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

[Andrea Righi <arighi@nvidia.com>]

On Wed, Apr 01, 2026 at 02:08:27PM +0200, Vincent Guittot wrote:
> On Wed, 1 Apr 2026 at 13:57, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
> >
> > On 31.03.26 11:04, Andrea Righi wrote:
> > > Hi Dietmar,
> > >
> > > On Tue, Mar 31, 2026 at 12:30:55AM +0200, Dietmar Eggemann wrote:
> > >> Hi Andrea,
> > >>
> > >> On 26.03.26 16:02, Andrea Righi wrote:
> >
> > [...]
> >
> > >> So does (2) with NO_SIS_UTIL performs worse than (1) with your smt
> > >> related add-ons in sic()?
> > >
> > > Thanks for running these experiments and sharing the data, this is very
> > > useful!
> > >
> > > I did a quick test on Vera using the NVBLAS benchmark, comparing NO
> > > ASYM_CPUCAPACITY with and without SIS_UTIL, but the difference seems to be
> > > within error range. I'll also run DCPerf MediaWiki with all the different
> >
> > I'm not familiar with the NVBLAS benchmark. Does it drive your system
> > into 'sd->shared->nr_idle_scan = 0' state?

It's something internally unfortunately... it's just running a single
CPU-intensive task for each SMT core (in practice half of the CPUs tasks).
I don't think we're hitting sd->shared->nr_idle_scan == 0 in this case.

> >
> > We just have to understand where this benefit of using sic() instead of
> > sis() is coming from. I'm doubtful that this is the best_cpu thing after
> > if (!choose_idle_cpu(cpu, p)) in sic()'s for_each_cpu_wrap(cpu, cpus,
> > target) loop given that the CPU capacity diffs are so small.
> >
> > > configurations to see if I get similar results.
> > >
> > > More in general, I agree that for small capacity differences (e.g., within
> > > ~5%) the benefits of using ASYM_CPUCAPACITY is questionable. And I'm also
> > > fine to go back to the idea of grouping together CPUS within the 5%
> > > capacity window, if we think it's a safer approach (results in your case
> > > are quite evident, and BTW, that means we also shouldn't have
> > > ASYM_CPU_CAPACITY on Grace, so in theory the 5% threshold should also
> > > improve performance on Grace, that doesn't have SMT).
> >
> > There shouldn't be so many machines with these binning-introduced small
> > CPU capacity diffs out there? In fact, I only know about your Grace
> > (!smt) and Vera (smt) machines.
> 
> In any case it's always better to add the support than enabling asym_packing
> 
> >
> > > That said, I still think there's value in adding SMT awareness to
> > > select_idle_capacity(). Even if we decide to avoid ASYM_CPUCAPACITY for
> > > small capacity deltas, we should ensure that the behavior remains
> > > reasonable if both features are enabled, for any reason. Right now, there
> > > are cases where the current behavior leads to significant performance
> > > degradation (~2x), so having a mechanism to prevent clearly suboptimal task
> > > placement still seems worthwhile. Essentially, what I'm saying is that one
> > > thing doesn't exclude the other.
> >
> > IMHO, in case we would know where this improvement is coming from using
> > sic() instead of default sis() (which already as smt support) then
> > maybe, it's a lot of extra code at the end ... And mobile big.LITTLE
> > (with larger CPU capacity diffs) doesn't have smt.
> 
> The last proposal based on  prateek proposal in sic() doesn't seems that large

Exactly, I was referring just to that patch, which would solve the big part
of the performance issue. We can ignore the ILB part for now.

Thanks,
-Andrea