linux-kernel - Re: [PATCH v4 03/18] PM: EM: Find first CPU online while updating OPP efficiency

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Message-ID: <749c927a-119a-41ea-adfd-9e778d640d16@arm.com>
Date:   Tue, 24 Oct 2023 08:50:18 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     dietmar.eggemann@....com, rui.zhang@...el.com, rafael@...nel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        amit.kucheria@...durent.com, amit.kachhap@...il.com,
        viresh.kumar@...aro.org, len.brown@...el.com, pavel@....cz,
        mhiramat@...nel.org, qyousef@...alina.io, wvw@...gle.com
Subject: Re: [PATCH v4 03/18] PM: EM: Find first CPU online while updating OPP
 efficiency

Hi Daniel,

Thanks for looking at the patches!

On 10/23/23 18:06, Daniel Lezcano wrote:
> 
> Hi Lukasz,
> 
> On 25/09/2023 10:11, Lukasz Luba wrote:
>> The Energy Model might be updated at runtime and the energy efficiency
>> for each OPP may change. Thus, there is a need to update also the
>> cpufreq framework and make it aligned to the new values. In order to
>> do that, use a first online CPU from the Performance Domain.
> 
> I'm failing to do the connection with the description and the change.
> 
> Perhaps, the changelog shall explain why 'cpu' must be replaced with the 
> first active cpu ?

It's not a big problem now for EM, since during the boot the first CPU
in the 'policy' is actually registering the EM. Although, this is an
assumption and for the new runtime update of EM, we cannot assume
that first is online. That's the motivation of the change. In a corner
case all CPUs might be put offline, but the EM is still there because
we never unregister EM for CPUs (to not race with task scheduler).

I will add that description to the patch header.

Thanks,
Lukasz