linux-kernel - Re: [PATCH 4/6 v5] sched: propagate load during synchronous attach/detach

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Date:   Wed, 26 Oct 2016 13:45:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, yuyang.du@...el.com,
        Morten.Rasmussen@....com, linaro-kernel@...ts.linaro.org,
        pjt@...gle.com, bsegall@...gle.com, kernellwp@...il.com
Subject: Re: [PATCH 4/6 v5] sched: propagate load during synchronous
 attach/detach

On Mon, Oct 17, 2016 at 11:14:11AM +0200, Vincent Guittot wrote:
> +/* 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
> +	 */

Does it make sense to do:

	if (!load)
		goto no_load;

and avoid the indent?

> +	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;
> +
> +		/*
> +		 * We need to compute a correction term in the case that the
> +		 * task group is consuming more cpu than a task of equal
> +		 * weight. A task with a weight equals to tg->shares will have
> +		 * a load less or equal to scale_load_down(tg->shares).
  +		 *
> +		 * Similarly, the sched_entities that represent the task group
> +		 * at parent level, can't have a load higher than
> +		 * scale_load_down(tg->shares). And the Sum of sched_entities'
> +		 * load must be <= scale_load_down(tg->shares).
> +		 */
> +		if (tg_load > scale_load_down(gcfs_rq->tg->shares)) {
> +			/* scale gcfs_rq's load into tg's shares*/
> +			load *= scale_load_down(gcfs_rq->tg->shares);
> +			load /= tg_load;
> +		}
> +	}
> +
no_load:
> +	delta = load - se->avg.load_avg;
> +
> +	/* Nothing to update */
> +	if (!delta)
> +		return;
> +
> +	/* Set new sched_entity's load */
> +	se->avg.load_avg = load;
> +	se->avg.load_sum = se->avg.load_avg * LOAD_AVG_MAX;
> +
> +	/* Update parent cfs_rq load */
> +	add_positive(&cfs_rq->avg.load_avg, delta);
> +	cfs_rq->avg.load_sum = cfs_rq->avg.load_avg * LOAD_AVG_MAX;
> +
> +	/*
> +	 * If the sched_entity is already enqueued, we also have to update the
> +	 * runnable load avg.
> +	 */
> +	if (se->on_rq) {
> +		/* Update parent cfs_rq runnable_load_avg */
> +		add_positive(&cfs_rq->runnable_load_avg, delta);
> +		cfs_rq->runnable_load_sum = cfs_rq->runnable_load_avg * LOAD_AVG_MAX;
> +	}
> +}