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
Date: Mon, 10 Feb 2020 14:21:33 +0100 [thread overview]
Message-ID: <20200210132133.GH14897@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <c49ca012-bb3e-580d-9b45-359caa67d7c1@arm.com>
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.
next prev parent reply other threads:[~2020-02-10 13:21 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-22 17:35 [RFC PATCH v4 0/6] sched/cpufreq: Make schedutil energy aware Douglas RAILLARD
2020-01-22 17:35 ` [RFC PATCH v4 1/6] PM: Introduce em_pd_get_higher_freq() Douglas RAILLARD
2020-01-22 17:35 ` [RFC PATCH v4 2/6] sched/cpufreq: Attach perf domain to sugov policy Douglas RAILLARD
2020-01-22 17:35 ` [RFC PATCH v4 3/6] sched/cpufreq: Hook em_pd_get_higher_power() into get_next_freq() Douglas RAILLARD
2020-01-23 16:16 ` Quentin Perret
2020-01-23 17:52 ` Douglas Raillard
2020-01-24 14:37 ` Quentin Perret
2020-01-24 14:58 ` Quentin Perret
2020-02-27 15:51 ` Douglas Raillard
2020-01-22 17:35 ` [RFC PATCH v4 4/6] sched/cpufreq: Introduce sugov_cpu_ramp_boost Douglas RAILLARD
2020-01-23 15:55 ` Rafael J. Wysocki
2020-01-23 17:21 ` Douglas Raillard
2020-01-23 21:02 ` Rafael J. Wysocki
2020-01-28 15:38 ` Douglas Raillard
2020-02-10 13:08 ` Peter Zijlstra
2020-02-13 10:49 ` Douglas Raillard
2020-01-22 17:35 ` [RFC PATCH v4 5/6] sched/cpufreq: Boost schedutil frequency ramp up Douglas RAILLARD
2020-01-22 17:35 ` [RFC PATCH v4 6/6] sched/cpufreq: Add schedutil_em_tp tracepoint Douglas RAILLARD
2020-01-22 18:14 ` [RFC PATCH v4 0/6] sched/cpufreq: Make schedutil energy aware Douglas Raillard
2020-02-10 13:21 ` Peter Zijlstra [this message]
2020-02-13 17:49 ` Douglas Raillard
2020-02-14 12:21 ` Peter Zijlstra
2020-02-14 12:52 ` Peter Zijlstra
2020-03-11 12:25 ` Douglas Raillard
2020-02-14 13:37 ` Peter Zijlstra
2020-03-11 12:40 ` Douglas Raillard
2020-01-23 15:43 ` Rafael J. Wysocki
2020-01-23 17:16 ` Douglas Raillard
2020-02-10 13:30 ` Peter Zijlstra
2020-02-13 11:55 ` Douglas Raillard
2020-02-13 13:20 ` Peter Zijlstra
2020-02-27 15:50 ` Douglas Raillard
2020-01-27 17:16 ` Vincent Guittot
2020-02-10 11:37 ` Douglas Raillard
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