Re: [PATCH] sched/fair: Prefer fully idle cores for NOHZ balancing

[K Prateek Nayak]

Hello Andrea,

On 7/29/2026 3:14 AM, Andrea Righi wrote:
> @@ -13974,19 +13974,32 @@ static inline int find_new_ilb(void)
>  		if (ilb_cpu == this_cpu)
>  			continue;
>  
> -		if (idle_cpu(ilb_cpu))
> +		if (!idle_cpu(ilb_cpu))
> +			continue;
> +
> +		/*
> +		 * Running the idle load balancer on an idle sibling of a busy
> +		 * SMT core can reduce the capacity available to its sibling. Prefer
> +		 * a CPU whose entire core is idle, but retain the first idle CPU as
> +		 * a fallback so idle balancing can still make progress when no fully
> +		 * idle core exists.
> +		 */
> +		if (!sched_smt_active() || is_core_idle(ilb_cpu))

nit.

is_core_idle() here would iterate all siblings and on systems with SMT-4
and SMT-8, that overhead is apparently visible when one thread per core
is occupied based on past optimizations like f8858d96061f ("sched/fair:
Optimize should_we_balance() for large SMT systems").

Copying the same approach from that optimization, can we do:

  (Only build tested)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d78467ec6ee1..814bce21ccf1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13849,21 +13849,35 @@ static inline int on_null_domain(struct rq *rq)
  */
 static inline int find_new_ilb(void)
 {
+	struct cpumask *ilb_cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
 	int this_cpu = smp_processor_id();
-	const struct cpumask *hk_mask;
-	int ilb_cpu;
+	int ilb_cpu, fallback = -1;
 
-	hk_mask = housekeeping_cpumask(HK_TYPE_KERNEL_NOISE);
+	cpumask_and(ilb_cpus, nohz.idle_cpus_mask, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE));
 
-	for_each_cpu_and(ilb_cpu, nohz.idle_cpus_mask, hk_mask) {
+	for_each_cpu(ilb_cpu, ilb_cpus) {
 		if (ilb_cpu == this_cpu)
 			continue;
 
-		if (idle_cpu(ilb_cpu))
-			return ilb_cpu;
+		if (!idle_cpu(ilb_cpu))
+			continue;
+
+		if (sched_smt_active() && !is_core_idle(ilb_cpu)) {
+			if (fallback == -1)
+				fallback = ilb_cpu;
+			/*
+			 * If the core is not idle, and first SMT sibling which is
+			 * idle has been found, then its not needed to check other
+			 * SMT siblings for idleness:
+			 */
+			cpumask_andnot(ilb_cpus, ilb_cpus, cpu_smt_mask(ilb_cpu));
+			continue;
+		}
+
+		return ilb_cpu;
 	}
 
-	return -1;
+	return fallback;
 }
 
 /*
---

It is safe to use "select_rq_mask" here since this is the tick handler
trying to find an ilb_cpu and "select_rq_mask" is only used in contexts
with IRQs disabled. It can probably be renamed to suggest that it is
safe to be used in any IRQ disabled context as a temporary mask.

Thoughts?

-- 
Thanks and Regards,
Prateek