linux-kernel - Re: [PATCH v3 3/3] [RFC] CPUFreq: Add support for cpu-perf-dependencies

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Date:   Tue, 17 Nov 2020 10:47:16 +0000
From:   Nicola Mazzucato <nicola.mazzucato@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Lukasz Luba <lukasz.luba@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pm@...r.kernel.org, devicetree@...r.kernel.org,
        sudeep.holla@....com, rjw@...ysocki.net, vireshk@...nel.org,
        robh+dt@...nel.org, sboyd@...nel.org, nm@...com,
        daniel.lezcano@...aro.org, morten.rasmussen@....com,
        chris.redpath@....com
Subject: Re: [PATCH v3 3/3] [RFC] CPUFreq: Add support for
 cpu-perf-dependencies

Hi Viresh,


On 11/17/20 10:11 AM, Viresh Kumar wrote:
> On 16-11-20, 11:33, Lukasz Luba wrote:
>> On 11/9/20 6:57 AM, Viresh Kumar wrote:
>>> On 06-11-20, 11:14, Lukasz Luba wrote:
>>>> I also had similar doubts, because if we make frequency requests
>>>> independently for each CPU, why not having N cooling devs, which
>>>> will set independently QoS max freq for them...
>>>>
>>>> What convinced me:
>>>> EAS and FIE would know the 'real' frequency of the cluster, IPA
>>>> can use it also and have only one cooling device per cluster.
>>>>
>>>> We would like to keep this old style 'one cooling device per cpuset'.
>>>> I don't have strong opinion and if it would appear that there are
>>>> some errors in freq estimation for cluster, then maybe it does make
>>>> more sense to have cdev per CPU...
>>>
>>> Let me rephrase my question. What is it that doesn't work _correctly_
>>> with cdev per cpufreq policy in your case? What doesn't work well if
>>> the thermal stuff keeps looking at only the related_cpus thing and not
>>> the cpu-perf-dependencies thing?
>>>
>>
>> We don't have a platform which would be this per-cpu freq request, yet.
>> Thus it's hard to answer your question. The EAS would work in 'old
>> style' - cluster mode. I don't know how IPA would work on such HW
>> and SW configuration. To figure this out I need a real platform.
> 
> Hmm, so who are going to be the users of this new stuff (dependent
> CPUs) ?

In general, any platform that has hardware coordination in place and some
components need to use the information.

I don't think cpufreq-cooling should be updated, unless there
> is a compelling reason to.
> 
> The other one in energy model ? Why does it need this information ?

The reasons has probably gone lost in the emails, but in a nutshell EM needs
accurate information on performance boundaries to achieve correct task placement.

> 
> Who else ?
> 

Freq-invariance has been mentioned. I suppose the fix will depend on which
strategy we prefer to solve this.

As a reminder, two solutions:
1) dependent_cpus cpumask in cpufreq and involved entities pick this info
or
2) dependent_cpus cpumask in driver but some entities' interfaces may need to change

Hope it helps,
Nicola