Re: [RFC][RFT][PATCH 0/3] arm64: Enable asympacking for minor CPPC asymmetry

[Christian Loehle <christian.loehle@arm.com>]

On 3/26/26 07:53, Vincent Guittot wrote:
> On Wed, 25 Mar 2026 at 19:13, Christian Loehle <christian.loehle@arm.com> wrote:
>>
>> The scheduler currently handles CPU performance asymmetry via either:
>>
>> - SD_ASYM_PACKING: simple priority-based task placement (x86 ITMT)
>> - SD_ASYM_CPUCAPACITY: capacity-aware scheduling
>>
>> On arm64, capacity-aware scheduling is used for any detected capacity
>> differences.
>>
>> Some systems expose small per-CPU performance differences via CPPC
>> highest_perf (e.g. due to chip binning), resulting in slightly different
>> capacities (<~5%). These differences are sufficient to trigger
>> SD_ASYM_CPUCAPACITY, even though the system is otherwise effectively
>> symmetric.
>>
>> For such small deltas, capacity-aware scheduling is unnecessarily
>> complex. A simpler priority-based approach, similar to x86 ITMT, is
>> sufficient.
> 
> I'm not convinced that moving to  SD_ASYM_PACKING is the right way to
> move forward.
> t
> 1st of all, do you target all kind of system or only SMT? It's not
> clear in your cover letter

AFAIK only Andrea has access to an unreleased asymmetric SMT system,
I haven't done any tests on such a system (as the cover-letter mentions
under RFT section).

> 
> Moving on asym pack for !SMT doesn't make sense to me. If you don't
> want EAS enabled, you can disable it with
> /proc/sys/kernel/sched_energy_aware

Sorry, what's EAS got to do with it? The system I care about here
(primarily nvidia grace) has no EM.

> 
> For SMT system and small capacity difference, I would prefer that we
> look at supporting SMT in SD_ASYM_CPUCAPACITY. Starting with
> select_idle_capacity

This series is actually targeted for primarily the !SMT case, although
it may or may not be useful for some of the SMT woes, too!
(Again, I wouldn't know, I don't have such a system to test with)

>[snip]