linux-kernel - Re: [PATCH 1/4] cpufreq: Add a cpufreq pressure feedback for the scheduler

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Message-ID: <54f3b98c-1f7d-4205-9e3c-a4a19ad3d941@arm.com>
Date:   Thu, 14 Dec 2023 09:08:04 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>
Cc:     catalin.marinas@....com, will@...nel.org, sudeep.holla@....com,
        rafael@...nel.org, agross@...nel.org, andersson@...nel.org,
        konrad.dybcio@...aro.org, mingo@...hat.com, peterz@...radead.org,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, rui.zhang@...el.com,
        mhiramat@...nel.org, daniel.lezcano@...aro.org,
        amit.kachhap@...il.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] cpufreq: Add a cpufreq pressure feedback for the
 scheduler



On 12/14/23 07:57, Vincent Guittot wrote:
> On Thu, 14 Dec 2023 at 06:43, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>>
>> On 12-12-23, 15:27, Vincent Guittot wrote:
>>> @@ -2618,6 +2663,9 @@ static int cpufreq_set_policy(struct cpufreq_policy *policy,
>>>        policy->max = __resolve_freq(policy, policy->max, CPUFREQ_RELATION_H);
>>>        trace_cpu_frequency_limits(policy);
>>>
>>> +     cpus = policy->related_cpus;
>>> +     cpufreq_update_pressure(cpus, policy->max);
>>> +
>>>        policy->cached_target_freq = UINT_MAX;
>>
>> One more question, why are you doing this from cpufreq_set_policy ? If
>> due to cpufreq cooling or from userspace, we end up limiting the
>> maximum possible frequency, will this routine always get called ?
> 
> Yes, any update of a FREQ_QOS_MAX ends up calling cpufreq_set_policy()
> to update the policy->max
> 

Agree, cpufreq sysfs scaling_max_freq is also important to handle
in this new design. Currently we don't reflect that as reduced CPU
capacity in the scheduler. There was discussion when I proposed to feed
that CPU frequency reduction into thermal_pressure [1].

The same applies for the DTPM which is missing currently the proper
impact to the CPU reduced capacity in the scheduler.

IMHO any limit set into FREQ_QOS_MAX should be visible in this
new design of capacity reduction signaling.

[1] https://lore.kernel.org/lkml/20220930094821.31665-2-lukasz.luba@arm.com/