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

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Date:   Mon, 18 Sep 2023 22:46:40 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     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,
        peterz@...radead.org, juri.lelli@...hat.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 15/09/2023 15:35, Vincent Guittot wrote:
> On Thu, 14 Sept 2023 at 23:07, Dietmar Eggemann
> <dietmar.eggemann@....com> wrote:
>>
>> On 01/09/2023 15:03, Vincent Guittot wrote:

[...]

>>> +#ifdef arch_scale_freq_ref
>>> +static __always_inline
>>> +unsigned long  arch_scale_freq_ref_em(int cpu, struct em_perf_domain *pd)
>>
>> Why is this function named with the arch prefix?
>>
>> So far we have 5 arch functions (arch_scale_freq_tick() <->
>> arch_scale_freq_ref()) and e.g. Arm/Arm64 defines them with there
>> topology_foo implementations.
>>
>> Isn't arch_scale_freq_ref_em() (as well as arch_scale_freq_ref_policy())
>> different in this sense and so a proper EM function which should
>> manifest in its name?
> 
> arch_scale_freq_ref_em() is there to handle cases where
> arch_scale_freq_ref() is not defined by arch. I keep arch_ prefix
> because this should be provided by architecture which wants to use EM.

That's correct, x86_64 with CONFIG_ENERGY_MODEL=y needs
arch_scale_freq_ref_em() returning highest perf_state of the perf_domain.
But this function as opposed to arch_scale_freq_ref() does not have to
be provided by the arch itself. It's provided by the EM instead.
That's why my doubt whether it should be named arch_scale_freq_ref_em().

> In the case of EM, it's only there for allyes|randconfig on arch that
> doesn't use arch_topology.c like x86_64

[...]

>>> @@ -241,11 +255,11 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
>>>        */
>>>       cpu = cpumask_first(to_cpumask(pd->cpus));
>>>       scale_cpu = arch_scale_cpu_capacity(cpu);
>>> -     ps = &pd->table[pd->nr_perf_states - 1];
>>> +     ref_freq = arch_scale_freq_ref_em(cpu, pd);
>>
>> Why not using existing `unsigned long freq` here like in schedutil's
>> get_next_freq()?
> 
> Find it easier to read and understand and will not make any difference
> in the compiled code

True but I thought it's easier to be able to detect the functional
similarity between em_cpu_energy() (*) and get_next_freq().

freq = arch_scale_freq_ref_{policy,em}({policy,(cpu, pd)});
... (in case of *)
freq = map_util_freq(util, freq, max);

Just a nitpick ...

[...]