linux-kernel - Re: [PATCH v4 2/3] sched/fair: Take thermal pressure into account while estimating energy

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Message-ID: <813b4ea6-97b0-f98b-5fe1-2ae2c1ff1ab0@arm.com>
Date:   Wed, 16 Jun 2021 19:31:19 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        peterz@...radead.org, rjw@...ysocki.net, viresh.kumar@...aro.org,
        vincent.guittot@...aro.org, qperret@...gle.com,
        vincent.donnefort@....com, Beata.Michalska@....com,
        mingo@...hat.com, juri.lelli@...hat.com, rostedt@...dmis.org,
        segall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        thara.gopinath@...aro.org, amit.kachhap@...il.com,
        amitk@...nel.org, rui.zhang@...el.com, daniel.lezcano@...aro.org
Subject: Re: [PATCH v4 2/3] sched/fair: Take thermal pressure into account
 while estimating energy



On 6/16/21 6:24 PM, Dietmar Eggemann wrote:
> On 15/06/2021 18:09, Lukasz Luba wrote:
>>
>> On 6/15/21 4:31 PM, Dietmar Eggemann wrote:
>>> On 14/06/2021 21:11, Lukasz Luba wrote:
> 
> [...]
> 
>>> It's important to highlight that this will only fix this issue between
>>> schedutil and EAS when it's due to `thermal pressure` (today only via
>>> CPU cooling). There are other places which could restrict policy->max
>>> via freq_qos_update_request() and EAS will be unaware of it.
>>
>> True, but for this I have some other plans.
> 
> As long as people are aware of the fact that this was developed to be
> beneficial for `EAS - IPA` integration, I'm fine with this.

Good. I had in mind that I will have to do some re-work on this
thermal pressure code in the cpufreq cooling, to satisfy our roadmap
goals...

> 
> [...]
> 
>>> IMHO, this means that this is catered for the IPA governor then. I'm not
>>> sure if this would be beneficial when another thermal governor is used?
>>
>> Yes, it will be, the cpufreq_set_cur_state() is called by
>> thermal exported function:
>> thermal_cdev_update()
>>    __thermal_cdev_update()
>>      thermal_cdev_set_cur_state()
>>        cdev->ops->set_cur_state(cdev, target)
>>
>> So it can be called not only by IPA. All governors call it, because
>> that's the default mechanism.
> 
> True, but I'm still not convinced that it is useful outside `EAS - IPA`.

It is. So in mainline thermal there is another governor: fair_share [1],
which uses 'weights' to split the cooling effort across cooling devices
in the thermal zone. That governor would manage CPUs and GPU and
set throttling like IPA.

> 
>>> The mechanical side of the code would allow for such benefits, I just
>>> don't know if their CPU cooling device + thermal zone setups would cater
>>> for this?
>>
>> Yes, it's possible. Even for custom vendor governors (modified clones
>> of IPA)
> 
> Let's stick to mainline here ;-) It's complicated enough ...

I agree, so there isn't only IPA in mainline.

> 
> [...]
> 
>>> Maybe shorter?
>>>
>>>           struct cpumask *pd_mask = perf_domain_span(pd);
>>> -       unsigned long cpu_cap =
>>> arch_scale_cpu_capacity(cpumask_first(pd_mask));
>>> +       int cpu = cpumask_first(pd_mask);
>>> +       unsigned long cpu_cap = arch_scale_cpu_capacity(cpu);
>>> +       unsigned long _cpu_cap = cpu_cap -
>>> arch_scale_thermal_pressure(cpu);
>>>           unsigned long max_util = 0, sum_util = 0;
>>> -       unsigned long _cpu_cap = cpu_cap;
>>> -       int cpu;
>>> -
>>> -       _cpu_cap -= arch_scale_thermal_pressure(cpumask_first(pd_mask));
>>
>> Could be, but still, the definitions should be sorted from longest on
>> top, to shortest at the bottom. I wanted to avoid modifying too many
>> lines with this simple patch.
> 
> Only if there are no dependencies, but here we have already `cpu_cap ->
> pd_mask`. OK, not a big deal.

True, those dependencies are tricky to sort them properly, so I coded
it this way.

[snip]

>> I see what you mean, but this might cause some issues in the design
>> (per-cpu scmi cpu perf control). Let's use this EM pointer gently ;)
> 
> OK, with the requirement that clients see the EM as ro:
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
> 

Thank you Dietmar for the review!

Regards,
Lukasz

[1] 
https://elixir.bootlin.com/linux/v5.13-rc6/source/drivers/thermal/gov_fair_share.c#L111