linux-kernel - Re: [PATCH v6 07/14] sched/topology: Introduce sched_energy

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Date:   Thu, 30 Aug 2018 10:57:45 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     peterz@...radead.org, 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, 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 v6 07/14] sched/topology: Introduce sched_energy_present
 static key

Hi Patrick,

On Thursday 30 Aug 2018 at 10:23:29 (+0100), Patrick Bellasi wrote:
> Yes, dunno if it's just me but perhaps a bit of rephrasing could help.

Ok, so what about something a little bit more explicit like:

/*
 * The complexity of the Energy Model is defined as:
 *
 *		C = nr_pd * (nr_cpus + nr_cs)
 *
 * with parameters defined as:
 *  - nr_pd:	the number of performance domains
 *  - nr_cpus:	the number of CPUs
 *  - nr_cs:	the sum of the number of capacity states of all performance
 *		domains (for example, on a system with 2 performance domains,
 *		with 10 capacity states each, nr_cs = 2 * 10 = 20).
 *
 * It is generally not a good idea to use such a model in the wake-up path on
 * very complex platforms because of the associated scheduling overheads. The
 * arbitrary constraint below prevents that. It makes EAS usable up to 16 CPUs
 * with per-CPU DVFS and less than 8 capacity states each, for example.
 */

> Alternatively, why not having this comment and check after patches
> 11 and 12 ?

Oh, you don't like it here ? What's wrong with it :-) ?

> Right... I was totally confused by the idea that we don't
> run EAS if we just have 1 CPU per PD... my bad!
> 
> Although on those systems, since we don't have idle costs, should not
> be a spreading strategy always the best from an energy efficiency
> standpoint ?

Even with per-CPU DVFS, if you have big and little CPUs it matters
_where_ you execute a task, and you'll still need an Energy Model to
make good decisions in a generic way. But yes, there is definitely some
room for improvement for those platforms. That's something we could
improve later on top of this series I suppose.

Thanks for looking at the patches !
Quentin