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

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Date:   Thu, 23 Apr 2020 16:30:02 +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] sched/pelt: sync util/runnable_sum with PELT window when
 propagating



On 22/04/2020 17:14, Vincent Guittot wrote:
> update_tg_cfs_util/runnable() propagate the impact of the attach/detach of
> an entity down into the cfs_rq hierarchy which must keep the sync with
> the current pelt window.
> 
> Even if we can't sync child rq and its group se, we can sync the group se

So we have

    gcfs --> tg --> gse
     ________________|
     |
     V

    cfs ---> tg (root)

     |
     V

    rq


here. What is 'child rq' for 'group se' here? I guess 'parent cfs_rq' is
cfs_rq.

> and parent cfs_rq with current PELT window. In fact, we must keep them sync
> in order to stay also synced with others se and group se that are already
> attached to the cfs_rq.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 26 ++++++--------------------
>  1 file changed, 6 insertions(+), 20 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 02f323b85b6d..ca6aa89c88f2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3441,52 +3441,38 @@ 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;
> +	u32 divider = LOAD_AVG_MAX - 1024 + cfs_rq->avg.period_contrib;
>  
>  	/* Nothing to update */
>  	if (!delta)
>  		return;
>  
> -	/*
> -	 * The relation between sum and avg is:
> -	 *
> -	 *   LOAD_AVG_MAX - 1024 + sa->period_contrib
> -	 *
> -	 * however, the PELT windows are not aligned between grq and gse.
> -	 */
> -
>  	/* Set new sched_entity's utilization */
>  	se->avg.util_avg = gcfs_rq->avg.util_avg;
> -	se->avg.util_sum = se->avg.util_avg * LOAD_AVG_MAX;
> +	se->avg.util_sum = se->avg.util_avg * divider;

divider uses cfs_rq but we sync se->avg.util_avg with gcfs_rq here.

But since avg.period_contrib of cfs_rq and gcfs_rq are the same this
should work.

>  	/* Update parent cfs_rq utilization */
>  	add_positive(&cfs_rq->avg.util_avg, delta);
> -	cfs_rq->avg.util_sum = cfs_rq->avg.util_avg * LOAD_AVG_MAX;
> +	cfs_rq->avg.util_sum = cfs_rq->avg.util_avg * divider;

Looks like that avg.last_update_time of se (group entity), it's gcfs_rq
and cfs_rq is always the same in update_tg_cfs_[util\|runnable].

So that means the PELT windows are aligned for cfs_rqs and group se's?

And if we want to enforce this for cfs_rq and task, we have
sync_entity_load_avg().

[...]