Re: [PATCH 0/4] sched/fair: SMT-aware asymmetric CPU capacity

[Andrea Righi <arighi@nvidia.com>]

On Thu, Mar 26, 2026 at 04:33:08PM +0000, Christian Loehle wrote:
> On 3/26/26 15:02, Andrea Righi wrote:
> > This series attempts to improve SD_ASYM_CPUCAPACITY scheduling by
> > introducing SMT awareness.
> > 
> > = Problem =
> > 
> > Nominal per-logical-CPU capacity can overstate usable compute when an SMT
> > sibling is busy, because the physical core doesn't deliver its full nominal
> > capacity. So, several SD_ASYM_CPUCAPACITY paths may pick high capacity CPUs
> > that are not actually good destinations.
> > 
> > = Proposed Solution =
> > 
> > This patch set aligns those paths with a simple rule already used
> > elsewhere: when SMT is active, prefer fully idle cores and avoid treating
> > partially idle SMT siblings as full-capacity targets where that would
> > mislead load balance.
> > 
> > Patch set summary:
> > 
> >  - [PATCH 1/4] sched/fair: Prefer fully-idle SMT cores in asym-capacity idle selection
> > 
> >    Prefer fully-idle SMT cores in asym-capacity idle selection. In the
> >    wakeup fast path, extend select_idle_capacity() / asym_fits_cpu() so
> >    idle selection can prefer CPUs on fully idle cores, with a safe fallback.
> > 
> >  - [PATCH 2/4] sched/fair: Reject misfit pulls onto busy SMT siblings on asym-capacity
> > 
> >    Reject misfit pulls onto busy SMT siblings on SD_ASYM_CPUCAPACITY.
> >    Provided for consistency with PATCH 1/4.
> > 
> >  - [PATCH 3/4] sched/fair: Enable EAS with SMT on SD_ASYM_CPUCAPACITY systems
> > 
> >    Enable EAS with SD_ASYM_CPUCAPACITY and SMT. Also provided for
> >    consistency with PATCH 1/4. I've also tested with/without
> >    /proc/sys/kernel/sched_energy_aware enabled (same platform) and haven't
> >    noticed any regression.
> 
> 
> There's a lot more to unpack, but just to confirm, Vera doesn't have an EM, right?
> There's no EAS with it?
> (To be more precise, CPPC should bail out of building an artifical EM if there's no
> or only one efficiency class:
> drivers/cpufreq/cppc_cpufreq.c:
> 
> if (bitmap_weight(used_classes, 256) <= 1) {
> 		pr_debug("Efficiency classes are all equal (=%d). "
> 			"No EM registered", class);
> 		return;
> 	}
> 
> This is the case, right?

Yes, that's correct, so my testing on Vera with EAS isn't that meaningful.

Thanks,
-Andrea