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: Fri, 14 Feb 2020 13:52:49 +0100 [thread overview]
Message-ID: <20200214125249.GL14879@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <4a664419-f5a6-882f-83ee-5bbf20ff33d3@arm.com>
On Thu, Feb 13, 2020 at 05:49:48PM +0000, Douglas Raillard wrote:
> On 2/10/20 1:21 PM, Peter Zijlstra wrote:
> > assuming cs[].cost ~ f^3, and given our cost_margin ~ f, that leaves a
> > factor f^2 on the table.
>
> I'm guessing that you arrived to `cost_margin ~ f` this way:
>
> cost_margin = util - util_est_enqueued
> cost_margin = util - constant
>
> # with constant small enough
> cost_margin ~ util
>
> # with util ~ 1/f
> cost_margin ~ 1/f
>
> In the case you describe, `constant` is actually almost equal to `util`
> so `cost_margin ~! util`, and that series assumes frequency invariant
> util_avg so `util !~ 1/f` (I'll probably have to fix that).
Nah, perhaps already clear from the other email; but it goes like:
boost = util_avg - util_est
cost_margin = boost * C = C * util_avg - C * util_est
And since u ~ f (per schedutil construction), cost_margin is a function
linear in either u or f.
> > So the higher the min_freq, the less effective the boost.
>
> Yes, since the boost is allowing a fixed amount of extra power. Higher
> OPPs are less efficient than lower ones, so if min_freq is high, we
> won't speed up as much as if min_freq was low.
>
> > Maybe it all works out in practise, but I'm missing a big picture
>
> Here is a big picture :)
>
> https://gist.github.com/douglas-raillard-arm/f76586428836ec70c6db372993e0b731#file-ramp_boost-svg
>
> The board is a Juno R0, with a periodic task pinned on a big CPU
> (capa=1024):
> * phase 1: 5% duty cycle (=51 PELT units)
> * phase 2: 75% duty cycle (=768 PELT units)
>
> Legend:
> * blue square wave: when the task executes (like in kernelshark)
> * base_cost = cost of frequency as selected by schedutil in normal
> operations
> * allowed_cost = base_cost + cost_margin
> * util = util_avg
>
> note: the small gaps right after the duty cycle transition between
> t=4.15 and 4.25 are due to sugov task executing, so there is no dequeue
> and no util_est update.
I'm confused by the giant drop in frequency (blue line) around 4.18
schedutil shouldn't select f < max(util_avg, util_est), which is
violated right about there.
I'm also confused by the base_cost line; how can that be flat until
somewhere around 4.16. Sadly there is no line for pure schedutil freq to
compare against.
Other than that, I can see the green line is consistent with
util_avg>util_est, and how it help grow the frequency (blue).
next prev parent reply other threads:[~2020-02-14 12:53 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
2020-02-13 17:49 ` Douglas Raillard
2020-02-14 12:21 ` Peter Zijlstra
2020-02-14 12:52 ` Peter Zijlstra [this message]
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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