Messages in this thread | | | Date | Tue, 31 May 2022 10:16:05 +0200 | Subject | Re: [PATCH v9 2/7] sched/fair: Decay task PELT values during wakeup migration | From | Dietmar Eggemann <> |
| |
- Vincent Donnefort <vincent.donnefort@arm.com>
On 23/05/2022 17:51, Vincent Donnefort wrote: > From: Vincent Donnefort <vincent.donnefort@arm.com>
[...]
> [1] https://lore.kernel.org/all/20190709115759.10451-1-chris.redpath@arm.com/
minor:
I get `WARNING: Possible unwrapped commit description (prefer a maximum 75 chars per line)`. If you use
https://lkml.kernel.org/r/20190709115759.10451-1-chris.redpath@arm.com
this warning disappears.
[...]
> +static inline void migrate_se_pelt_lag(struct sched_entity *se) > +{ > + u64 throttled = 0, now, lut; > + struct cfs_rq *cfs_rq; > + struct rq *rq; > + bool is_idle; > + > + if (load_avg_is_decayed(&se->avg)) > + return; > + > + cfs_rq = cfs_rq_of(se); > + rq = rq_of(cfs_rq); > + > + rcu_read_lock(); > + is_idle = is_idle_task(rcu_dereference(rq->curr)); > + rcu_read_unlock(); > + > + /* > + * The lag estimation comes with a cost we don't want to pay all the > + * time. Hence, limiting to the case where the source CPU is idle and > + * we know we are at the greatest risk to have an outdated clock. > + */ > + if (!is_idle) > + return; > + > + /* > + * Estimated "now" is: last_update_time + cfs_idle_lag + rq_idle_lag, where: > + * > + * last_update_time (the cfs_rq's last_update_time) > + * = cfs_rq_clock_pelt() > + * = rq_clock_pelt() - cfs->throttled_clock_pelt_time
So this line is always:
= rq_clock_pelt()@cfs_rq_idle - cfs->throttled_clock_pelt_time@cfs_rq_idle
since we only execute this code when idle. Which then IMHO explains (1) better.
> + * > + * cfs_idle_lag (delta between cfs_rq's update and rq's update) > + * = rq_clock_pelt()@rq_idle - rq_clock_pelt()@cfs_rq_idle > + * > + * rq_idle_lag (delta between rq's update and now) > + * = sched_clock_cpu() - rq_clock()@rq_idle > + * > + * The rq_clock_pelt() from last_update_time being the same as > + * rq_clock_pelt()@cfs_rq_idle, we can write:
--> (1) ^^^
> + * > + * now = rq_clock_pelt()@rq_idle - cfs->throttled_clock_pelt_time + > + * sched_clock_cpu() - rq_clock()@rq_idle > + * Where: > + * rq_clock_pelt()@rq_idle is rq->clock_pelt_idle > + * rq_clock()@rq_idle is rq->enter_idle > + * cfs->throttled_clock_pelt_time is cfs_rq->throttled_pelt_idle
To understand this better:
cfs->throttled_clock_pelt_time@cfs_rq_idle is cfs_rq->throttled_pelt_idle
[...]
> + /* > + * Paired with _update_idle_rq_clock_pelt. It ensures at the worst case
minor:
s/_update_idle_rq_clock_pelt/_update_idle_rq_clock_pelt()
> + * is observed the old clock_pelt_idle value and the new enter_idle, > + * which lead to an understimation. The opposite would lead to an
s/understimation/underestimation
[...]
> @@ -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?
[...]
> +/* 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().
Besides these small issues:
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
| |