linux-kernel - Re: [PATCH v2] sched/pelt: sync util/runnable

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Message-ID: <c1beb50b-d385-524b-56e0-eae16d3700df@arm.com>
Date:   Tue, 19 May 2020 12:28:28 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/pelt: sync util/runnable_sum with PELT window
 when propagating

On 06/05/2020 17:53, Vincent Guittot wrote:

[...]

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 02f323b85b6d..df3923a65162 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3441,52 +3441,46 @@ static inline void
>  update_tg_cfs_util(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq *gcfs_rq)
>  {
>  	long delta = gcfs_rq->avg.util_avg - se->avg.util_avg;
> +	/*
> +	 * cfs_rq->avg.period_contrib can be used for both cfs_rq and se.
> +	 * See ___update_load_avg() for details.
> +	 */
> +	u32 divider = LOAD_AVG_MAX - 1024 + cfs_rq->avg.period_contrib;

Why not doing the assignment (like in update_tg_cfs_load()) after the
next condition? Same question for update_tg_cfs_runnable().

[...]

>  static inline void
>  update_tg_cfs_runnable(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq *gcfs_rq)
>  {
>  	long delta = gcfs_rq->avg.runnable_avg - se->avg.runnable_avg;
> +	/*
> +	 * cfs_rq->avg.period_contrib can be used for both cfs_rq and se.
> +	 * See ___update_load_avg() for details.
> +	 */
> +	u32 divider = LOAD_AVG_MAX - 1024 + cfs_rq->avg.period_contrib;

We know have 6 assignments like this in fair.c and 1 in pelt.c. Could
this not be refactored by using something like this in pelt.h:

+static inline u32 get_divider(struct sched_avg *avg)
+{
+       return LOAD_AVG_MAX - 1024 + avg->period_contrib;
+}

[...]

> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> index b647d04d9c8b..1feff80e7e45 100644
> --- a/kernel/sched/pelt.c
> +++ b/kernel/sched/pelt.c
> @@ -237,6 +237,30 @@ ___update_load_sum(u64 now, struct sched_avg *sa,
>  	return 1;
>  }
>  
> +/*
> + * When syncing *_avg with *_sum, we must take into account the current
> + * position in the PELT segment otherwise the remaining part of the segment
> + * will be considered as idle time whereas it's not yet elapsed and this will
> + * generate unwanted oscillation in the range [1002..1024[.
> + *
> + * The max value of *_sum varies with the position in the time segment and is
> + * equals to :
> + *
> + *   LOAD_AVG_MAX*y + sa->period_contrib
> + *
> + * which can be simplified into:
> + *
> + *   LOAD_AVG_MAX - 1024 + sa->period_contrib
> + *
> + * because LOAD_AVG_MAX*y == LOAD_AVG_MAX-1024

Isn't this rather '~' instead of '==', even for y^32 = 0.5 ?

47742 * 0.5^(1/32) ~ 47742 - 1024


Apart from that, LGTM

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>