linux-kernel - Re: [RFC v3 2/6] Improve the tracking of active utilisation

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Message-ID: <20161118165647.46f8c867@sweethome>
Date:   Fri, 18 Nov 2016 16:56:47 +0100
From:   luca abeni <luca.abeni@...tn.it>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@....com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC v3 2/6] Improve the tracking of active utilisation

On Fri, 18 Nov 2016 16:36:15 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, Oct 24, 2016 at 04:06:34PM +0200, Luca Abeni wrote:
> > @@ -514,7 +556,20 @@ static void update_dl_entity(struct
> > sched_dl_entity *dl_se, struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
> >  	struct rq *rq = rq_of_dl_rq(dl_rq);
> >  
> > +	if (hrtimer_is_queued(&dl_se->inactive_timer)) {
> > +		hrtimer_try_to_cancel(&dl_se->inactive_timer);
> > +		WARN_ON(dl_task_of(dl_se)->nr_cpus_allowed > 1);  
> 
> Isn't that always so? That is, DL tasks cannot be but 'global', right?
Well, if I understand well in general (that is, if admission control is
enabled) nr_cpus_allowed is equal to the number of CPUs in the
cpuset...
This is generally > 1 (and in this case select_task_rq_dl() is invoked
first, and tries to cancel the timer - so I think the timer cannot be
queued), or can be = 1 if we do partitioned scheduling (cpusets
containing only 1 CPU, or disabled admission control). If
nr_cpus_allowed is 1, then select_task_rq_dl() is not invoked, so the
timer can be queued.
In some of my tests I used partitioned scheduling; in some other tests
I disabled admission control to mix tasks with different affinities, so
I made the warning conditional to the number of CPUs being > 1.


> Also, you could use the return value of hrtimer_try_to_cancel() to
> determine hrtimer_is_queued() I suppose.

Ah, ok... I was under the impression that
"if (hrtimer_is_queued()) hrtimer_try_to_cancel()"
is less overhead than a simple "hrtimer_try_to_cancel()", but this was
just an uneducated guess... I'll change the code to avoid the check on
hrtimer_is_queued().

> 
> > +	} else {
> > +		/*
> > +		 * The "inactive timer" has been cancelled in
> > +		 * select_task_rq_dl() (and the acvive utilisation
> > has
> > +		 * been decreased). So, increase the active
> > utilisation.
> > +		 * If select_task_rq_dl() could not cancel the
> > timer,
> > +		 * inactive_task_timer() will * find the task
> > state as  
>                                              ^^^
> superfluous '*'?

Yes, sorry... Something went wrong when I re-indented the comment :(


			Thanks,
				Luca