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

[Shrikanth Hegde]

On 3/26/26 8:32 PM, Andrea Righi 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.
>
> 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
> +		if (!idle_cpu(ilb_cpu))
> +			continue;
> +
> +		if (fallback < 0)
> +			fallback = ilb_cpu;
> +
> +		if (!sched_smt_active() || is_core_idle(ilb_cpu))

is_core_idle does loop for all sublings and nohz.idle_cpus_mask
will have all siblings likely.

So that might turn out be a bit expensive on large SMT system such as SMT=4
Also, this is with interrupt disabled.

Will try to run this on powerpc system and see if simple benchmarks show anything.

> +			return ilb_cpu;
> +#else
>   		if (idle_cpu(ilb_cpu))
>   			return ilb_cpu;
> +#endif
>   	}
>   
> -	return -1;
> +	return fallback;
>   }
>   
>   /*