linux-kernel - Re: [PATCH v4] sched/fair: unlink misfit task from cpu overutilized

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Date:   Wed, 25 Jan 2023 09:37:02 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Kajetan Puchalski <kajetan.puchalski@....com>
Cc:     mingo@...nel.org, peterz@...radead.org, dietmar.eggemann@....com,
        qyousef@...alina.io, rafael@...nel.org, viresh.kumar@...aro.org,
        vschneid@...hat.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, lukasz.luba@....com, wvw@...gle.com,
        xuewen.yan94@...il.com, han.lin@...iatek.com,
        Jonathan.JMChen@...iatek.com
Subject: Re: [PATCH v4] sched/fair: unlink misfit task from cpu overutilized

On Wed, 25 Jan 2023 at 02:46, Kajetan Puchalski
<kajetan.puchalski@....com> wrote:
>
> Hi,
>
> > By taking into account uclamp_min, the 1:1 relation between task misfit
> > and cpu overutilized is no more true as a task with a small util_avg may
> > not fit a high capacity cpu because of uclamp_min constraint.
> >
> > Add a new state in util_fits_cpu() to reflect the case that task would fit
> > a CPU except for the uclamp_min hint which is a performance requirement.
> >
> > Use -1 to reflect that a CPU doesn't fit only because of uclamp_min so we
> > can use this new value to take additional action to select the best CPU
> > that doesn't match uclamp_min hint.
> >
>
> Just wanted to include some more test data here to flag potential issues
> with how all these changes use thermal pressure in the scheduler.
>
> For the tables below, 'baseline' is pre the already merged "uclamp fits
> capacity" patchset.
> 'baseline_ufc' is the current mainline with said patchset applied.
> The 'no_thermal' variants are variants with thermal handling taken out
> of util_fits_cpu akin to the v1 variant of the patchset.
> The 'patched' variants are the above but with the v4 of the 'unlink misfit
> task' patch applied as well.
>
> Geekbench 5:
>
> +-----------------+-------------------------+--------+-----------+
> |     metric      |         kernel          | value  | perc_diff |
> +-----------------+-------------------------+--------+-----------+
> | multicore_score |        baseline         | 2765.4 |   0.0%    |
> | multicore_score |      baseline_ufc       | 2704.3 |  -2.21%   | <-- mainline score regression
> | multicore_score | baseline_ufc_no_thermal | 2870.8 |   3.81%   | <-- ~170 pts better without thermal
> | multicore_score |     ufc_patched_v4      | 2839.8 |   2.69%   | <-- no more regression but worse than above
> | multicore_score | ufc_patched_no_thermal  | 2891.0 |   4.54%   | <-- best score
> +-----------------+-------------------------+--------+-----------+
>
> +--------------+--------+-------------------------+--------+-----------+
> |  chan_name   | metric |         kernel          | value  | perc_diff |
> +--------------+--------+-------------------------+--------+-----------+
> | total_power  | gmean  |        baseline         | 2664.0 |   0.0%    |
> | total_power  | gmean  |      baseline_ufc       | 2621.5 |   -1.6%   |
> | total_power  | gmean  | baseline_ufc_no_thermal | 2710.0 |   1.73%   |
> | total_power  | gmean  |     ufc_patched_v4      | 2729.0 |   2.44%   |
> | total_power  | gmean  | ufc_patched_no_thermal  | 2778.1 |   4.28%   |
> +--------------+--------+-------------------------+--------+-----------+
>
> (Jankbench scores show more or less no change, Jankbench power below)
>
> +--------------+--------+------------------------------+-------+-----------+
> |  chan_name   | metric |            kernel            | value | perc_diff |
> +--------------+--------+------------------------------+-------+-----------+
> | total_power  | gmean  |        baseline_60hz         | 135.9 |   0.0%    |
> | total_power  | gmean  |      baseline_ufc_60hz       | 155.7 |  14.61%   | <-- mainline power usage regression
> | total_power  | gmean  | baseline_ufc_no_thermal_60hz | 134.5 |  -1.01%   | <-- no more regression without the thermal bit
> | total_power  | gmean  |     ufc_patched_v4_60hz      | 131.4 |  -3.26%   | <-- no more regression with the patch either
> | total_power  | gmean  | ufc_patched_no_thermal_60hz  | 140.4 |   3.37%   | <-- both combined increase power usage
> +--------------+--------+------------------------------+-------+-----------+
>
> Specifically the swing of +15% power to -1% power by taking out thermal
> handling from util_fits_cpu on the original patchset is noteworthy. It
> shows that there's some subtle thermal-related interaction there taking
> place that can have adverse effects on power usage.
>
> Even more interestingly, the 'unlink misfit task' patch appears to be
> preventing said interaction from happening down the line as it has a
> similar effect to that of just taking out the thermal bits.
>
> My only concern here is that without taking a closer look at the thermal
> parts this power issue shown above could easily accidentally be
> reintroduced at some point in the future.

Yes, the handling of thermal pressure needs some closer look. It has
been agreed to keep the current behavior for now and have a closer
look on thermal pressure as a next step. And your results for
ufc_patched_v4_* provide some reasonably good perf and power figures.

Thanks

>
> > --
> > 2.34.1
> >