Re: [PATCH] sched/fair: Prefer fully-idle SMT cores in asym-capacity idle selection

[Andrea Righi <arighi@nvidia.com>]

Hi Christian,

On Wed, Mar 18, 2026 at 03:43:26PM +0000, Christian Loehle wrote:
> On 3/18/26 10:31, Andrea Righi wrote:
> > Hi Vincent,
> > 
> > On Wed, Mar 18, 2026 at 10:41:15AM +0100, Vincent Guittot wrote:
> >> On Wed, 18 Mar 2026 at 10:22, Andrea Righi <arighi@nvidia.com> wrote:
> >>>
> >>> On systems with asymmetric CPU capacity (e.g., ACPI/CPPC reporting
> >>> different per-core frequencies), the wakeup path uses
> >>> select_idle_capacity() and prioritizes idle CPUs with higher capacity
> >>> for better task placement. However, when those CPUs belong to SMT cores,
> >>
> >> Interesting, which kind of system has both SMT and SD_ASYM_CPUCAPACITY
> >> ? I thought both were never set simultaneously and SD_ASYM_PACKING was
> >> used for system involving SMT like x86
> > 
> > It's an NVIDIA platform (not publicly available yet), where the firmware
> > exposes different CPU capacities and has SMT enabled, so both
> > SD_ASYM_CPUCAPACITY and SMT are present. I'm not sure whether the final
> > firmware release will keep this exact configuration (there's a good chance
> > it will), so I'm targeting it to be prepared.
> 
> 
> Andrea,
> that makes me think, I've played with a nvidia grace available to me recently,
> which sets slightly different CPPC highest_perf values (~2%) which automatically
> will set SD_ASYM_CPUCAPACITY and run the entire capacity-aware scheduling
> machinery for really almost negligible capacity differences, where it's
> questionable how sensible that is.

That looks like the same system that I've been working with. I agree that
treating small CPPC differences as full asymmetry can be a bit overkill.

I've been experimenting with flattening the capacities (to force the
"regular" idle CPU selection policy), which performs better than the
current asym-capacity CPU selection. However, adding the SMT awareness to
the asym-capacity, seems to give a consistent +2-3% (same set of
CPU-intensive benchmarks) compared to flatening alone, which is not bad.

> I have an arm64 + CPPC implementation for asym-packing for this machine, maybe
> we can reuse that for here too?

Sure, that sounds interesting, if it's available somewhere I'd be happy to
do some testing.

Thanks,
-Andrea