linux-kernel - [PATCH v2 0/3] EM / PM: Inefficient OPPs

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Date:   Fri, 21 May 2021 17:54:21 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     peterz@...radead.org, rjw@...ysocki.net, viresh.kumar@...aro.org,
        vincent.guittot@...aro.org, qperret@...gle.com
Cc:     linux-kernel@...r.kernel.org, ionela.voinescu@....com,
        lukasz.luba@....com, dietmar.eggemann@....com,
        Vincent Donnefort <vincent.donnefort@....com>
Subject: [PATCH v2 0/3] EM / PM: Inefficient OPPs

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

Vincent Donnefort (3):
  PM / EM: Fix inefficient state detection
  PM / EM: Extend em_perf_domain with a flag field
  PM / EM: Skip inefficient OPPs

 include/linux/energy_model.h     | 95 ++++++++++++++++++++++++++++++++++++----
 kernel/power/energy_model.c      | 34 +++++++-------
 kernel/sched/cpufreq_schedutil.c |  4 ++
 3 files changed, 106 insertions(+), 27 deletions(-)

-- 
2.7.4