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

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Date:   Thu, 14 Dec 2023 10:41:05 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        catalin.marinas@....com, will@...nel.org, sudeep.holla@....com,
        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 09:40, Rafael J. Wysocki wrote:
> On Thu, Dec 14, 2023 at 10:07 AM Lukasz Luba <lukasz.luba@....com> wrote:
>>
>> 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/
> 
> Actually, freq_qos_read_value(&policy->constraints, FREQ_QOS_MAX) will
> return the requisite limit.

Yes, but we need to translate that information from freq domain
into capacity domain and plumb ii into scheduler as stolen CPU capacity.
Ideally, w/o any 'smoothing' but just instant value.
That's the hope of this patch set re-design.