linux-kernel - Re: [PATCH V4 3/3] thermal: cpufreq_cooling: Reuse sched_cpu

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Date:   Tue, 8 Dec 2020 13:31:26 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Amit Daniel Kachhap <amit.kachhap@...il.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Javi Merino <javi.merino@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Amit Kucheria <amitk@...nel.org>, linux-kernel@...r.kernel.org,
        Quentin Perret <qperret@...gle.com>,
        Lukasz Luba <lukasz.luba@....com>, linux-pm@...r.kernel.org
Subject: Re: [PATCH V4 3/3] thermal: cpufreq_cooling: Reuse sched_cpu_util()
 for SMP platforms

On 07/12/2020 13:17, Viresh Kumar wrote:
> On 03-12-20, 12:54, Dietmar Eggemann wrote:
>> On 24/11/2020 07:26, Viresh Kumar wrote:

[...]

>> When I ran schbench (-t 16 -r 5) on hikey960 I get multiple (~50)
>> instances of ~80ms task activity phase and then ~20ms idle phase on all
>> CPUs.
>>
>> So I assume that scenario 1 is at the beginning (but you mentioned the
>> task were sleeping?)
> 
> I am not able to find the exact values I used, but I did something
> like this to create a scenario where the old computations shall find
> the CPU as idle in the last IPA window:
> 
> - schbench -m 2 -t 4 -s 25000 -c 20000 -r 60
> 
> - sampling rate of IPA to 10 ms
> 
> With this IPA wakes up many times and finds the CPUs to have been idle
> in the last IPA window (i.e. 10ms).

Ah, this makes sense.

So with this there are only 8 worker threads w/ 20ms runtime and 75ms
period (30ms message thread time (-C) and 25 latency (-c)).

So much more idle time between two invocations of the worker/message
threads and more IPA sampling.

[...]

>>>  Old: thermal_power_cpu_get_power: cpus=00000000,000000ff freq=1200000 total_load=800 load={{0x64,0x64,0x64,0x64,0x64,0x64,0x64,0x64}} dynamic_power=5280
>>>  New: thermal_power_cpu_get_power: cpus=00000000,000000ff freq=1200000 total_load=708 load={{0x4d,0x5c,0x5c,0x5b,0x5c,0x5c,0x51,0x5b}} dynamic_power=4672
>>>
>>> As can be seen, the idle time based load is 100% for all the CPUs as it
>>> took only the last window into account, but in reality the CPUs aren't
>>> that loaded as shown by the utilization numbers.
>>
>> Is this an IPA sampling at the end of the ~20ms idle phase?
> 
> This is during the phase where the CPUs were fully busy for the last
> period.

OK.