Re: [PATCH 2/2] sched/fair: Honor asymmetric SMT priority in idle selection

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Andrea,

On 9/8/2026 1:53 PM, Andrea Righi wrote:
> @@ -9747,8 +9796,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  	}
>  
>  	/* Slow path */
> -	if (unlikely(sd))
> -		return sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag);
> +	if (unlikely(sd)) {
> +		new_cpu = sched_balance_find_dst_cpu(sd, p, cpu, prev_cpu, sd_flag);
> +		return select_idle_smt_cpu(p, new_cpu);

nit. I personally feel this can be better integrated into the
sched_balance_find_dst_cpu(). Something like the following:

  (Only build tested)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b8bd308c2d5b..1012dfb33f08 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12353,6 +12353,17 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd,
 
 	}
 
+	/*
+	 * If we are on a SD_SHARE_CPUCAPACITY | SD_ASYM_PACKING
+	 * domain, use the group_asym_packing classification to
+	 * decide placement based on rankings of idle siblings.
+	 */
+	if (unlikely(sched_smt_asym_active() &&
+		     (sd->flags & SD_SHARE_CPUCAPACITY) &&
+		     (sd->flags & SD_ASYM_PACKING) &&
+		     sgs->idle_cpus))
+		sgs->group_asym_packing = 1;
+
 	sgs->group_capacity = group->sgc->capacity;
 
 	sgs->group_weight = group->group_weight;
@@ -12393,9 +12404,15 @@ static bool update_pick_idlest(struct sched_group *idlest,
 			return false;
 		break;
 
+	case group_asym_packing:
+		/*
+		 * Only possible for sched_smt_asym_active().
+		 * Select the idle SMT that is more preferred.
+		 */
+		return sched_asym_prefer(idlest->asym_prefer_cpu,
+					 group->asym_prefer_cpu);
 	case group_llc_balance:
 	case group_imbalanced:
-	case group_asym_packing:
 	case group_smt_balance:
 		/* Those types are not used in the slow wakeup path */
 		return false;
---

It leads to slightly more branches but they are super predictable when
iterating at a particular sched_domain level so the overhead should be
negligible.

I don't have any strong feelings either ways. Thoughts?

-- 
Thanks and Regards,
Prateek