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

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Date:   Wed, 7 Mar 2018 11:31:49 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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 06-Mar 19:58, Peter Zijlstra wrote:
> 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?

By away you mean go over 1024 or overflow the unsigned int storage?

In the first case, I think we don't care about exceeding 1024 since:
- we cap to capacity_orig_of in cpu_util_est
- by directly reading the cfs_rq->avg.util_est.enqueued we can
  actually detect conditions in which a CPU is over-saturated.

In the second case, with an unsigned int we can enqueue up to few
millions of 100% tasks on a single CPU without overflowing.

> Should we put a max in enqueue to avoid this?

IMO the capping from the cpu_util_est getter should be enough...

Maybe I'm missing your point here?

-- 
#include <best/regards.h>

Patrick Bellasi