linux-kernel - Re: [PATCH v1] Revert "cpufreq: schedutil: Move max CPU capacity to sugov

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Date:   Tue, 22 Nov 2022 08:58:12 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Sam Wu <wusamuel@...gle.com>
Cc:     Saravana Kannan <saravanak@...gle.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        "Isaac J . Manjarres" <isaacmanjarres@...gle.com>,
        kernel-team@...roid.com,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] Revert "cpufreq: schedutil: Move max CPU capacity to
 sugov_policy"

Hi Rafael and Sam

On 11/21/22 19:18, Rafael J. Wysocki wrote:
> On Fri, Nov 18, 2022 at 2:00 AM Sam Wu <wusamuel@...gle.com> wrote:
>>
>> On Wed, Nov 16, 2022 at 3:35 AM Lukasz Luba <lukasz.luba@....com> wrote:
>>> Which mainline kernel version you use in pixel6?
>> I am using kernel version 6.1-rc5.
>>>
>>> Could you elaborate a bit how is it possible?
>>> Do you have the sg_policy setup properly (and at right time)?
>>> Do you have the cpu capacity from arch_scale_cpu_capacity()
>>> set correctly and at the right time during this cpufreq
>>> governor setup?
>>>
>>> IIRC in Android there is a different code for setting up the
>>> cpufreq sched governor clones. In mainline we don't have to do
>>> those tricks, so this might be the main difference.
>> This behavior is seen on the mainline kernel. There isn't any vendor code
>> modifying the behavior, and the schedutil governor is being used.
>>>
>>> Could you trace the value that is read from
>>> arch_scale_cpu_capacity() and share it with us?
>>> I suspect this value changes in time in your kernel.
>> There's an additional CPU capacity normalization step during
>> init_cpu_capacity_callback() that does not happen until all the CPUs come
>> online. However, the sugov_start() function can be called for a subset of
>> CPUs before all the CPUs are brought up and before the normalization of
>> the CPU capacity values, so there could be a stale value stored
>> in sugov_policy.max field.
> 
> OK, the revert has been applied as 6.1-rc material, thanks!

I was on a business trip last week so couldn't check this.
Now I'm back and I've checked the booting sequence.
Yes, there is some race condition and the mechanism
using blocking_notifier_call_chain() in the cpufreq_online()
doesn't help while we are registering that schedutil
new policy.

I will have to go through those mechanisms and check them.
I agree, for now the best option is to revert the patch.

My apologies for introducing this issues.
Thanks Sam for capturing it.

Regards,
Lukasz