linux-kernel - Re: [PATCH v2 4/5] sched/pelt: Add a new runnable average signal

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Date:   Wed, 19 Feb 2020 09:08:22 +0000
From:   Mel Gorman <mgorman@...e.de>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        linux-kernel@...r.kernel.org, pauld@...hat.com,
        parth@...ux.ibm.com, hdanton@...a.com
Subject: Re: [PATCH v2 4/5] sched/pelt: Add a new runnable average signal

On Tue, Feb 18, 2020 at 09:19:16PM +0000, Valentin Schneider wrote:
> On 14/02/2020 15:27, Vincent Guittot wrote:
> > Now that runnable_load_avg has been removed, 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 runnable _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.
> > 
> 
> I did a bit of playing around with tracepoints and it seems to be behaving
> fine. For instance, if I spawn 12 always runnable tasks (sysbench --test=cpu)
> on my Juno (6 CPUs), I get to a system-wide runnable value (\Sum cpu_runnable())
> of about 12K. I've only eyeballed them, but migration of the signal values
> seem fine too.
> 
> I have a slight worry that the rq-wide runnable signal might be too easy to
> inflate, since we aggregate for *all* runnable tasks, and that may not play
> well with your group_is_overloaded() change (despite having the imbalance_pct
> on the "right" side).
> 
> In any case I'll need to convince myself of it with some messing around, and
> this concerns patch 5 more than patch 4. So FWIW for this one:
> 
> Tested-by: Valentin Schneider <valentin.schneider@....com>
> 
> I also have one (two) more nit(s) below.
> 

Thanks.

> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> > diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> > @@ -227,14 +231,14 @@ ___update_load_sum(u64 now, struct sched_avg *sa,
> >  	 * Step 1: accumulate *_sum since last_update_time. If we haven't
> >  	 * crossed period boundaries, finish.
> >  	 */
> > -	if (!accumulate_sum(delta, sa, load, running))
> > +	if (!accumulate_sum(delta, sa, load, runnable, running))
> >  		return 0;
> >  
> >  	return 1;
> >  }
> >  
> >  static __always_inline void
> > -___update_load_avg(struct sched_avg *sa, unsigned long load)
> > +___update_load_avg(struct sched_avg *sa, unsigned long load, unsigned long runnable)
> >  {
> >  	u32 divider = LOAD_AVG_MAX - 1024 + sa->period_contrib;
> >  
> > @@ -242,6 +246,7 @@ ___update_load_avg(struct sched_avg *sa, unsigned long load)
> >  	 * Step 2: update *_avg.
> >  	 */
> >  	sa->load_avg = div_u64(load * sa->load_sum, divider);
> > +	sa->runnable_avg =	div _u64(runnable * sa->runnable_sum, divider);
>                           ^^^^^^        ^^^^^^^^
>                             a)             b)
> a) That's a tab
> 

Fixed and I'll post a v4 of my own series with Vincent's included.

> b) The value being passed is always 1, do we really need it to expose it as a
>    parameter?

This does appear to be an oversight but I'm not familiar enough with
pelt to be sure.

___update_load_avg() is called when sum of the load has changed because
a pelt period has passed and it has lost sight and does not care if an
individual sched entity is runnable or not. The parameter was added by
this patch but I cannot find any useful meaning for it.

Vincent, what was your thinking here? Should the parameter be removed?

-- 
Mel Gorman
SUSE Labs