[PATCH v2 00/17] Introduce runtime modifiable Energy Model

From: Lukasz Luba
Date: Fri May 12 2023 - 05:58:32 EST

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 or EM can be adjusted and reflect the fact of increased
power usage. That power increase is due to a factor called 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
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'.

Some design details:
The internal mechanisms for the memory allocation are handled internally in the
EM. Kernel modules can just call the new API to update the EM data and the
new memory would be provided and owned by the EM. The EM memory is used by
EAS, which impacts those design decisions. The EM writers are protected by
a mutex. This new runtime modified EM table is protected using RCU mechanism,
which fits the current EAS hot path (which already uses RCU read lock).
The unregister API handles only non-CPU (e.g. GPU, ISP) devices and uses the
same mutex as EM modifiers to make sure the memory is safely freed.

More detailed explanation and background can be found in presentations
during LPC2022 [1][2] or in the documentation patches.

- solved build warning of unused variable in patch 13/17 when EM is
not compiled in, e.g. on Intel platform for this cpufreq_cooling
- re-based on top of v6.4-rc1
- implementation can be found here [4]

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/20230314103357.26010-1-lukasz.luba@xxxxxxx/

Lukasz Luba (17):
PM: EM: Refactor em_cpufreq_update_efficiencies() arguments
PM: EM: Find first CPU online while updating OPP efficiency
PM: EM: Refactor em_pd_get_efficient_state() to be more flexible
PM: EM: Create a new function em_compute_costs()
trace: energy_model: Add trace event for EM runtime modifications
PM: EM: Add update_power() callback for runtime modifications
PM: EM: Check if the get_cost() callback is present in
PM: EM: Introduce runtime modifiable table
PM: EM: Add RCU mechanism which safely cleans the old data
PM: EM: Add runtime update interface to modify EM power
PM: EM: Use runtime modified EM for CPUs energy estimation in EAS
PM: EM: Add argument to get_cost() for runtime modification
PM: EM: Refactor struct em_perf_domain and add default_table
Documentation: EM: Add a new section about the design
Documentation: EM: Add a runtime modifiable EM design description
Documentation: EM: Add example with driver modifying the EM
Documentation: EM: Describe the API of runtime modifications

Documentation/power/energy-model.rst | 134 +++++++++++-
drivers/cpufreq/cppc_cpufreq.c | 2 +-
drivers/powercap/dtpm_cpu.c | 27 ++-
drivers/powercap/dtpm_devfreq.c | 23 +-
drivers/thermal/cpufreq_cooling.c | 23 +-
drivers/thermal/devfreq_cooling.c | 23 +-
include/linux/energy_model.h | 93 ++++++--
include/trace/events/energy_model.h | 46 ++++
kernel/power/energy_model.c | 305 +++++++++++++++++++++++----
9 files changed, 580 insertions(+), 96 deletions(-)
create mode 100644 include/trace/events/energy_model.h