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

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Date:   Wed, 16 Jun 2021 19:24:36 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Lukasz Luba <lukasz.luba@....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 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.

[...]

>> 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`.

>> 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 ...

[...]

>> 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.

[...]

>> There is IPA specific code in cpufreq_set_cur_state() ->
>> get_state_freq() which accesses the EM:
>>
>>      ...
>>      return cpufreq_cdev->em->table[idx].frequency;
>>      ...
>>
>> Has it been discussed that the `per-PD max (allowed) CPU capacity` (1)
>> could be stored in the EM from there so that code like the EAS wakeup
>> code (compute_energy()) could retrieve this information from the EM?
> 
> No, we haven't think about this approach in these patch sets.
> The EM structure given to the cpufreq_cooling device and stored in:
> cpufreq_cdev->em should not be modified. There are a few places which
> receive the EM, but they all should not touch it. For those clients
> it's a read-only data structure.
> 
>> And there wouldn't be any need to pass (1) into the EM (like now via
>> em_cpu_energy()).
>> This would be signalling within the EM compared to external signalling
>> via `CPU cooling -> thermal pressure <- EAS wakeup -> EM`.
> 
> 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>