Re: [PATCH 5/5] sched/fair: Add SIS_UTIL support to select_idle_capacity()

[Vincent Guittot]

On Sat, 9 May 2026 at 20:10, Andrea Righi <arighi@xxxxxxxxxx> wrote:
>
> From: K Prateek Nayak <kprateek.nayak@xxxxxxx>
>
> Add to select_idle_capacity() the same SIS_UTIL-controlled idle-scan
> mechanism, already used by select_idle_cpu(): when sched_feat(SIS_UTIL)
> is enabled and the LLC domain has sched_domain_shared data, derive the
> per-attempt scan limit from sd->shared->nr_idle_scan.
>
> That bounds the walk on large LLCs: once nr_idle_scan is exhausted,
> return the best CPU seen so far. The early exit is gated on
> !has_idle_core so an active idle-core search (SMT with idle cores
> reported by test_idle_cores()) isn't cut short before it gets a chance
> to find one.
>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Co-developed-by: Andrea Righi <arighi@xxxxxxxxxx>
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>


> ---
>  kernel/sched/fair.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2ddba8bd27e59..494149f14d98f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8084,6 +8084,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>         int fits, best_fits = ASYM_IDLE_COMPLETE_MISFIT;
>         int cpu, best_cpu = -1;
>         struct cpumask *cpus;
> +       int nr = INT_MAX;
>
>         cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
>         cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> @@ -8092,10 +8093,28 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>         util_min = uclamp_eff_value(p, UCLAMP_MIN);
>         util_max = uclamp_eff_value(p, UCLAMP_MAX);
>
> +       if (sched_feat(SIS_UTIL) && sd->shared) {
> +               /*
> +                * Same nr_idle_scan hint as select_idle_cpu(), nr only limits
> +                * the scan when not preferring an idle core.
> +                */
> +               nr = READ_ONCE(sd->shared->nr_idle_scan) + 1;
> +               /* overloaded domain is unlikely to have idle cpu/core */
> +               if (nr == 1)
> +                       return -1;
> +       }
> +
>         for_each_cpu_wrap(cpu, cpus, target) {
>                 bool preferred_core = !has_idle_core || is_core_idle(cpu);
>                 unsigned long cpu_cap = capacity_of(cpu);
>
> +               /*
> +                * Stop when the nr_idle_scan is exhausted (mirrors
> +                * select_idle_cpu() logic).
> +                */
> +               if (!has_idle_core && --nr <= 0)
> +                       return best_cpu;
> +
>                 if (!choose_idle_cpu(cpu, p))
>                         continue;
>
> --
> 2.54.0
>