On Thu, Jul 8, 2021 at 12:09 PM Vincent Donnefort
[off-list ref] wrote:

Hi all,

Here's the new patch-set version that brings support for skipping
inefficiencies found by the Energy Model. This version doesn't bring
changes for all the drivers that could benefit from this work at the
moment. I'll do that in the next version or in a separated patch-set.
Also, it's been discussed that enabling RELATION_E should be a driver
flag. This sadly needs to be read in functions that do not have access to
cpufreq_driver. Hence, I created a new policy flag instead.

A bit of context:

We (Power team in Arm) are working with an experimental kernel for the
Google's Pixel4 to evaluate and improve the current mainline performance
and energy consumption on a real life device with Android.

The SD855 SoC found in this phone has several OPPs that are inefficient.
I.e. despite a lower frequency, they have a greater cost. (That cost being
fmax * OPP power / OPP freq). This issue is twofold. First of course,
running a specific workload at an inefficient OPP is counterproductive
since it wastes wasting energy. But also, inefficient OPPs make a
performance domain less appealing for task placement than it really is.

We evaluated the change presented here by running 30 iterations of Android
PCMark "Work 2.0 Performance". While we did not see any statistically
significant performance impact, this change allowed to drastically improve
the idle time residency.


                           |   Running   |  WFI [1]  |    Idle   |
   ------------------------+-------------+-----------+-----------+
   Little cluster (4 CPUs) |    -0.35%   |   +0.35%  |   +0.79%  |
   ------------------------+-------------+-----------+-----------+
   Medium cluster (3 CPUs) |    -6.3%    |    -18%   |    +12%   |
   ------------------------+-------------+-----------+-----------+
   Big cluster    (1 CPU)  |    -6.4%    |    -6.5%  |    +2.8%  |
   ------------------------+-------------+-----------+-----------+

On the SD855, the inefficient OPPs are found on the little cluster. By
removing them from the Energy Model, we make the most efficient CPUs more
appealing for task placement, helping to reduce the running time for the
medium and big CPUs. Increasing idle time is crucial for this platform due
to the substantial energy cost differences among the clusters. Also,
despite not appearing in the statistics (the idle driver used here doesn't
report it), we can speculate that we also improve the cluster idle time.

[1] WFI: Wait for interrupt.

Changelog since v3:
  - New freq-table relation CPUFREQ_RELATION_E.
  - New CPUFreq driver flag CPUFREQ_READ_ENERGY_MODEL.
  - EM flag to skip or not inefficiencies (driven by CPUFreq).
  - Fix infinite loop in set_freq_table_efficiencies().

Changelog since v2:
  - Add separated support for inefficiencies into CPUFreq.
  - Collect Reviewed-by for the first patch.

Changelog since v1:
  - Remove the Look-up table as the numbers weren't strong enough to
    justify the implementation.
  - Split the patch.

Vincent Donnefort (9):
  PM / EM: Fix inefficient states detection
  PM / EM: Mark inefficient states
  PM / EM: Extend em_perf_domain with a flag field
  PM / EM: Allow skipping inefficient states
  cpufreq: Add an interface to mark inefficient frequencies
  cpufreq: Add a new freq-table relation CPUFREQ_RELATION_E
  cpufreq: CPUFREQ_RELATION_E in schedutil ondemand and conservative
  cpufreq: Add driver flag CPUFREQ_READ_ENERGY_MODEL
  cpufreq: dt: Add CPUFREQ_READ_ENERGY_MODEL

The cpufreq changes are mostly fine by me now, but I would like to
hear from Viresh regarding this.

 drivers/cpufreq/cpufreq-dt.c           |  2 +-
 drivers/cpufreq/cpufreq.c              | 67 ++++++++++++++++++++++++++++++++-
 drivers/cpufreq/cpufreq_conservative.c |  2 +-
 drivers/cpufreq/cpufreq_ondemand.c     |  4 +-
 drivers/cpufreq/freq_table.c           | 57 +++++++++++++++++++++++++++-
 include/linux/cpufreq.h                | 50 +++++++++++++++++++++++--
 include/linux/energy_model.h           | 68 +++++++++++++++++++++++++++++-----
 kernel/power/energy_model.c            | 46 ++++++++++++++---------
 kernel/sched/cpufreq_schedutil.c       |  2 +-
 9 files changed, 260 insertions(+), 38 deletions(-)

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