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Date:   Mon, 10 Feb 2020 14:21:33 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Douglas Raillard <douglas.raillard@arm.com>
Cc:     linux-kernel@vger.kernel.org, rjw@rjwysocki.net,
        viresh.kumar@linaro.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        qperret@google.com, linux-pm@vger.kernel.org
Subject: Re: [RFC PATCH v4 0/6] sched/cpufreq: Make schedutil energy aware
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On Wed, Jan 22, 2020 at 06:14:24PM +0000, Douglas Raillard wrote:
> Hi Peter,
> 
> Since the v3 was posted a while ago, here is a short recap of the hanging
> comments:
> 
> * The boost margin was relative, but we came to the conclusion it would make
>   more sense to make it absolute (done in that v4).

As per (patch #1):

+       max_cost = pd->table[pd->nr_cap_states - 1].cost;
+       cost_margin = (cost_margin * max_cost) / EM_COST_MARGIN_SCALE;

So we'll allow the boost to double energy consumption (or rather, since
you cannot go above the max OPP, we're allowed that).

> * The main remaining blur point was why defining boost=(util - util_est) makes
>   sense. The justification for that is that we use PELT-shaped signal to drive
>   the frequency, so using a PELT-shaped signal for the boost makes sense for the
>   same reasons.

As per (patch #4):

+       unsigned long boost = 0;

+       if (util_est_enqueued == sg_cpu->util_est_enqueued &&
+           util_avg >= sg_cpu->util_avg &&
+           util_avg > util_est_enqueued)
+               boost = util_avg - util_est_enqueued;

The result of that is not, strictly speaking, a PELT shaped signal.
Although when it is !0 the curves are similar, albeit offset.

> AFAIK there is no specific criteria to meet for frequency selection signal shape
> for anything else than periodic tasks (if we don't add other constraints on
> top), so (util - util_est)=(util - constant) seems as good as anything else.
> Especially since util is deemed to be a good fit in practice for frequency
> selection. Let me know if I missed anything on that front.


Given:

  sugov_get_util() <- cpu_util_cfs() <- UTIL_EST ? util_est.enqueued : util_avg.

our next_f becomes:

  next_f = 1.25 * util_est * max_freq / max;

so our min_freq in em_pd_get_higher_freq() will already be compensated
for the offset.

So even when:

  boost = util_avg - util_est

is small, despite util_avg being huge (~1024), due to large util_est,
we'll still get an effective boost to max_cost ASSUMING cs[].cost and
cost_margin have the same curve.

They have not.

assuming cs[].cost ~ f^3, and given our cost_margin ~ f, that leaves a
factor f^2 on the table.

So the higher the min_freq, the less effective the boost.

Maybe it all works out in practise, but I'm missing a big picture
description of it all somewhere.