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

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Message-ID: <20230826200829.gnha7xcrjbyipjr2@airbuntu>
Date:   Sat, 26 Aug 2023 21:08:29 +0100
From:   Qais Yousef <qyousef@...alina.io>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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 08/21/23 18:39, Dietmar Eggemann wrote:
> 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.

Sorry, I would very strongly disagree here. What you're saying the effective
range of uclamp_max is 800 and anything above that will always go to max. How
can this be acceptable?

> 
> > 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()`

:-O

So when I set the system uclamp_max to 800 it will still run at max; and this
is normal?!!

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

-ENOPARSE.


Cheers

--
Qais Yousef