
>
> But who cares? :>.
>
>
> Thanks,
> Kayra