linux-kernel - Re: [RFC PATCH v4 09/12] sched/fair: Introduce an energy estimation helper function

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Date:   Fri, 6 Jul 2018 17:49:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Quentin Perret <quentin.perret@....com>
Cc:     rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, gregkh@...uxfoundation.org,
        mingo@...hat.com, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com,
        patrick.bellasi@....com, valentin.schneider@....com,
        vincent.guittot@...aro.org, thara.gopinath@...aro.org,
        viresh.kumar@...aro.org, tkjos@...gle.com, joel@...lfernandes.org,
        smuckle@...gle.com, adharmap@...cinc.com, skannan@...cinc.com,
        pkondeti@...eaurora.org, juri.lelli@...hat.com,
        edubezval@...il.com, srinivas.pandruvada@...ux.intel.com,
        currojerez@...eup.net, javi.merino@...nel.org
Subject: Re: [RFC PATCH v4 09/12] sched/fair: Introduce an energy estimation
 helper function

On Fri, Jul 06, 2018 at 04:12:12PM +0100, Quentin Perret wrote:
> On Friday 06 Jul 2018 at 15:12:43 (+0200), Peter Zijlstra wrote:

> > Did you want to use sugov_get_util() here? There is no way we're going
> > to duplicate all that.
> 
> I need to look into how we can do that ... Sugov looks at the current
> util landscape while EAS tries to predict the _future_ util landscape.
> Merging the two means I need to add a task and a dst_cpu as parameters
> of sugov_get_util() and call cpu_util_next() from there, which doesn't
> feel so clean ...

Just pass in the util_cfs as computed by cpu_util_next(), then schedutil
will pass in cpu_util_cfs(), the rest is all the same I think.

See below.

> Also, if we merge sugov_get_util() and sugov_aggregate_util() with
> Vincent's patch-set I'll need to make sure to return two values with
> sugov_get_util(): 1) the sum of the util of all classes; and 2) the util
> that will be used to request an OPP. 1) should be used in sum_util and
> 2) could (but I don't think it's is a good idea) be used for max_util.

I am confused, the max/sum thing is composed of the same values, just a
different operator. Both take 'util':


+	util = schedutil_get_util(cpu, cpu_util_next(cpu, p, dst_cpu))
+	max_util = max(util, max_util);
+	sum_util += util;


unsigned long schedutil_get_util(int cpu, unsigned long util_cfs)
{
	struct rq *rq = cpu_rq(cpu);
	unsigned long util, irq, max;

	max = arch_scale_cpu_capacity(NULL, cpu);

	if (rt_rq_is_runnable(&rq->rt))
		return max;

	/*
	 * Early check to see if IRQ/steal time saturates the CPU, can be
	 * because of inaccuracies in how we track these -- see
	 * update_irq_load_avg().
	 */
	irq = cpu_util_irq(rq);
	if (unlikely(irq >= max))
		return max;

	/*
	 * Because the time spend on RT/DL tasks is visible as 'lost' time to
	 * CFS tasks and we use the same metric to track the effective
	 * utilization (PELT windows are synchronized) we can directly add them
	 * to obtain the CPU's actual utilization.
	 */
	util = util_cfs;
	util += cpu_util_rt(rq);

	/*
	 * We do not make cpu_util_dl() a permanent part of this sum because we
	 * want to use cpu_bw_dl() later on, but we need to check if the
	 * CFS+RT+DL sum is saturated (ie. no idle time) such that we select
	 * f_max when there is no idle time.
	 *
	 * NOTE: numerical errors or stop class might cause us to not quite hit
	 * saturation when we should -- something for later.
	 */
	if ((util + cpu_util_dl(rq)) >= max)
		return max;

	/*
	 * There is still idle time; further improve the number by using the
	 * irq metric. Because IRQ/steal time is hidden from the task clock we
	 * need to scale the task numbers:
	 *
	 *              1 - irq
	 *   U' = irq + ------- * U
	 *                max
	 */
	util *= (max - irq);
	util /= max;
	util += irq;

	/*
	 * Bandwidth required by DEADLINE must always be granted while, for
	 * FAIR and RT, we use blocked utilization of IDLE CPUs as a mechanism
	 * to gracefully reduce the frequency when no tasks show up for longer
	 * periods of time.
	 *
	 * Ideally we would like to set bw_dl as min/guaranteed freq and util +
	 * bw_dl as requested freq. However, cpufreq is not yet ready for such
	 * an interface. So, we only do the latter for now.
	 */
	return min(max, util + cpu_bw_dl(rq));
}