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

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

Hello Andrea,

On 9/4/2026 2:48 PM, Andrea Righi wrote:
> +/*
> + * Return true when @cpu has a higher asymmetric-packing priority than
> + * @other in their shared SMT scheduling domain.
> + */
> +static bool sched_smt_asym_prefer(int cpu, int other)
> +{
> +	struct sched_domain *sd = rcu_dereference_all(cpu_rq(cpu)->sd);
> +
> +	if (!sd)
> +		return false;
> +
> +	if (!(sd->flags & SD_SHARE_CPUCAPACITY) ||
> +	    !(sd->flags & SD_ASYM_PACKING))
> +		return false;
> +
> +	if (!cpumask_test_cpu(other, sched_domain_span(sd)))
> +		return false;
> +
> +	return sched_asym_prefer(cpu, other);
> +}
> +
> +/*
> + * Return the highest-priority available CPU in @cpu's SMT core that is also in @cpus.
> + */
> +static int __select_idle_smt_cpu(struct task_struct *p, int cpu, const struct cpumask *cpus)
> +{
> +	int best = cpu;
> +	int sibling;
> +
> +	for_each_cpu_and(sibling, cpu_smt_mask(cpu), cpus) {
> +		if (sibling == best || !choose_idle_cpu(sibling, p))
> +			continue;
> +
> +		if (sched_smt_asym_prefer(sibling, best))
> +			best = sibling;

nit. Since sched_smt_asym_prefer() is only used here, and we know rq->sd
is the one that can have SD_SHARE_CPUCAPACITY | SD_ASYM_PACKING, perhaps
you can inline the check here do a:

    sd = rcu_dereference_all(cpu_rq(cpu)->sd);

    if (!sd)
         return cpu;

   if (!(sd->flags & SD_SHARE_CPUCAPACITY) || !(sd->flags & SD_ASYM_PACKING))
       return cpu;

   for_each_cpu_and (sibling, sched_domain_span(sd), cpus) {
       ...
   }

  ...


That way, you don't need to dereference cpu_rq(cpu)->sd every time in
sched_smt_asym_prefer() and check cpumask_test_cpu(). Both, domain
span and task affinity will be covered at once.

Thoughts?

> +	}
> +
> +	return best;
> +}
> +
> +static inline int
> +select_idle_smt_cpu(struct task_struct *p, int cpu, const struct cpumask *cpus)
> +{
> +	if (!sched_smt_asym_active())
> +		return cpu;
> +
> +	return __select_idle_smt_cpu(p, cpu, cpus);
> +}
> +
> +/*
> + * Redirect an available SMT CPU to a higher-priority available sibling allowed by task affinity.
> + */
> +static inline int select_idle_smt_priority(struct task_struct *p, int cpu)
> +{
> +	return select_idle_smt_cpu(p, cpu, p->cpus_ptr);
> +}
> +
>  /*
>   * Scans the local SMT mask to see if the entire core is idle, and records this
>   * information in sd_balance_shared->has_idle_cores.
> @@ -8645,7 +8702,7 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
>  	}
>  
>  	if (idle)
> -		return core;
> +		return select_idle_smt_cpu(p, core, cpus);
>  
>  	cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
>  	return -1;
> @@ -8668,7 +8725,7 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
>  		if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
>  			continue;
>  		if (choose_idle_cpu(cpu, p))
> -			return cpu;
> +			return select_idle_smt_priority(p, cpu);
>  	}
>  
>  	return -1;
> @@ -8720,7 +8777,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  						return -1;
>  					idle_cpu = __select_idle_cpu(cpu, p);
>  					if ((unsigned int)idle_cpu < nr_cpumask_bits)
> -						return idle_cpu;
> +						return select_idle_smt_priority(p, idle_cpu);

Question for Shrikanth: On larger SMT (SMT-4, SMT-8), does the ranking
make that big of a difference if the core is already busy?

Does the overehead of additional search get offset by the benefit of
being placed on a better ranked thread? If not, maybe the paths for
!has_idle_core can stay as is?

