linux-kernel - Re: [PATCH v5 07/10] sched/irq: add irq utilization tracking

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Message-ID: <72473e6f-8ade-8e26-3282-276fcae4c4c7@arm.com>
Date:   Wed, 30 May 2018 17:55:24 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
        mingo@...nel.org, linux-kernel@...r.kernel.org, rjw@...ysocki.net
Cc:     juri.lelli@...hat.com, Morten.Rasmussen@....com,
        viresh.kumar@...aro.org, valentin.schneider@....com,
        quentin.perret@....com
Subject: Re: [PATCH v5 07/10] sched/irq: add irq utilization tracking

On 05/25/2018 03:12 PM, Vincent Guittot wrote:
> interrupt and steal time are the only remaining activities tracked by
> rt_avg. Like for sched classes, we can use PELT to track their average
> utilization of the CPU. But unlike sched class, we don't track when
> entering/leaving interrupt; Instead, we take into account the time spent
> under interrupt context when we update rqs' clock (rq_clock_task).
> This also means that we have to decay the normal context time and account
> for interrupt time during the update.
> 
> That's also important to note that because
>    rq_clock == rq_clock_task + interrupt time
> and rq_clock_task is used by a sched class to compute its utilization, the
> util_avg of a sched class only reflects the utilization of the time spent
> in normal context and not of the whole time of the CPU. The utilization of
> interrupt gives an more accurate level of utilization of CPU.
> The CPU utilization is :
>    avg_irq + (1 - avg_irq / max capacity) * /Sum avg_rq
> 
> Most of the time, avg_irq is small and neglictible so the use of the
> approximation CPU utilization = /Sum avg_rq was enough

[...]

> @@ -7362,6 +7363,7 @@ static void update_blocked_averages(int cpu)
>   	}
>   	update_rt_rq_load_avg(rq_clock_task(rq), rq, 0);
>   	update_dl_rq_load_avg(rq_clock_task(rq), rq, 0);
> +	update_irq_load_avg(rq, 0);

So this one decays the signals only in case the update_rq_clock_task() 
didn't call update_irq_load_avg() because 'irq_delta + steal' is 0, right?

[...]

> diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> index 3d5bd3a..d2e4f21 100644
> --- a/kernel/sched/pelt.c
> +++ b/kernel/sched/pelt.c
> @@ -355,3 +355,41 @@ int update_dl_rq_load_avg(u64 now, struct rq *rq, int running)
>   
>   	return 0;
>   }
> +
> +/*
> + * irq:
> + *
> + *   util_sum = \Sum se->avg.util_sum but se->avg.util_sum is not tracked
> + *   util_sum = cpu_scale * load_sum
> + *   runnable_load_sum = load_sum
> + *
> + */
> +
> +int update_irq_load_avg(struct rq *rq, u64 running)
> +{
> +	int ret = 0;
> +	/*
> +	 * We know the time that has been used by interrupt since last update
> +	 * but we don't when. Let be pessimistic and assume that interrupt has
> +	 * happened just before the update. This is not so far from reality
> +	 * because interrupt will most probably wake up task and trig an update
> +	 * of rq clock during which the metric si updated.
> +	 * We start to decay with normal context time and then we add the
> +	 * interrupt context time.
> +	 * We can safely remove running from rq->clock because
> +	 * rq->clock += delta with delta >= running

This is true as long update_irq_load_avg() with a 'running != 0' is 
called only after rq->clock moved forward (rq->clock += delta) (which is 
true for update_rq_clock()->update_rq_clock_task()).

> +	 */
> +	ret = ___update_load_sum(rq->clock - running, rq->cpu, &rq->avg_irq,
> +				0,
> +				0,
> +				0);
> +	ret += ___update_load_sum(rq->clock, rq->cpu, &rq->avg_irq,
> +				1,
> +				1,
> +				1);

So you decay the signal in [sa->lut, rq->clock - running] (assumed to be 
the portion of delta used by the task scheduler) and you increase it in 
[rq->clock - running, rq->clock] (irq and virt portion of delta).

That means that this signal is updated on rq->clock whereas the others 
are on rq->clock_task.

What about the ever growing clock diff between them? I see e.g ~6s after 
20min uptime and up to 1.5ms 'running'.

It should be still safe to sum the sched class and irq signal in 
sugov_aggregate_util() because they are independent, I guess.

[...]