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

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Date:   Tue, 18 Feb 2020 21:19:16 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     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,
        mgorman@...e.de, linux-kernel@...r.kernel.org
Cc:     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 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.

> 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

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