linux-kernel - Re: [PATCH 2/4] sched: cpufreq: Fix apply_dvfs

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Message-ID: <7839dd5f-e7fd-7065-934d-436d012ac9d6@arm.com>
Date:   Mon, 21 Aug 2023 18:39:35 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qyousef@...alina.io>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Lukasz Luba <lukasz.luba@....com>
Subject: Re: [PATCH 2/4] sched: cpufreq: Fix apply_dvfs_headroom() escaping
 uclamp constraints

On 20/08/2023 23:06, Qais Yousef wrote:
> DVFS headroom is applied after we calculate the effective_cpu_util()
> which is where we honour uclamp constraints. It makes more sense to
> apply the headroom there once and let all users naturally get the right
> thing without having to sprinkle the call around in various places.

uclamp is applied in effective_cpu_util(..., FREQUENCY_UTIL, ...) which
IMHO currently has 2 power callers: (1) schedutil: sugov_get_util() and
(2) EAS: eenv_pd_max_util()

> Before this fix running
> 
> 	uclampset -M 800 cat /dev/zero > /dev/null
> 
> Will cause the test system to run at max freq of 2.8GHz. After the fix
> it runs at 2.2GHz instead which is the correct value that matches the
> capacity of 800.

IMHO, a system at util = 800 (w/o uclamp) would also run at 2.8Ghz since
we would call map_util_to_perf() on 800, no matter from where we call it.

> Note that similar problem exist for uclamp_min. If util was 50, and
> uclamp_min is 100. Since we apply_dvfs_headroom() after apply uclamp
> constraints, we'll end up with util of 125 instead of 100. IOW, we get
> boosted twice, first time by uclamp_min, and second time by dvfs
> headroom.

I see what you want to change here but:

So far we have `util -> uclamp -> map_util_to_perf()`

which is fine when we see uclamp as an entity which constrains util, not
the util after being mapped to a capacity constraint.

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