linux-kernel - Re: [PATCH v5 1/4] sched/fair: add util

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Date:   Tue, 6 Mar 2018 19:58:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH v5 1/4] sched/fair: add util_est on top of PELT

On Thu, Feb 22, 2018 at 05:01:50PM +0000, Patrick Bellasi wrote:
> +static inline void util_est_enqueue(struct cfs_rq *cfs_rq,
> +				    struct task_struct *p)
> +{
> +	unsigned int enqueued;
> +
> +	if (!sched_feat(UTIL_EST))
> +		return;
> +
> +	/* Update root cfs_rq's estimated utilization */
> +	enqueued  = READ_ONCE(cfs_rq->avg.util_est.enqueued);
> +	enqueued += _task_util_est(p);
> +	WRITE_ONCE(cfs_rq->avg.util_est.enqueued, enqueued);
> +}

> +static inline void util_est_dequeue(struct cfs_rq *cfs_rq,
> +				    struct task_struct *p,
> +				    bool task_sleep)
> +{
> +	long last_ewma_diff;
> +	struct util_est ue;
> +
> +	if (!sched_feat(UTIL_EST))
> +		return;
> +
> +	/*
> +	 * Update root cfs_rq's estimated utilization
> +	 *
> +	 * If *p is the last task then the root cfs_rq's estimated utilization
> +	 * of a CPU is 0 by definition.
> +	 */
> +	ue.enqueued = 0;
> +	if (cfs_rq->nr_running) {
> +		ue.enqueued  = READ_ONCE(cfs_rq->avg.util_est.enqueued);
> +		ue.enqueued -= min_t(unsigned int, ue.enqueued,
> +				     _task_util_est(p));
> +	}
> +	WRITE_ONCE(cfs_rq->avg.util_est.enqueued, ue.enqueued);

It appears to me this isn't a stable situation and completely relies on
the !nr_running case to recalibrate. If we ensure that doesn't happen
for a significant while the sum can run-away, right?

Should we put a max in enqueue to avoid this?