linux-kernel - Re: [PATCH v9 13/15] sched/fair: Introduce an energy estimation helper function

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Date:   Wed, 21 Nov 2018 16:05:27 +0000
From:   Quentin Perret <quentin.perret@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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@...eaurora.org,
        skannan@...eaurora.org, pkondeti@...eaurora.org,
        juri.lelli@...hat.com, edubezval@...il.com,
        srinivas.pandruvada@...ux.intel.com, currojerez@...eup.net,
        javi.merino@...nel.org
Subject: Re: [PATCH v9 13/15] sched/fair: Introduce an energy estimation
 helper function

On Wednesday 21 Nov 2018 at 15:28:03 (+0100), Peter Zijlstra wrote:
> On Mon, Nov 19, 2018 at 02:18:55PM +0000, Quentin Perret wrote:
> > +static long
> > +compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
> > +{
> > +	long util, max_util, sum_util, energy = 0;
> > +	int cpu;
> > +
> > +	for (; pd; pd = pd->next) {
> > +		max_util = sum_util = 0;
> > +		/*
> > +		 * The capacity state of CPUs of the current rd can be driven by
> > +		 * CPUs of another rd if they belong to the same performance
> > +		 * domain. So, account for the utilization of these CPUs too
> > +		 * by masking pd with cpu_online_mask instead of the rd span.
> > +		 *
> > +		 * If an entire performance domain is outside of the current rd,
> > +		 * it will not appear in its pd list and will not be accounted
> > +		 * by compute_energy().
> > +		 */
> > +		for_each_cpu_and(cpu, perf_domain_span(pd), cpu_online_mask) {
> 
> Should that not be cpu_active_mask ?

Hmm, I must admit I'm sometimes a bit confused by the exact difference
between these masks, so maybe yeah ...

IIUC, cpu_active_mask is basically the set of CPUs on which the
scheduler is actually allowed to migrate tasks. Is that correct ?

I have always seen cpu_online_mask as a superset of cpu_active_mask
which can also include CPUs which are still running 'special' tasks
(kthreads and things like that I assume) although not allowed for
migration any more (or not yet) because we're in the process of
hotplugging that CPU.

So, the thing is, I'm not trying to select a CPU candidate for my task
here, I'm trying to understand what's the energy impact of a migration.
That involves all CPUs that are running _something_ in a perf domain
no matter if they're allowed to run more tasks or not. I mean, raising
the OPP will make running online && !active CPUs more expensive as well.
That's why I thought cpu_online_mask was good match here.

Or maybe I'm confused again :-)

> 
> > +			util = cpu_util_next(cpu, p, dst_cpu);
> > +			util = schedutil_energy_util(cpu, util);
> > +			max_util = max(util, max_util);
> > +			sum_util += util;
> > +		}
> > +
> > +		energy += em_pd_energy(pd->em_pd, max_util, sum_util);
> > +	}
> > +
> > +	return energy;
> > +}

Thanks,
Quentin