Date:   Fri, 28 Oct 2022 15:22:59 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Kajetan Puchalski <kajetan.puchalski@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>, daniel.lezcano@...aro.org,
        lukasz.luba@....com, Dietmar.Eggemann@....com, dsmythies@...us.net,
        yu.chen.surf@...il.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 0/1] cpuidle: teo: Introduce optional util-awareness

Hi,

On Thu, Oct 20, 2022 at 6:21 PM Kajetan Puchalski
<kajetan.puchalski@....com> wrote:
>
> Hi Rafael,
>
> > The avg_util value tells us nothing about how much the CPU is going to
> > be idle this time and it also tells us nothing about the
> > latency-sensitivity of the workload.
> >
> > Yes, it tells us how much idle time there was on the given CPU in the
> > past, on the average, but there is zero information about the
> > distribution of that idle time in it.
> >
> > So in the first place please tell me why it fundamentally makes sense
> > to use avg_util in CPU idle time management at all.
>
> I have an alternative suggestion that could be a reasonable way forward
> here. Instead of applying util-awareness on top of TEO where it would
> have to be reconciled with how TEO is currently expected to work, I just
> wrote a simple completely new governor which operates only on timer
> events alongside util values.
>
> The idea is this:
> 1. Find the deepest state based on residency and time until the next timer event
> 2. If sched_cpu_util() is above the threshold, select a shallower non-polling state
>
> There's no other metrics or anything else under the current
> implementation. I can't say how it would work on Intel systems and in
> the presence of more idle states but having a completely separate
> governor would be very useful for us to tune it specifically for our use
> cases and types of systems (ie. ones with 2 idle states and no polling
> states).

So this is not a totally bad idea IMV and the simplicity of this new
governor is certainly attractive.

However, it is likely to underperform in the cases when the interrupt
activity is not directly related to the average CPU load, like when
CPUs do much work, but they are not interrupted very often.

> As it stands it performs quite well and achieves better results
> (especially in terms of latency) than both menu & TEO but slightly worse
> than the previously suggested TEO + util.

Well, precisely, because teo takes other factors into account too.

