Re: [RFC][PATCH v1 3/3] cpufreq: intel_pstate: Set asymmetric CPU capacity on hybrid systems

Dietmar Eggemann
2024-05-03T18:30:10Z

On 03/05/2024 05:32, Ricardo Neri wrote:
> On Thu, May 02, 2024 at 12:42:54PM +0200, Dietmar Eggemann wrote:
>> On 25/04/2024 21:06, Rafael J. Wysocki wrote:
>>> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>>
>>> Make intel_pstate use the HWP_HIGHEST_PERF values from
>>> MSR_HWP_CAPABILITIES to set asymmetric CPU capacity information
>>> via the previously introduced arch_set_cpu_capacity() on hybrid
>>> systems without SMT.
>>
>> Are there such systems around? My i7-13700K has P-cores (CPU0..CPU15)
>> with SMT.
>
> We have been experimenting with nosmt in the kernel command line.

OK.

[...]

>>> If the given system is hybrid and non-SMT, the new code disables ITMT
>>> support in the scheduler (because it may get in the way of asymmetric CPU
>>> capacity code in the scheduler that automatically gets enabled by setting
>>> asymmetric CPU capacity) after initializing all online CPUs and finds
>>> the one with the maximum HWP_HIGHEST_PERF value. Next, it computes the
>>> capacity number for each (online) CPU by dividing the product of its
>>> HWP_HIGHEST_PERF and SCHED_CAPACITY_SCALE by the maximum HWP_HIGHEST_PERF.
>>
>> SO either CAS at wakeup and in load_balance or SIS at wakeup and ITMT in
>> load balance.
>
> May I know what CAS and SIS stand for?

Capacity Aware Scheduling and Select_Idle_Sibling().

Either select_idle_sibling() -> select_idle_capacity() (1)

or select_idle_sibling() -> select_idle_cpu() /* nosmt */ (2)

So my system with now 'nosmt' goes (1).