Re: [PATCH v4] sched/freq: move call to cpufreq_update_util

[Vincent Guittot]

On Fri, 15 Nov 2019 at 14:25, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Nov 14, 2019 at 06:07:31PM +0100, Vincent Guittot wrote:
>
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 69a81a5..3be44e1 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3504,9 +3504,6 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
> >       cfs_rq->load_last_update_time_copy = sa->last_update_time;
> >  #endif
> >
> > -     if (decayed)
> > -             cfs_rq_util_change(cfs_rq, 0);
> > -
> >       return decayed;
> >  }
>
> This removes the call from the for_each_leaf_cfs_rq_safe() loop.
>
> > @@ -7543,18 +7544,19 @@ static void update_blocked_averages(int cpu)
> >       const struct sched_class *curr_class;
> >       struct rq_flags rf;
> >       bool done = true;
> > +     int decayed;
> >
> >       rq_lock_irqsave(rq, &rf);
> >       update_rq_clock(rq);
> >
> >       /*
> > -      * update_cfs_rq_load_avg() can call cpufreq_update_util(). Make sure
> > -      * that RT, DL and IRQ signals have been updated before updating CFS.
> > +      * update_load_avg() can call cpufreq_update_util(). Make sure that RT,
> > +      * DL and IRQ signals have been updated before updating CFS.
> >        */
> >       curr_class = rq->curr->sched_class;
> > -     update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> > -     update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> > -     update_irq_load_avg(rq, 0);
> > +     decayed = update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> > +     decayed |= update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> > +     decayed |= update_irq_load_avg(rq, 0);
>
> Should not all 3 have their windows aligned and thus alway return the
> exact same value?

rt and dl yes but not irq

But having aligned window doesn't mean that they will all decay.
One can have been updated just before (during a dequeue as an example)
or at least less than 1ms before

>
> >
> >       /* Don't need periodic decay once load/util_avg are null */
> >       if (others_have_blocked(rq))
> > @@ -7567,9 +7569,13 @@ static void update_blocked_averages(int cpu)
> >       for_each_leaf_cfs_rq_safe(rq, cfs_rq, pos) {
> >               struct sched_entity *se;
> >
> > -             if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq))
> > +             if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq)) {
> >                       update_tg_load_avg(cfs_rq, 0);
> >
> > +                     if (cfs_rq == &rq->cfs)
> > +                             decayed = 1;
>
> And that restores it.
>
> But should not also rq->cfs's window be aligned with the above 3?
> Meaning that this one, with exception of the list_del, covers all 4.
>
> > +             }
> > +
> >               /* Propagate pending load changes to the parent, if any: */
> >               se = cfs_rq->tg->se[cpu];
> >               if (se && !skip_blocked_update(se))
> > @@ -7588,6 +7594,9 @@ static void update_blocked_averages(int cpu)
> >       }
> >
> >       update_blocked_load_status(rq, !done);
> > +
> > +     if (decayed)
> > +             cpufreq_update_util(rq, 0);
> >       rq_unlock_irqrestore(rq, &rf);
> >  }
> >
> > @@ -7644,22 +7653,22 @@ static inline void update_blocked_averages(int cpu)
> >       struct cfs_rq *cfs_rq = &rq->cfs;
> >       const struct sched_class *curr_class;
> >       struct rq_flags rf;
> > +     int decayed;
> >
> >       rq_lock_irqsave(rq, &rf);
> >       update_rq_clock(rq);
> >
> > -     /*
> > -      * update_cfs_rq_load_avg() can call cpufreq_update_util(). Make sure
> > -      * that RT, DL and IRQ signals have been updated before updating CFS.
> > -      */
> >       curr_class = rq->curr->sched_class;
> > -     update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> > -     update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> > -     update_irq_load_avg(rq, 0);
> > +     decayed = update_rt_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &rt_sched_class);
> > +     decayed |= update_dl_rq_load_avg(rq_clock_pelt(rq), rq, curr_class == &dl_sched_class);
> > +     decayed |= update_irq_load_avg(rq, 0);
> >
> > -     update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq);
> > +     decayed |= update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq);
>
> And that thus this one makes all 3 above redundant.
>
> >
> >       update_blocked_load_status(rq, cfs_rq_has_blocked(cfs_rq) || others_have_blocked(rq));
> > +
> > +     if (decayed)
> > +             cpufreq_update_util(rq, 0);
> >       rq_unlock_irqrestore(rq, &rf);
> >  }
>
> That is, I'm almost tempted to prefer a variant of your initial hack,
> that refuses to remove rq->cfs from the list.
>
> That avoids having to care about the rt,dl,irq decays (their windows
> align with rq->cfs) and makes smp/up similar.
>
>
> I still don't actually understand how any of this makes intel_pstate
> happy though.