> As far as we're concerned
> that's okay, we can work from there to try to find a way of doing
> metrics or improving the algorithm that would be more tailored to using
> the util approach. I think it's much cleaner than what we were
> discussing previously since that was effectively overriding most of what
> TEO was doing.
>
> Here are some numbers to visualize the results. They were all obtained
> in the same way as the ones in the cover letter so you can refer to that
> in case something isn't clear.
>
> 'teo_util' is of course TEO + util as in the patchset.
> 'idleutil' is this entirely new proposed minimal governor.
>
> 1. Geekbench 5 (latency-sensitive, heavy load test)
>
> +-----------------+----------+---------+-------------+
> | metric          | kernel   |   value | perc_diff   |
> |-----------------+----------+---------+-------------|
> | multicore_score | menu     |  2832.3 | 0.0%        |
> | multicore_score | teo      |  2815.3 | -0.6%       |
> | multicore_score | teo_util |  2880.6 | 1.7%        |
> | multicore_score | idleutil |  2859.3 | 0.95%       |
> +-----------------+----------+---------+-------------+
>
> Percentages & types of idle misses
>
> +-----------+-------------+--------------+
> | kernel    | type        |   percentage |
> |-----------+-------------+--------------|
> | menu      | too deep    |      15.613% |
> | teo       | too deep    |       9.376% |
> | teo_util  | too deep    |       4.581% |
> | idleutil  | too deep    |       5.464% |
> | menu      | too shallow |       2.611% |
> | teo       | too shallow |       6.099% |
> | teo_util  | too shallow |      14.141% |
> | idleutil  | too shallow |      13.282% |
> +-----------+-------------+--------------+
>
> Power usage [mW]
>
> +--------------+----------+----------+---------+-------------+
> | chan_name    | metric   | kernel   |   value | perc_diff   |
> |--------------+----------+----------+---------+-------------|
> | total_power  | gmean    | menu     |  2705.9 | 0.0%        |
> | total_power  | gmean    | teo      |  2668.2 | -1.39%      |
> | total_power  | gmean    | teo_util |  2710.2 | 0.16%       |
> | total_power  | gmean    | idleutil |  2657.9 | -1.78%      |
> +--------------+----------+----------+---------+-------------+
>
> Wakeup latency
>
> +-----------------+----------+----------+-------------+-------------+
> | comm            | metric   | kernel   |       value | perc_diff   |
> |-----------------+----------+----------+-------------+-------------|
> | AsyncTask #1    | gmean    | menu     | 66.85μs     | 0.0%        |
> | AsyncTask #1    | gmean    | teo      | 66.79μs     | -0.09%      |
> | AsyncTask #1    | gmean    | teo_util | 57.84μs     | -13.47%     |
> | AsyncTask #1    | gmean    | idleutil | 62.61μs     | -6.35%      |
> | labs.geekbench5 | gmean    | menu     | 80.62μs     | 0.0%        |
> | labs.geekbench5 | gmean    | teo      | 94.75μs     | 17.52%      |
> | labs.geekbench5 | gmean    | teo_util | 52.98μs     | -34.28%     |
> | labs.geekbench5 | gmean    | idleutil | 68.58μs     | -14.93%     |
> +-----------------+----------+----------+-------------+-------------+
>
> 2. PCMark Web Browsing (non latency-sensitive, normal usage test)
>
> +----------------+----------+---------+-------------+
> | metric         | kernel   |   value | perc_diff   |
> |----------------+----------+---------+-------------|
> | PcmaWebV2Score | menu     |  5232   | 0.0%        |
> | PcmaWebV2Score | teo      |  5219.8 | -0.23%      |
> | PcmaWebV2Score | teo_util |  5249.7 | 0.34%       |
> | PcmaWebV2Score | idleutil |  5215.7 | -0.31%      |
> +----------------+----------+---------+-------------+
>
> Percentages & types of idle misses
>
> +-----------+-------------+--------------+
> | kernel    | type        |   percentage |
> |-----------+-------------+--------------|
> | menu      | too deep    |      24.814% |
> | teo       | too deep    |       11.65% |
> | teo_util  | too deep    |       3.753% |
> | idleutil  | too deep    |       4.304% |
> | menu      | too shallow |       3.101% |
> | teo       | too shallow |       8.578% |
> | teo_util  | too shallow |      18.309% |
> | idleutil  | too shallow |      17.638% |
> +-----------+-------------+--------------+
>
> Power usage [mW]
>
> +--------------+----------+----------+---------+-------------+
> | chan_name    | metric   | kernel   |   value | perc_diff   |
> |--------------+----------+----------+---------+-------------|
> | total_power  | gmean    | menu     |   179.2 | 0.0%        |
> | total_power  | gmean    | teo      |   184.8 | 3.1%        |
> | total_power  | gmean    | teo_util |   180.5 | 0.71%       |
> | total_power  | gmean    | idleutil |   185   | 3.24%       |
> +--------------+----------+----------+---------+-------------+
>
> Wakeup latency
>
> +-----------------+----------+----------+-------------+-------------+
> | comm            | metric   | kernel   |       value | perc_diff   |
> |-----------------+----------+----------+-------------+-------------|
> | CrRendererMain  | gmean    | menu     | 236.63μs    | 0.0%        |
> | CrRendererMain  | gmean    | teo      | 201.85μs    | -14.7%      |
> | CrRendererMain  | gmean    | teo_util | 111.76μs    | -52.77%     |
> | CrRendererMain  | gmean    | idleutil | 105.55μs    | -55.39%     |
> | chmark:workload | gmean    | menu     | 100.30μs    | 0.0%        |
> | chmark:workload | gmean    | teo      | 80.20μs     | -20.04%     |
> | chmark:workload | gmean    | teo_util | 53.81μs     | -46.35%     |
> | chmark:workload | gmean    | idleutil | 71.29μs     | -28.92%     |
> | RenderThread    | gmean    | menu     | 37.97μs     | 0.0%        |
> | RenderThread    | gmean    | teo      | 31.69μs     | -16.54%     |
> | RenderThread    | gmean    | teo_util | 34.32μs     | -9.63%      |
> | RenderThread    | gmean    | idleutil | 35.78μs     | -5.77%      |
> | surfaceflinger  | gmean    | menu     | 97.57μs     | 0.0%        |
> | surfaceflinger  | gmean    | teo      | 98.86μs     | 1.31%       |
> | surfaceflinger  | gmean    | teo_util | 72.59μs     | -25.6%      |
> | surfaceflinger  | gmean    | idleutil | 56.23μs     | -42.37%     |
> +-----------------+----------+----------+-------------+-------------+
>
> I also have similar data for Jankbench & Speedometer with right about
> the same results, I'll skip those for now for brevity.
> Would you like me to send a patch with this new governor instead? What
> would you think about this instead of the previously suggested approach?

I would still kind of prefer to improve teo so it covers the known use
cases better, especially that modified teo is likely to give you
better results than the new simplistic one.

Please see my other reply for the possible direction of improvement.

Thanks!

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