Re: [PATCH 4/4] sched/fair: Prefer fully-idle SMT core for NOHZ idle load balancer

[Vincent Guittot]

On Thu, 26 Mar 2026 at 16:12, Andrea Righi <arighi@xxxxxxxxxx> wrote:
>
> When choosing which idle housekeeping CPU runs the idle load balancer,
> prefer one on a fully idle core if SMT is active, so balance can migrate
> work onto a CPU that still offers full effective capacity. Fall back to
> any idle candidate if none qualify.

This one isn't straightforward for me. The ilb cpu will check all
other idle CPUs 1st and finish with itself so unless the next CPU in
the idle_cpus_mask is a sibling, this should not make a difference

Did you see any perf diff ?


>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Christian Loehle <christian.loehle@xxxxxxx>
> Cc: Koba Ko <kobak@xxxxxxxxxx>
> Reported-by: Felix Abecassis <fabecassis@xxxxxxxxxx>
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 593a89f688679..a1ee21f7b32f6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12733,11 +12733,15 @@ static inline int on_null_domain(struct rq *rq)
>   * - When one of the busy CPUs notices that there may be an idle rebalancing
>   *   needed, they will kick the idle load balancer, which then does idle
>   *   load balancing for all the idle CPUs.
> + *
> + * - When SMT is active, prefer a CPU on a fully idle core as the ILB
> + *   target, so that when it runs balance it becomes the destination CPU
> + *   and can accept migrated tasks with full effective capacity.
>   */
>  static inline int find_new_ilb(void)
>  {
>         const struct cpumask *hk_mask;
> -       int ilb_cpu;
> +       int ilb_cpu, fallback = -1;
>
>         hk_mask = housekeeping_cpumask(HK_TYPE_KERNEL_NOISE);
>
> @@ -12746,11 +12750,22 @@ static inline int find_new_ilb(void)
>                 if (ilb_cpu == smp_processor_id())
>                         continue;
>
> +#ifdef CONFIG_SCHED_SMT

you can probably get rid of the CONFIG and put this special case below
sched_smt_active()


> +               if (!idle_cpu(ilb_cpu))
> +                       continue;
> +
> +               if (fallback < 0)
> +                       fallback = ilb_cpu;
> +
> +               if (!sched_smt_active() || is_core_idle(ilb_cpu))
> +                       return ilb_cpu;
> +#else
>                 if (idle_cpu(ilb_cpu))
>                         return ilb_cpu;
> +#endif
>         }
>
> -       return -1;
> +       return fallback;
>  }
>
>  /*
> --
> 2.53.0
>