Re: [PATCH v6 2/2] cpuidle: teo: Introduce util-awareness

[Lukasz Luba <lukasz.luba@arm.com>]

On 1/5/23 15:07, Rafael J. Wysocki wrote:
> On Thu, Jan 5, 2023 at 3:52 PM Kajetan Puchalski
> <kajetan.puchalski@arm.com> wrote:
>>
>> Modern interactive systems, such as recent Android phones, tend to have power
>> efficient shallow idle states. Selecting deeper idle states on a device while a
>> latency-sensitive workload is running can adversely impact performance due to
>> increased latency. Additionally, if the CPU wakes up from a deeper sleep before
>> its target residency as is often the case, it results in a waste of energy on
>> top of that.
>>
>> At the moment, none of the available idle governors take any scheduling
>> information into account. They also tend to overestimate the idle
>> duration quite often, which causes them to select excessively deep idle
>> states, thus leading to increased wakeup latency and lower performance with no
>> power saving. For 'menu' while web browsing on Android for instance, those
>> types of wakeups ('too deep') account for over 24% of all wakeups.
>>
>> At the same time, on some platforms idle state 0 can be power efficient
>> enough to warrant wanting to prefer it over idle state 1. This is because
>> the power usage of the two states can be so close that sufficient amounts
>> of too deep state 1 sleeps can completely offset the state 1 power saving to the
>> point where it would've been more power efficient to just use state 0 instead.
>> This is of course for systems where state 0 is not a polling state, such as
>> arm-based devices.
>>
>> Sleeps that happened in state 0 while they could have used state 1 ('too shallow') only
>> save less power than they otherwise could have. Too deep sleeps, on the other
>> hand, harm performance and nullify the potential power saving from using state 1 in
>> the first place. While taking this into account, it is clear that on balance it
>> is preferable for an idle governor to have more too shallow sleeps instead of
>> more too deep sleeps on those kinds of platforms.
>>
>> This patch specifically tunes TEO to prefer shallower idle states in
>> order to reduce wakeup latency and achieve better performance.
>> To this end, before selecting the next idle state it uses the avg_util signal
>> of a CPU's runqueue in order to determine to what extent the CPU is being utilized.
>> This util value is then compared to a threshold defined as a percentage of the
>> cpu's capacity (capacity >> 6 ie. ~1.5% in the current implementation). If the
>> util is above the threshold, the idle state selected by TEO metrics will be
>> reduced by 1, thus selecting a shallower state. If the util is below the threshold,
>> the governor defaults to the TEO metrics mechanism to try to select the deepest
>> available idle state based on the closest timer event and its own correctness.
>>
>> The main goal of this is to reduce latency and increase performance for some
>> workloads. Under some workloads it will result in an increase in power usage
>> (Geekbench 5) while for other workloads it will also result in a decrease in
>> power usage compared to TEO (PCMark Web, Jankbench, Speedometer).
>>
>> It can provide drastically decreased latency and performance benefits in certain
>> types of workloads that are sensitive to latency.
>>
>> Example test results:
>>
>> 1. GB5 (better score, latency & more power usage)
>>
>> | metric                                | menu           | teo               | teo-util-aware    |
>> | ------------------------------------- | -------------- | ----------------- | ----------------- |
>> | gmean score                           | 2826.5 (0.0%)  | 2764.8 (-2.18%)   | 2865 (1.36%)      |
>> | gmean power usage [mW]                | 2551.4 (0.0%)  | 2606.8 (2.17%)    | 2722.3 (6.7%)     |
>> | gmean too deep %                      | 14.99%         | 9.65%             | 4.02%             |
>> | gmean too shallow %                   | 2.5%           | 5.96%             | 14.59%            |
>> | gmean task wakeup latency (asynctask) | 78.16μs (0.0%) | 61.60μs (-21.19%) | 54.45μs (-30.34%) |
>>
>> 2. Jankbench (better score, latency & less power usage)
>>
>> | metric                                | menu           | teo               | teo-util-aware    |
>> | ------------------------------------- | -------------- | ----------------- | ----------------- |
>> | gmean frame duration                  | 13.9 (0.0%)    | 14.7 (6.0%)       | 12.6 (-9.0%)      |
>> | gmean jank percentage                 | 1.5 (0.0%)     | 2.1 (36.99%)      | 1.3 (-17.37%)     |
>> | gmean power usage [mW]                | 144.6 (0.0%)   | 136.9 (-5.27%)    | 121.3 (-16.08%)   |
>> | gmean too deep %                      | 26.00%         | 11.00%            | 2.54%             |
>> | gmean too shallow %                   | 4.74%          | 11.89%            | 21.93%            |
>> | gmean wakeup latency (RenderThread)   | 139.5μs (0.0%) | 116.5μs (-16.49%) | 91.11μs (-34.7%)  |
>> | gmean wakeup latency (surfaceflinger) | 124.0μs (0.0%) | 151.9μs (22.47%)  | 87.65μs (-29.33%) |
>>
>> Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>
> 
> This looks good enough for me.
> 
> There are still a couple of things I would change in it, but I may as
> well do that when applying it, so never mind.
> 
> The most important question for now is what the scheduler people think
> about calling sched_cpu_util() from a CPU idle governor.  Peter,
> Vincent?
> 

We have a precedence in thermal framework for purpose of thermal
governor - IPA. It's been there for a while to estimate the power
of CPUs in the frequency domain for cpufreq_cooling device [1].
That's how this API sched_cpu_util() got created. Then it was also
adopted to PowerCap DTPM [2] (for the same power estimation purpose).

It's a function available with form include/linux/sched.h so I don't
see reasons why to not use it.

[1] 
https://elixir.bootlin.com/linux/latest/source/drivers/thermal/cpufreq_cooling.c#L151
[2] 
https://elixir.bootlin.com/linux/latest/source/drivers/powercap/dtpm_cpu.c#L83