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

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

On 3/26/26 13:45, Andrea Righi wrote:
> On Thu, Mar 26, 2026 at 02:04:42PM +0100, Vincent Guittot wrote:
>> On Thu, 26 Mar 2026 at 10:24, Christian Loehle <christian.loehle@arm.com> wrote:
>>>
>>> On 3/26/26 08:24, Vincent Guittot wrote:
>>>> On Thu, 26 Mar 2026 at 09:16, Christian Loehle <christian.loehle@arm.com> wrote:
>>>>>
>>>>> 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.
>>>>
>>>> I tried to understand the end goal of this patch
>>>>
>>>> SD_ASYM_CPUCAPACITY works fine with !SMT system so why enabling
>>>> SD_ASYM_PACKING for <5% diff ?
>>>>
>>>> That doesn't make sense to me
>>> I don't know if "works fine" describes the situation accurately.
>>> I guess I should've included the context in the cover letter, but you
>>> are aware of them (you've replied to them anyway):
>>> https://lore.kernel.org/lkml/20260324005509.1134981-1-arighi@nvidia.com/
>>> https://lore.kernel.org/lkml/20260318092214.130908-1-arighi@nvidia.com/
>>>
>>> Andrea sees an improvement even when force-equalizing CPUs to remove
>>> SD_ASYM_CPUCAPACITY, so I'd argue it doesn't "work fine" on these platforms.
>>
>> IIUC this was for SMT systems not for !SMT ones but I might have
>> missed some emails in the thread.
> 
> Right, the issue I'm trying to solve is SD_ASYM_CPUCAPACITY + SMT. Removing
> SD_ASYM_CPUCAPACITY from the equation fixes my issue, because we fall back
> into the regular idle CPU selection policy, which avoids allocating both
> SMT siblings when possible.
> 
> Thanks,
> -Andrea

Could you also report how Grace baseline vs ASYM_PACKING works for your
benchmark? (or Vera nosmt)