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

Lukasz Luba
2023-11-29T11:08:06Z

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(-)

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2.25.1