linux-kernel - Re: [PATCH V3 3/3] thermal/cpu-cooling: Update thermal pressure in case of a maximum frequency capping

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Message-ID: <5CC30C09.4020005@linaro.org>
Date:   Fri, 26 Apr 2019 09:47:53 -0400
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Quentin Perret <quentin.perret@....com>
Cc:     mingo@...hat.com, peterz@...radead.org, rui.zhang@...el.com,
        linux-kernel@...r.kernel.org, amit.kachhap@...il.com,
        viresh.kumar@...aro.org, javi.merino@...nel.org,
        edubezval@...il.com, daniel.lezcano@...aro.org,
        vincent.guittot@...aro.org, nicolas.dechesne@...aro.org,
        bjorn.andersson@...aro.org, dietmar.eggemann@....com
Subject: Re: [PATCH V3 3/3] thermal/cpu-cooling: Update thermal pressure in
 case of a maximum frequency capping

On 04/25/2019 06:45 AM, Quentin Perret wrote:
> On Tuesday 23 Apr 2019 at 18:38:46 (-0400), Thara Gopinath wrote:
>> I think there is one major difference between user-defined frequency
>> constraints and frequency constraints due to thermal events in terms of
>> the time period the system spends in the the constraint state.
>> Typically, a user constraint lasts for seconds if not minutes and I
>> think in this case cpu_capacity_orig should reflect this constraint and
>> not cpu_capacity like this patch set.
> 
> That might not always be true I think. There's tons of userspace thermal
> deamons out there, and I wouldn't be suprised if they were writing into
> the cpufreq sysfs files, although I'm not sure.
> 
> Another thing is, if you want to change the capacity_orig value, you'll
> need to rebuild the sched domains and all I believe. Otherwise there is
> a risk to 'break' the sd_asym flags. So we need to make sure we're happy
> to pay that price.

Hi Quentin,

I saw Vincent's reply on this and my answer is similar. I completely
agree that this will involve a rebuild of sched domains. My thought on
cpufreq capping max frequency from the user space is that the capping is
for long term basis and hence we could live with re-building sched
domains. If user space wants to control the max frequency of a cpu for
thermal reasons then the cooling device sys interface should be used. In
practical scenario, I am interested in knowing why thermal daemons
control cpufreq sysfs files instead of cooling device files.

Regards
Thara

> 
>> Also, in case of the user
>> constraint, there is possibly no need to accumulate and average the
>> capacity constraints and instantaneous values can be directly applied to
>> cpu_capacity_orig. On the other hand thermal pressure is more spiky and
>> sometimes in the order of ms and us requiring the accumulating and
>> averaging.
>>>
>>> Perhaps the Intel boost stuff could be factored in there ? That is,
>>> at times when the boost freq is not reachable capacity_of() would appear
>>> smaller ... Unless this wants to be reflected instantaneously ?
>> Again, do you think intel boost is more applicable to be reflected in
>> cpu_capacity_orig and not cpu_capacity?
> 
> I'm not even sure if we want to reflect it at all TBH, but I'd be
> interested to see what Intel folks think :-)
> 
> Thanks,
> Quentin
> 


-- 
Regards
Thara