>  				}
>  			}
>  			cpumask_andnot(cpus, cpus, sched_group_span(sg));
> @@ -8745,7 +8802,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  	if (has_idle_core)
>  		set_idle_cores(target, false);
>  
> -	return idle_cpu;
> +	return (unsigned int)idle_cpu < nr_cpumask_bits ?
> +		select_idle_smt_priority(p, idle_cpu) : idle_cpu;

Since every path does a select_idle_smt_priority() - be it coming from
select_idle_core(), the early-return from the cluster scan, or just an
idle CPU from the LLc scan, can't we simply just do it once in
select_idle_sibling()?

Something like:

  (Only build tested)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f79fcba4afec..7c97585141dd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8964,7 +8964,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 
 	if (choose_idle_cpu(target, p) &&
 	    asym_fits_cpu(task_util, util_min, util_max, target))
-		return target;
+		goto out;
 
 	/*
 	 * If the previous CPU is cache affine and idle, don't be stupid:
@@ -8974,8 +8974,10 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	    asym_fits_cpu(task_util, util_min, util_max, prev)) {
 
 		if (!static_branch_unlikely(&sched_cluster_active) ||
-		    cpus_share_resources(prev, target))
-			return prev;
+		    cpus_share_resources(prev, target)) {
+			target = prev;
+			goto out;
+		}
 
 		prev_aff = prev;
 	}
@@ -8993,7 +8995,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	    prev == smp_processor_id() &&
 	    this_rq()->nr_running <= 1 &&
 	    asym_fits_cpu(task_util, util_min, util_max, prev)) {
-		return prev;
+		target = prev;
+		goto out;
 	}
 
 	/* Check a recently used CPU as a potential idle candidate: */
@@ -9007,8 +9010,10 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	    asym_fits_cpu(task_util, util_min, util_max, recent_used_cpu)) {
 
 		if (!static_branch_unlikely(&sched_cluster_active) ||
-		    cpus_share_resources(recent_used_cpu, target))
-			return recent_used_cpu;
+		    cpus_share_resources(recent_used_cpu, target)) {
+			target = recent_used_cpu;
+			goto out;
+		}
 
 	} else {
 		recent_used_cpu = -1;
@@ -9030,7 +9035,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 		 */
 		if (sd) {
 			i = select_idle_capacity(p, sd, target);
-			return ((unsigned)i < nr_cpumask_bits) ? i : target;
+			target = ((unsigned)i < nr_cpumask_bits) ? i : target;
+			goto out;
 		}
 	}
 
@@ -9043,27 +9049,31 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 
 		if (!has_idle_core && cpus_share_cache(prev, target)) {
 			i = select_idle_smt(p, sd, prev);
-			if ((unsigned int)i < nr_cpumask_bits)
-				return i;
+			if ((unsigned int)i < nr_cpumask_bits) {
+				target = i;
+				goto out;
+			}
 		}
 	}
 
 	i = select_idle_cpu(p, sd, has_idle_core, target);
 	if ((unsigned)i < nr_cpumask_bits)
-		return i;
-
+		target = i;
 	/*
 	 * For cluster machines which have lower sharing cache like L2 or
 	 * LLC Tag, we tend to find an idle CPU in the target's cluster
 	 * first. But prev_cpu or recent_used_cpu may also be a good candidate,
 	 * use them if possible when no idle CPU found in select_idle_cpu().
 	 */
-	if ((unsigned int)prev_aff < nr_cpumask_bits)
-		return prev_aff;
-	if ((unsigned int)recent_used_cpu < nr_cpumask_bits)
-		return recent_used_cpu;
+	else if ((unsigned int)prev_aff < nr_cpumask_bits)
+		target = prev_aff;
+	else if ((unsigned int)recent_used_cpu < nr_cpumask_bits)
+		target = recent_used_cpu;
+out:
+	if (!sched_smt_asym_active())
+		return target;
 
-	return target;
+	return select_idle_smt_priority(p, target);
 }
 
 /**
---

That way, it lives in a single place, and we don't have to pepper
select_idle_smt_priority() everywhere. Thoughts?

-- 
Thanks and Regards,
Prateek