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

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Date:   Wed, 7 Mar 2018 15:24:58 +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 07-Mar 13:24, Peter Zijlstra wrote:
> On Wed, Mar 07, 2018 at 11:31:49AM +0000, Patrick Bellasi wrote:
> > > 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?
> 
> the later, I think you can make it arbitrarily large. Have a busy task
> on CPU0, this ensure !nr_running never happens.
> 
> Start a busy task on CPU1, wait for it to hit u=1, then migrate it to
> CPU0, 

At this point util_est(CPU0) = 2048, which is:

   +1024 for the busy running task
         assuming it has been enqueued with the utilization since the beginning
   +1024 for the newly migrated task from CPU1
         which is enqueued with the value he reached at dequeued time
         from CPU1

> then wait for it to hit u=.5 then kill it,

... but when we kill it, the task is dequeued, and thus we remove
1024.

Maybe that's the tricky bit: we remove the value we enqueued, _not_
the current util_avg. Notice we use _task_util_est(p)... with the
leading "_".

> this effectively adds
> .5 to the enqueued value, repeat indefinitely.

Thus this should not happen.

Basically, the RQ's util_est is the sum of the RUNNABLE tasks's
util_est at their enqueue time... which has been update at their last
dequeue time, hence the usage of name "dequeued" for both tasks and
rqs.

Does it make sense now?

-- 
#include <best/regards.h>

Patrick Bellasi