[Bug 2077470] Re: [SRU][Noble] Bad EPP defaults cause performance regressions on select Intel CPUs

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** Tags added: kernel-spammed-noble-linux-xilinx-v2 verification-needed-noble-linux-xilinx

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Title:
[SRU][Noble] Bad EPP defaults cause performance regressions on select
Intel CPUs

Status in linux package in Ubuntu:
New
Status in linux source package in Noble:
Fix Committed

Bug description:
[ Impact ]

* Intel currently provides an Energy Performance Preference (EPP) tunable via its intel_pstate
to control power management. It ranges from 0 (performance) to 255 (powersaving).
* When the governor is set to powersave, which is the default, the EPP value of balance_performance
controls how the CPU frequency scaling occurs. However, on some platforms (Emerald Rapids, Meteor
Lake, Arrow Lake) it is set to a very conservative value out of the box, resulting in degraded
performance by default.
* These EPP tuning values work as a black-box and vary per-generation, so having the latest values
available is important to keep systems running at maximum achievable performance. Both Intel and
Canonical perform this kinds of tests and supply values.

[ Test Plan ]

* Perform a benchmark on a stock Ubuntu 24.04 image

* On Emerald Rapids:
* Change the EPP to 32 by executing
echo 32 | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/energy_performance_preference

* Observe the performance difference. In our internal testing, we found the following results
when setting EPP to 32:
POV-Ray: 32% faster | 12% less energy
OpenSSL: 12% faster | energy within 1%
Build Linux Kernel: 29% faster | 18% less energy

* On Meteor Lake and Arrow Lake:
* Change the EPP to 64 by executing
echo 64 | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/energy_performance_preference

* Observe the performance difference. Intel found out the following for Meteor Lake:
> Speedometer 2.1
> score: +19%
> Perf/watt: +5.25%
>
> Webxprt 4 score
> score: +12%
> Perf/watt: +6.12%
>
> 3DMark Wildlife extreme unlimited score
> score: +3.2%
> Perf/watt: +11.5%
>
> Geekbench6 MT
> score: +2.14%
> Perf/watt: +0.32%
>
> Also update balance_power EPP default to 179. With this change:
> Video Playback power is reduced by 52%
> Team video conference power is reduced by 35%
* The same outcome should be obtained from Arrow Lake as per Intel.

[ Where problems could occur ]

* The regression risks are very low. The CPU should perform in a more efficient zone, resulting in
overall power savings, despite the more aggressive scaling. This was verified using a Yokogawa WT310
power meter and an Emerald Rapids reference platform.
* Tests on Meteor Lake and Arrow Lake were performed by Intel. They were not independently verified by
Canonical.

[ Other Info ]

* Patch for Emerald Rapids: https://lore.kernel.org/lkml/Zqu6zjVMoiXwROBI@capivara/ (merged in 6.11-rc3 https://lore.kernel.org/lkml/CAJZ5v0hQ3O7E3ygvFKbzjkJbx7pddyD9Qc+rE4EFYzJS12oTMg@mail.gmail.com/)
* Patch for Meteor Lake: https://lore.kernel.org/lkml/CAJZ5v0i8RnWs9OgxxqVOCG4RHhN7X3ELcruPoGrS1iP8hNSGsw@mail.gmail.com/
* Patch for Arrow Lake: https://lore.kernel.org/lkml/CAJZ5v0g+R6JK=T9EjmweRPKmcorw93JmPM4u-Z=pywv6kH7zUA@mail.gmail.com/ (both merged in 6.11-rc1 https://lore.kernel.org/lkml/CAJZ5v0h7OHR7ipj6ubgTLEz238JZkTshh93CYe_gDx5j+zBwfA@mail.gmail.com/)

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