linux-kernel - Re: [PATCH 4/7 v3] sched: propagate load during synchronous attach/detach

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Date:   Thu, 15 Sep 2016 14:11:49 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
        mingo@...nel.org, linux-kernel@...r.kernel.org,
        yuyang.du@...el.com, Morten.Rasmussen@....com
Cc:     linaro-kernel@...ts.linaro.org, pjt@...gle.com, bsegall@...gle.com
Subject: Re: [PATCH 4/7 v3] sched: propagate load during synchronous
 attach/detach

On 12/09/16 08:47, Vincent Guittot wrote:
> When a task moves from/to a cfs_rq, we set a flag which is then used to
> propagate the change at parent level (sched_entity and cfs_rq) during
> next update. If the cfs_rq is throttled, the flag will stay pending until
> the cfs_rw is unthrottled.
> 
> For propagating the utilization, we copy the utilization of child cfs_rq to

s/child/group ?

> the sched_entity.
> 
> For propagating the load, we have to take into account the load of the
> whole task group in order to evaluate the load of the sched_entity.
> Similarly to what was done before the rewrite of PELT, we add a correction
> factor in case the task group's load is less than its share so it will
> contribute the same load of a task of equal weight.

What about cfs_rq->runnable_load_avg?

[...]

> +/* Take into account change of load of a child task group */
> +static inline void
> +update_tg_cfs_load(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +{
> +	struct cfs_rq *gcfs_rq = group_cfs_rq(se);
> +	long delta, load = gcfs_rq->avg.load_avg;
> +
> +	/* If the load of group cfs_rq is null, the load of the
> +	 * sched_entity will also be null so we can skip the formula
> +	 */
> +	if (load) {
> +		long tg_load;
> +
> +		/* Get tg's load and ensure tg_load > 0 */
> +		tg_load = atomic_long_read(&gcfs_rq->tg->load_avg) + 1;
> +
> +		/* Ensure tg_load >= load and updated with current load*/
> +		tg_load -= gcfs_rq->tg_load_avg_contrib;
> +		tg_load += load;
> +
> +		/* scale gcfs_rq's load into tg's shares*/
> +		load *= scale_load_down(gcfs_rq->tg->shares);
> +		load /= tg_load;
> +
> +		/*
> +		 * we need to compute a correction term in the case that the
> +		 * task group is consuming <1 cpu so that we would contribute
> +		 * the same load as a task of equal weight.

Wasn't 'consuming <1' related to 'NICE_0_LOAD' and not
scale_load_down(gcfs_rq->tg->shares) before the rewrite of PELT (v4.2,
__update_group_entity_contrib())?

> +		*/
> +		if (tg_load < scale_load_down(gcfs_rq->tg->shares)) {
> +			load *= tg_load;
> +			load /= scale_load_down(gcfs_rq->tg->shares);
> +		}
> +	}

[...]