Re: [PATCH v2 2/2] sched/cpufreq: Consider reduced CPU capacity in energy calculation

From: Lukasz Luba
Date: Thu Jun 10 2021 - 04:19:15 EST

On 6/9/21 4:01 PM, Rafael J. Wysocki wrote:
On Fri, Jun 4, 2021 at 10:10 AM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:

Energy Aware Scheduling (EAS) needs to predict the decisions made by
SchedUtil. The map_util_freq() exists to do that.

There are corner cases where the max allowed frequency might be reduced
(due to thermal). SchedUtil as a CPUFreq governor, is aware of that
but EAS is not. This patch aims to address it.

SchedUtil stores the maximum allowed frequency in
'sugov_policy::next_freq' field. EAS has to predict that value, which is
the real used frequency. That value is made after a call to
cpufreq_driver_resolve_freq() which clamps to the CPUFreq policy limits.
In the existing code EAS is not able to predict that real frequency.
This leads to energy estimation errors.

To avoid wrong energy estimation in EAS (due to frequency miss prediction)
make sure that the step which calculates Performance Domain frequency,
is also aware of the allowed CPU capacity.

Furthermore, modify map_util_freq() to not extend the frequency value.
Instead, use map_util_perf() to extend the util value in both places:
SchedUtil and EAS, but for EAS clamp it to max allowed CPU capacity.
In the end, we achieve the same desirable behavior for both subsystems
and alignment in regards to the real CPU frequency.

Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>

For the schedutil part

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

Thank you Rafael!