Re: [PATCH 1/4] sched/fair: Prefer fully-idle SMT cores in asym-capacity idle selection

[Vincent Guittot]

On Mon, 30 Mar 2026 at 12:17, K Prateek Nayak <kprateek.nayak@xxxxxxx> wrote:
>
> Hello Andrea,
>
> On 3/27/2026 10:09 PM, Andrea Righi wrote:
> >> My naive eyes say it should be equivalent of what you have but maybe
> >> I'm wrong?
> >
> > It seems correct to my naive eyes as well. Will test this out to make sure.
>
> So I found one small problem with fits > 0 && !preferred_core where even
> though it is an ideal target, we don't end up preferring it because of
> the larger "fits" value.
>
> Here is an updated diff:
>
>   (Only build tested)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 226509231e67..580218656865 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7949,6 +7949,7 @@ 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;
> @@ -7962,6 +7963,7 @@ 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 (!choose_idle_cpu(cpu, p))
> @@ -7970,7 +7972,7 @@ 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.
> @@ -7978,9 +7980,30 @@ 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 an 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) ||
> ---
>
> Sorry for the oversight but this should now be equivalent to your
> Patch 1. I'll let Vincent comment if he prefers this to the original
> or not :-)

Yes, I prefer this version which keeps the same logic for selecting the best cpu

Thanks
Vincent

>
> --
> Thanks and Regards,
> Prateek
>