[SRU][OEM-6.8][Noble][PATCH V2 0/1] UBUNTU: SAUCE: cpufreq: intel_pstate: Use HWP to initialize ITMT if CPPC is missing

[Aaron Rainbolt]

BugLink: https://bugs.launchpad.net/bugs/2069988

V2: Resubmitted using a different email application in the hopes of
not mangling things this time.

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.