Re: [PATCH v4 0/9] Inefficient OPPs

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Jul 8, 2021 at 12:09 PM Vincent Donnefort
<vincent.donnefort@arm.com> 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(-)
>
> --