linux-kernel - Re: [PATCH v9 2/7] sched/fair: Decay task PELT values during wakeup migration

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Date:   Mon, 6 Jun 2022 10:31:16 +0100
From:   Vincent Donnefort <vdonnefort@...gle.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, morten.rasmussen@....com,
        chris.redpath@....com, qperret@...gle.com, tao.zhou@...ux.dev,
        kernel-team@...roid.com
Subject: Re: [PATCH v9 2/7] sched/fair: Decay task PELT values during wakeup
 migration

[...]
> > @@ -8114,6 +8212,10 @@ static bool __update_blocked_fair(struct rq *rq, bool *done)
> >  		if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq)) {
> >  			update_tg_load_avg(cfs_rq);
> >  
> > +			/* sync clock_pelt_idle with last update */
> 
> update_idle_cfs_rq_clock_pelt() syncs cfs_rq->throttled_pelt_idle with
> cfs_rq->throttled_clock_pelt_time. Not sure what `clock_pelt_idle` and
> `last update` here mean?


Indeed, this comment is not helpful at all. What matters here is that the cfs_rq
is idle and we need to update the throttled_pelt_idle accordingly.

> 
> [...]
> 
> > +/* The rq is idle, we can sync to clock_task */
> > +static inline void _update_idle_rq_clock_pelt(struct rq *rq)
> > +{
> > +	rq->clock_pelt  = rq_clock_task(rq);
> > +
> > +	u64_u32_store(rq->enter_idle, rq_clock(rq));
> > +	/* Paired with smp_rmb in migrate_se_pelt_lag */
> 
> minor:
> 
> s/migrate_se_pelt_lag/migrate_se_pelt_lag()
> 
> [...]
> 
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index bf4a0ec98678..97bc26e5c8af 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -648,6 +648,10 @@ struct cfs_rq {
> >  	int			runtime_enabled;
> >  	s64			runtime_remaining;
> >  
> > +	u64			throttled_pelt_idle;
> > +#ifndef CONFIG_64BIT
> > +	u64                     throttled_pelt_idle_copy;
> > +#endif
> >  	u64			throttled_clock;
> >  	u64			throttled_clock_pelt;
> >  	u64			throttled_clock_pelt_time;
> > @@ -1020,6 +1024,12 @@ struct rq {
> >  	u64			clock_task ____cacheline_aligned;
> >  	u64			clock_pelt;
> >  	unsigned long		lost_idle_time;
> > +	u64			clock_pelt_idle;
> > +	u64			enter_idle;
> > +#ifndef CONFIG_64BIT
> > +	u64			clock_pelt_idle_copy;
> > +	u64			enter_idle_copy;
> > +#endif
> >  
> >  	atomic_t		nr_iowait;
> 
> `throttled_pelt_idle`, `clock_pelt_idle` and `enter_idle` are clock
> snapshots when cfs_rq resp. rq go idle. But the naming does not really
> show this relation. And this makes reading those equations rather difficult.
> 
> What about something like `throttled_clock_pelt_time_enter_idle`,
> `clock_pelt_enter_idle`, `clock_enter_idle`? Especially the first one is
> too long but something which shows that those are clock snapshots when
> enter idle would IMHO augment readability in migrate_se_pelt_lag().

What if I drop the "enter"?

 clock_idle;
 clock_pelt_idle;
 throttled_clock_pelt_time_idle;

> 
> Besides these small issues:
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>

Thanks!