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

[Aaron Rainbolt]

On 6/24/24 00:45, Stefan Bader wrote:
> 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.
D'oh. I tried to follow the whole process and then botched the last bit. Resubmitting the right way.
>
> -Stefan