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

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

Hi Rafael,

Thank you for having a loot at the series.

On 12/12/23 18:49, Rafael J. Wysocki wrote:
> 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.

Let me contact offline some of the partners who were keen to have this
in mainline (when I presented some first implementation in 2021 at
Android kernel review systems).

Regards,
Lukasz