NACK/Cmnt: [SRU][OEM-6.8][Noble][PATCH 0/1] UBUNTU: SAUCE: cpufreq: intel_pstate: Use HWP to initialize ITMT if CPPC is missing

[Stefan Bader]

On 22.06.24 03:29, Aaron Rainbolt wrote:
> BugLink: https://bugs.launchpad.net/bugs/2069988
> 
> SRU Justification:
> 
> [Impact]
> 
> The new EEVDF scheduler, introduced in Linux 6.6 and present in the Noble
> 6.8 kernel, requires ITMT in order to determine which cores are P-cores
> and which cores are E-cores on hybrid-core Intel-based systems. Currently,
> the intel_pstate requires CPPC v2 to be present in order to enable ITMT.
> Not all systems that support ITMT also support CPPC v2 however. On such
> systems, ITMT will be mistakenly disabled and the scheduler will no longer
> be able to schedule intensive single-core workloads on P-cores reliably.
> This results in a serious drop in single-core performance (machines
> capable of getting Geekbench 5 single-core scores of over 1800 get scores
> of less than 1000).
> 
> [Fix]
> 
> The proposed patch (which I have shamelessly "stolen" from the linux-acpi
> mailing list where Rafael J. Wysocki proposed it) fixes this issue by
> using information from MSR_HWP_CAPABILITIES to determine whether or not to
> enable ITMT, in the event CPPC v2 is not present.
> 
> [Test Plan]
> 
> Internally, Kubuntu Focus will be using a thorough kernel testing suite on
> our hardware to ensure the kernel functions correctly in a wide variety of
> different use case scenarios, while also benchmarking the systems to
> ensure that single-core Geekbench 5 scores rise to their expected level.
> For public testing, it should be noted that this patch will only provide
> a performance improvement on hardware without CPPC v2 and with a
> hybrid-core Intel processor. If you do not have an affected system, the
> patch should provide no change in performance at all. With this in mind,
> the following plan should be sufficient to ensure the kernel is functional
> and that performance is not negatively impacted:
> 
> * Install `7z`.
> * Using an older kernel, run `7z b -mmt1` and record the results.
> * Repeat the above three times.
> * Install the kernel from proposed.
> * Again, run `7z b -mmt1` and record the results three times.
> * Compare these results and ensure that performance is approximately
>    the same both before and after patching.
> * Run the system with usual day-to-day workloads for a day or two to
>    ensure that it is stable.
> 
> [Where problems could occur]
> 
> It is theoretically possible that kernel code that deals with ITMT could
> behave strangely when ITMT is active without CPPC v2 present. This could
> result in kernel instability, or (if the intel_pstate driver itself
> misbehaves) performance degredation. The above test plan is intended to
> catch either of these issues.
> 

Rejected for the following reasons:
Patch is not submitted as a threaded reply to the cover email. This 
makes it rather hard to keep pieces together.

-Stefan
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