Re: [PATCH 1/2] sched/fair: Prefer fully-idle SMT cores in asym-capacity idle selection

[Andrea Righi <arighi@nvidia.com>]

Hi Vincent,

On Fri, Apr 17, 2026 at 11:39:21AM +0200, Vincent Guittot wrote:
> On Fri, 3 Apr 2026 at 07:37, 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, their effective capacity
> > can be much lower than the nominal capacity when the sibling thread is
> > busy: SMT siblings compete for shared resources, so a "high capacity"
> > CPU that is idle but whose sibling is busy does not deliver its full
> > capacity. This effective capacity reduction cannot be modeled by the
> > static capacity value alone.
> >
> > When SMT is active, teach asym-capacity idle selection to treat a
> > logical CPU as a weaker target if its physical core is only partially
> > idle: select_idle_capacity() no longer returns on the first idle CPU
> > whose static capacity fits the task when that CPU still has a busy
> > sibling, it keeps scanning for an idle CPU on a fully-idle core and only
> > if none qualify does it fall back to partially-idle cores, using shifted
> > fit scores so fully-idle cores win ties; asym_fits_cpu() applies the
> > same fully-idle core requirement when asym capacity and SMT are both
> > active.
> >
> > This improves task placement, since partially-idle SMT siblings deliver
> > less than their nominal capacity. Favoring fully idle cores, when
> > available, can significantly enhance both throughput and wakeup latency
> > on systems with both SMT and CPU asymmetry.
> >
> > No functional changes on systems with only asymmetric CPUs or only SMT.
> >
> > Cc: K Prateek Nayak <kprateek.nayak@amd.com>
> > Cc: Vincent Guittot <vincent.guittot@linaro.org>
> > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > Cc: Christian Loehle <christian.loehle@arm.com>
> > Cc: Koba Ko <kobak@nvidia.com>
> > Reported-by: Felix Abecassis <fabecassis@nvidia.com>
> > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > ---
> >  kernel/sched/fair.c | 36 ++++++++++++++++++++++++++++++++----
> >  1 file changed, 32 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index bf948db905ed1..7f09191014d18 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7774,6 +7774,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >  static int
> >  select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> >  {
> > +       bool prefers_idle_core = sched_smt_active() && test_idle_cores(target);
> >         unsigned long task_util, util_min, util_max, best_cap = 0;
> >         int fits, best_fits = 0;
> >         int cpu, best_cpu = -1;
> > @@ -7787,6 +7788,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> >         util_max = uclamp_eff_value(p, UCLAMP_MAX);
> >
> >         for_each_cpu_wrap(cpu, cpus, target) {
> > +               bool preferred_core = !prefers_idle_core || is_core_idle(cpu);
> >                 unsigned long cpu_cap = capacity_of(cpu);
> >
> >                 if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
> > @@ -7795,7 +7797,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> >                 fits = util_fits_cpu(task_util, util_min, util_max, cpu);
> >
> >                 /* This CPU fits with all requirements */
> > -               if (fits > 0)
> > +               if (fits > 0 && preferred_core)
> >                         return cpu;
> >                 /*
> >                  * Only the min performance hint (i.e. uclamp_min) doesn't fit.
> > @@ -7803,9 +7805,30 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> >                  */
> >                 else if (fits < 0)
> >                         cpu_cap = get_actual_cpu_capacity(cpu);
> > +               /*
> > +                * fits > 0 implies we are not on a preferred core
> > +                * but the util fits CPU capacity. Set fits to -2 so
> > +                * the effective range becomes [-2, 0] where:
> > +                *    0 - does not fit
> > +                *   -1 - fits with the exception of UCLAMP_MIN
> > +                *   -2 - fits with the exception of preferred_core
> > +                */
> > +               else if (fits > 0)
> > +                       fits = -2;
> > +
> > +               /*
> > +                * If we are on a preferred core, translate the range of fits
> > +                * of [-1, 0] to [-4, -3]. This ensures that an idle core
> > +                * is always given priority over (partially) busy core.
> > +                *
> > +                * A fully fitting idle core would have returned early and hence
> > +                * fits > 0 for preferred_core need not be dealt with.
> > +                */
> > +               if (preferred_core)
> > +                       fits -= 3;
> >
> >                 /*
> > -                * First, select CPU which fits better (-1 being better than 0).
> > +                * First, select CPU which fits better (lower is more preferred).
> >                  * Then, select the one with best capacity at same level.
> >                  */
> >                 if ((fits < best_fits) ||
> 
> You have to clear idle_core if you were looking of an idle core but
> didn't find one while looping on CPUs.
> 
> You need the following to clear idle core:
> 
> @@ -7739,6 +7739,11 @@ select_idle_capacity(struct task_struct *p,
> struct sched_domain *sd, int target)
>                 }
>         }
> 
> +       /* The range [-4, -3] implies at least one idle core, the values above
> +        * imply that we didn't find anyone while looping CPUs */
> +       if (prefers_idle_core && fits > -3)
> +                       set_idle_cores(target, false);
> +
>         return best_cpu;
>  }

That makes sense! But it should be best_fits instead of fits, right?

Thanks,
-Andrea