Re: [PATCH v5 00/23] Introduce runtime modifiable Energy Model

Rafael J. Wysocki
2023-12-12T18:49:58Z

Hi Lukasz,

On Wed, Nov 29, 2023 at 12:08 PM Lukasz Luba <lukasz.luba@arm.com> wrote:
>
> Hi all,
>
> This patch set adds a new feature which allows to modify Energy Model (EM)
> power values at runtime. It will allow to better reflect power model of
> a recent SoCs and silicon. Different characteristics of the power usage
> can be leveraged and thus better decisions made during task placement in EAS.
>
> It's part of feature set know as Dynamic Energy Model. It has been presented
> and discussed recently at OSPM2023 [3]. This patch set implements the 1st
> improvement for the EM.
>
> The concepts:
> 1. The CPU power usage can vary due to the workload that it's running or due
> to the temperature of the SoC. The same workload can use more power when the
> temperature of the silicon has increased (e.g. due to hot GPU or ISP).
> In such situation the EM can be adjusted and reflect the fact of increased
> power usage. That power increase is due to static power
> (sometimes called simply: leakage). The CPUs in recent SoCs are different.
> We have heterogeneous SoCs with 3 (or even 4) different microarchitectures.
> They are also built differently with High Performance (HP) cells or
> Low Power (LP) cells. They are affected by the temperature increase
> differently: HP cells have bigger leakage. The SW model can leverage that
> knowledge.
>
> 2. It is also possible to change the EM to better reflect the currently
> running workload. Usually the EM is derived from some average power values
> taken from experiments with benchmark (e.g. Dhrystone). The model derived
> from such scenario might not represent properly the workloads usually running
> on the device. Therefore, runtime modification of the EM allows to switch to
> a different model, when there is a need.
>
> 3. The EM can be adjusted after boot, when all the modules are loaded and
> more information about the SoC is available e.g. chip binning. This would help
> to better reflect the silicon characteristics. Thus, this EM modification
> API allows it now. It wasn't possible in the past and the EM had to be
> 'set in stone'.
>
> More detailed explanation and background can be found in presentations
> during LPC2022 [1][2] or in the documentation patches.
>
> Some test results.
> The EM can be updated to fit better the workload type. In the case below the EM
> has been updated for the Jankbench test on Pixel6 (running v5.18 w/ mainline backports
> for the scheduler bits). The Jankbench was run 10 times for those two configurations,
> to get more reliable data.
>
> 1. Janky frames percentage
> +--------+-----------------+---------------------+-------+-----------+
> | metric | variable | kernel | value | perc_diff |
> +--------+-----------------+---------------------+-------+-----------+
> | gmean | jank_percentage | EM_default | 2.0 | 0.0% |
> | gmean | jank_percentage | EM_modified_runtime | 1.3 | -35.33% |
> +--------+-----------------+---------------------+-------+-----------+
>
> 2. Avg frame render time duration
> +--------+---------------------+---------------------+-------+-----------+
> | metric | variable | kernel | value | perc_diff |
> +--------+---------------------+---------------------+-------+-----------+
> | gmean | mean_frame_duration | EM_default | 10.5 | 0.0% |
> | gmean | mean_frame_duration | EM_modified_runtime | 9.6 | -8.52% |
> +--------+---------------------+---------------------+-------+-----------+
>
> 3. Max frame render time duration
> +--------+--------------------+---------------------+-------+-----------+
> | metric | variable | kernel | value | perc_diff |
> +--------+--------------------+---------------------+-------+-----------+
> | gmean | max_frame_duration | EM_default | 251.6 | 0.0% |
> | gmean | max_frame_duration | EM_modified_runtime | 115.5 | -54.09% |
> +--------+--------------------+---------------------+-------+-----------+
>
> 4. OS overutilized state percentage (when EAS is not working)
> +--------------+---------------------+------+------------+------------+
> | metric | wa_path | time | total_time | percentage |
> +--------------+---------------------+------+------------+------------+
> | overutilized | EM_default | 1.65 | 253.38 | 0.65 |
> | overutilized | EM_modified_runtime | 1.4 | 277.5 | 0.51 |
> +--------------+---------------------+------+------------+------------+
>
> 5. All CPUs (Little+Mid+Big) power values in mW
> +------------+--------+---------------------+-------+-----------+
> | channel | metric | kernel | value | perc_diff |
> +------------+--------+---------------------+-------+-----------+
> | CPU | gmean | EM_default | 142.1 | 0.0% |
> | CPU | gmean | EM_modified_runtime | 131.8 | -7.27% |
> +------------+--------+---------------------+-------+-----------+
>
> The time cost to update the EM decreased in this v5 vs v4:
> big: 5us vs 2us -> 2.6x faster
> mid: 9us vs 3us -> 3x faster
> little: 16us vs 16us -> no change
>
> We still have to update the inefficiency in the cpufreq framework, thus
> a bit of overhead will be there.
>
> Changelog:
> v5:
> - removed 2 tables design
> - have only one table (runtime_table) used also in thermal (Wei, Rafael)
> - refactored update function and removed callback call for each opp
> - added faster EM table swap, using only the RCU pointer update
> - added memory allocation API and tracking with kref
> - avoid overhead for computing 'cost' for each OPP in update, it can be
> pre-computed in device drivers EM earlier
> - add support for device drivers providing EM table
> - added API for computing 'cost' values in EM for EAS
> - added API for thermal/powercap to use EM (using RCU wrappers)
> - switched to single allocation and 'state[]' array (Rafael)
> - changed documentation to align with current design
> - added helper API for computing cost values
> - simplified EM free in unregister path (thanks to kref)
> - split patch updating EM clients and changed them separetly
> - added seperate patch removing old static EM table
> - added EM debugfs change patch to dump the runtime_table
> - addressed comments in v4 for spelling/comments/headers
> - added review tags

I like this one more than the previous one and thanks for taking my
feedback into account.

I would still like other people having a vested interest in the EM to
look at it and give feedback (or just tags), so I'm not inclined to
apply it just yet. However, I don't have any specific comments on it.