linux-kernel - Re: [PATCH 4/4] energy_model: use a fixed reference frequency

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Message-ID: <20230905113308.GF28319@noisy.programming.kicks-ass.net>
Date:   Tue, 5 Sep 2023 13:33:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Pierre Gondois <pierre.gondois@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org,
        paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, sudeep.holla@....com,
        gregkh@...uxfoundation.org, rafael@...nel.org, mingo@...hat.com,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, viresh.kumar@...aro.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-pm@...r.kernel.org,
        conor.dooley@...rochip.com, suagrfillet@...il.com,
        ajones@...tanamicro.com, lftan@...nel.org
Subject: Re: [PATCH 4/4] energy_model: use a fixed reference frequency

On Tue, Sep 05, 2023 at 12:05:30PM +0200, Pierre Gondois wrote:
> Hello Vincent,
> I tried the patch-set on a platform using cppc_cpufreq and that has boosting
> frequencies,
> 
> 1-
> On such platform, the CPU capacity comes from the CPPC highest_frequency
> field. The CPU capacity is set to the capacity of the boosting frequency.
> This behaviour is different from DT platforms where the CPU capacity is
> updated whenever the boosting mode is enabled (it seems).
> 
> Wouldn't it be better to have CPU max capacities set to their boosting
> capacity as for CPPC base platforms ? It seems the max frequency is always
> available somehow for all the cpufreq drivers with boosting available, i.e.
> acpi-cpufreq, amd-pstate, cppc_cpufreq.

So on Intel we don't use the max (turbo) boost value, but typically end
up picking the 4-core turbo value or something. There's a comment in
arch/x86/kernel/cpu/aperfmperf.c.

Per that comment it probably makes sense to be able to differentiate
between a mobile device and a server, or perhaps we can (ab)use the EAS
enable knob for this distinction?

That is, I'm not sure it makes sense to always pick the highest boost
freqency for ARM64 servers, very much analogous to  how we don't do that
on Intel.