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Message-ID: <710b460f-20b9-21c4-3077-8cea35600550@arm.com>
Date: Mon, 21 Mar 2022 18:23:37 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Vincent Donnefort <vincent.donnefort@....com>,
peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org
Cc: linux-kernel@...r.kernel.org, morten.rasmussen@....com,
chris.redpath@....com, qperret@...gle.com
Subject: Re: [PATCH v3 2/7] sched/fair: Decay task PELT values during
migration
On 08/03/2022 19:19, Vincent Donnefort wrote:
Patch header:
s/during migration/during wakeup migration ?
To make sure it's not related to lb migration (TASK_ON_RQ_MIGRATING).
[...]
> Neither clock_task, clock_pelt nor clock can be accessed without the
> runqueue lock. The new cfs_rq last_update_lag is therefore created and
> encode those three values when the last_update_time value for that very
s/encode/encodes ... It's not really encoding?
[...]
> Now we have an estimation for A + B, we can create an estimator for the
> PELT value at the time of the migration. We need for this purpose to
> inject last_update_time which is a combination of both clock_pelt and
> lost_idle_time. The latter is a time value which is completely lost form a
s/form/from
> PELT point of view and must be ignored. And finally, we can write:
>
> rq_clock_pelt_estimator() = last_update_time + A + B
> = last_update_time +
> sched_clock_cpu() - last_update_lag
rq_clock_pelt_estimator() did exist in v2 but does not in v3 anymore.
Might be misleading when people search for it.
[...]
> @@ -3688,6 +3704,7 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
> cfs_rq->last_update_time_copy,
> sa->last_update_time);
> #endif
> + update_cfs_rq_lag(cfs_rq);
>
> return decayed;
> }
> @@ -6852,6 +6869,44 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>
> static void detach_entity_cfs_rq(struct sched_entity *se);
>
> +#ifdef CONFIG_NO_HZ_COMMON
Couldn't you put update_cfs_rq_lag() and migrate_se_pelt_lag() under one
CONFIG_NO_HZ_COMMON?
> +static inline void migrate_se_pelt_lag(struct sched_entity *se)
[...]
> + /*
> + * 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 + * we know we are at the greatest risk to have an outdated clock.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;
> +
> + last_update_lag = u64_u32_load(cfs_rq->last_update_lag);
So each taskgroup has its own last_update_lag. I guess it works since we
sync se in migrate_task_rq_fair() -> remove_entity_load_avg() ->
sync_entity_load_avg() with its cfs_rq.
[...]
> @@ -6859,6 +6914,9 @@ static void detach_entity_cfs_rq(struct sched_entity *se);
> */
> static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> {
> + struct sched_entity *se = &p->se;
> + struct cfs_rq *cfs_rq = cfs_rq_of(se);
This line can stay in the if condition below since cfs_rq is only used
there. The brackets are also still there (A).
[...]
> @@ -6866,8 +6924,6 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> * the task on the new runqueue.
> */
> if (READ_ONCE(p->__state) == TASK_WAKING) {
> - struct sched_entity *se = &p->se; <--- (A)
> - struct cfs_rq *cfs_rq = cfs_rq_of(se);
[...]
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