linux-kernel - Re: [RFC 3/4] sched/fair: replace runnable load average by runnable average

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Date:   Fri, 14 Feb 2020 08:42:38 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Phil Auld <pauld@...hat.com>, Parth Shah <parth@...ux.ibm.com>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [RFC 3/4] sched/fair: replace runnable load average by runnable average

On Wed, 12 Feb 2020 at 15:30, Mel Gorman <mgorman@...e.de> wrote:
>
> On Tue, Feb 11, 2020 at 06:46:50PM +0100, Vincent Guittot wrote:
> > Now that runnable_load_avg is not more used, we can replace it by a new
> > signal that will highlight the runnable pressure on a cfs_rq. This signal
> > track the waiting time of tasks on rq and can help to better define the
> > state of rqs.
> >
> > At now, only util_avg is used to define the state of a rq:
> >   A rq with more that around 80% of utilization and more than 1 tasks is
> >   considered as overloaded.
> >
> > But the util_avg signal of a rq can become temporaly low after that a task
> > migrated onto another rq which can bias the classification of the rq.
> >
> > When tasks compete for the same rq, their runnable average signal will be
> > higher than util_avg as it will include the waiting time and we can use
> > this signal to better classify cfs_rqs.
> >
> > The new runnable_avg will track the runnable time of a task which simply
> > adds the waiting time to the running time. The runnbale _avg of cfs_rq
> > will be the /Sum of se's runnable_avg and the runnable_avg of group entity
> > will follow the one of the rq similarly to util_avg.
> >
>
> s/runnbale/runnable/
>
> Otherwise, all I can do is give a heads-up that I will not be able to
> review this patch and the next patch properly in the short-term. While the
> new metric appears to have a sensible definition, I've not spent enough
> time comparing/contrasting the pro's and con's of PELT implementation
> details or their consequences. I am not confident I can accurately
> predict whether this is better or if there are corner cases that make
> poor placement decisions based on fast changes of runnable_avg. At least
> not within a reasonable amount of time.

ok. understood

>
> This caught my attention though
>
> > @@ -4065,8 +4018,8 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> >        *   - Add its new weight to cfs_rq->load.weight
> >        */
> >       update_load_avg(cfs_rq, se, UPDATE_TG | DO_ATTACH);
> > +     se_update_runnable(se);
> >       update_cfs_group(se);
> > -     enqueue_runnable_load_avg(cfs_rq, se);
> >       account_entity_enqueue(cfs_rq, se);
> >
> >       if (flags & ENQUEUE_WAKEUP)
>
> I don't think the ordering matters any more because of what was removed
> from update_cfs_group. Unfortunately, I'm not 100% confident so am
> bringing it to your attention in case it does.

Yes. I have just tried to keep the same order with
se_update_runnable() just below update_load_avg() like for other
places

>
> --
> Mel Gorman
> SUSE Labs