Date:   Wed, 13 Dec 2023 13:16:18 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     adharmap@...cinc.com
Cc:     dietmar.eggemann@....com, rui.zhang@...el.com,
        amit.kucheria@...durent.com, amit.kachhap@...il.com,
        daniel.lezcano@...aro.org, viresh.kumar@...aro.org,
        len.brown@...el.com, pavel@....cz, mhiramat@...nel.org,
        qyousef@...alina.io, wvw@...gle.com, rafael@...nel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 00/23] Introduce runtime modifiable Energy Model

Hi Abhijeet,

It's been a while when we discussed an EM feature presented on some
Android common kernel Gerrit (Nov 2021).

On 11/29/23 11:08, Lukasz Luba 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
> v4 changes are here [4]
> 
> Regards,
> Lukasz Luba
> 
> [1] https://lpc.events/event/16/contributions/1341/attachments/955/1873/Dynamic_Energy_Model_to_handle_leakage_power.pdf
> [2] https://lpc.events/event/16/contributions/1194/attachments/1114/2139/LPC2022_Energy_model_accuracy.pdf
> [3] https://www.youtube.com/watch?v=2C-5uikSbtM&list=PL0fKordpLTjKsBOUcZqnzlHShri4YBL1H
> [4] https://lore.kernel.org/lkml/20230925081139.1305766-1-lukasz.luba@arm.com/
> 
> 
> Lukasz Luba (23):
>    PM: EM: Add missing newline for the message log
>    PM: EM: Refactor em_cpufreq_update_efficiencies() arguments
>    PM: EM: Find first CPU active while updating OPP efficiency
>    PM: EM: Refactor em_pd_get_efficient_state() to be more flexible
>    PM: EM: Refactor a new function em_compute_costs()
>    PM: EM: Check if the get_cost() callback is present in
>      em_compute_costs()
>    PM: EM: Refactor how the EM table is allocated and populated
>    PM: EM: Introduce runtime modifiable table
>    PM: EM: Use runtime modified EM for CPUs energy estimation in EAS
>    PM: EM: Add API for memory allocations for new tables
>    PM: EM: Add API for updating the runtime modifiable EM
>    PM: EM: Add helpers to read under RCU lock the EM table
>    PM: EM: Add performance field to struct em_perf_state
>    PM: EM: Support late CPUs booting and capacity adjustment
>    PM: EM: Optimize em_cpu_energy() and remove division
>    powercap/dtpm_cpu: Use new Energy Model interface to get table
>    powercap/dtpm_devfreq: Use new Energy Model interface to get table
>    drivers/thermal/cpufreq_cooling: Use new Energy Model interface
>    drivers/thermal/devfreq_cooling: Use new Energy Model interface
>    PM: EM: Change debugfs configuration to use runtime EM table data
>    PM: EM: Remove old table
>    PM: EM: Add em_dev_compute_costs() as API for device drivers
>    Documentation: EM: Update with runtime modification design
> 
>   Documentation/power/energy-model.rst | 206 +++++++++++-
>   drivers/powercap/dtpm_cpu.c          |  35 +-
>   drivers/powercap/dtpm_devfreq.c      |  31 +-
>   drivers/thermal/cpufreq_cooling.c    |  40 ++-
>   drivers/thermal/devfreq_cooling.c    |  43 ++-
>   include/linux/energy_model.h         | 163 +++++----
>   kernel/power/energy_model.c          | 479 +++++++++++++++++++++++----
>   7 files changed, 813 insertions(+), 184 deletions(-)
> 

You've been interested in this feature back then.

I have a gentle ask, if you are still interested in. It would be nice if
you (or some other Qcom engineer) could leave a feedback comment
(similar what you have made for the Gerrit original series). I will be
really grateful.

In this cover letter, there are some power saving numbers from
a real phone, with also performance metrics (janky frames). You might
be interested in those scenarios as well.

Regards,
Lukasz

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