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

[Andrea Righi <arighi@nvidia.com>]

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