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

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Message-ID: <20160915144337.GF5016@twins.programming.kicks-ass.net>
Date:   Thu, 15 Sep 2016 16:43:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        yuyang.du@...el.com, Morten.Rasmussen@....com,
        linaro-kernel@...ts.linaro.org, dietmar.eggemann@....com,
        pjt@...gle.com, bsegall@...gle.com
Subject: Re: [PATCH 4/7 v3] sched: propagate load during synchronous
 attach/detach

On Mon, Sep 12, 2016 at 09:47:49AM +0200, Vincent Guittot wrote:
> +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.
> +		*/
> +		if (tg_load < scale_load_down(gcfs_rq->tg->shares)) {
> +			load *= tg_load;
> +			load /= scale_load_down(gcfs_rq->tg->shares);
> +		}

Note that you're reversing the exact scaling you just applied.

That is:
               shares    tg_load
	load * ------- * ------- == load
               tg_load   shares

> +	}

So something like:

	shares = scale_load_down(gcfs_rq->tg->shares);

	if (tg_load >= shares) {
		load *= shares;
		load /= tg_load;
	}

should be the same as the above and saves a bunch of math, no?