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

[Vincent Guittot]

On Thu, 26 Mar 2026 at 10:24, Christian Loehle <christian.loehle@xxxxxxx> wrote:
>
> On 3/26/26 08:24, Vincent Guittot wrote:
> > On Thu, 26 Mar 2026 at 09:16, Christian Loehle <christian.loehle@xxxxxxx> wrote:
> >>
> >> On 3/26/26 07:53, Vincent Guittot wrote:
> >>> On Wed, 25 Mar 2026 at 19:13, Christian Loehle <christian.loehle@xxxxxxx> 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@xxxxxxxxxx/
> https://lore.kernel.org/lkml/20260318092214.130908-1-arighi@xxxxxxxxxx/
>
> 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.

> To me it seems more reasonable to attempt to get these minor improvements
> of minor asymmetries through asympacking and leave SD_ASYM_CPUCAPACITY
> to the actual 'true' asymmetry (e.g. different uArch or vastly different
> performance levels).
> SD_ASYM_CPUCAPACITY handling is also arguably broken if no CPU pair in
> the system fulfills capacity_greater(), the call sites in fair.c give
> a good overview.
> Is $subject the right approach to deal with these platforms instead?
> I don't know, that's why it's marked RFC and RFT.