Re: [PATCH 1/2] sched/fair: Prefer fully-idle SMT cores in asym-capacity idle selection

[K Prateek Nayak]

Hello Andrea,

On 4/18/2026 1:54 PM, Andrea Righi wrote:
>>> @@ -7774,6 +7774,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>>>  static int
>>>  select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
>>>  {
>>> +	bool prefers_idle_core = sched_smt_active() && test_idle_cores(target);
>>
>> Somehow I miss a:
>>
>>     if (prefers_idle_core)
>>         set_idle_cores(target, false)
>>
>> The one in select_idle_sibling() -> select_idle_cpu() isn't executed
>> anymore in with ASYM_CPUCAPACITY.
>>
> 
> Right, we need to add this as also pointed by Vincent.
> 
>>
>> Another thing is that sic() iterates over CPUs sd_asym_cpucapacity
>> whereas the idle core thing lives in sd_llc/sd_llc_shared. Both sd's are
>> probably th same on your system.
> 
> Hm... they're the same on my machine, but if they're different, clearing
> has_idle_cores here is not right and it might lead to false positives. We should
> only clear it only when both domains span the same CPUs (or just check if
> sd_asym_cpucapacity and sd_llc are the same).
> 
> However, if they're not the same, I'm not sure exactly what we should do...
> maybe ignore has_idle_cores and always do the scan for now?

With your changes, only two places actually care about test_idle_cores():

- select_idle_capacity()
- select_idle_cpu()

If we go into select_idle_capacity(), we don't do select_idle_cpu() so
the two paths are mutually exclusive.

In nohz_balancer_kick(), if we find, sd_asym_cpucapacity, we simply
don't care about the sd_llc_shared->nr_busy_cpus during balancing so
that begs the question if we can simply track idle_cores at
sd_asym_cpucapacity for these systems?

Following is only build tested for now but I'll try to spoof asym
cpucapacity on my system and check if it holds up or not:

  (On top of tip:sched/core at sched-core-2026-04-13 + this series)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 78f2d2c4e24f..509146c486ac 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7913,7 +7913,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
 	int i, cpu, idle_cpu = -1, nr = INT_MAX;
 
-	if (sched_feat(SIS_UTIL)) {
+	if (sched_feat(SIS_UTIL) && sd->shared) {
 		/*
 		 * Increment because !--nr is the condition to stop scan.
 		 *
@@ -12856,7 +12856,8 @@ static void set_cpu_sd_state_busy(int cpu)
 		goto unlock;
 	sd->nohz_idle = 0;
 
-	atomic_inc(&sd->shared->nr_busy_cpus);
+	if (sd->shared)
+		atomic_inc(&sd->shared->nr_busy_cpus);
 unlock:
 	rcu_read_unlock();
 }
@@ -12885,7 +12886,8 @@ static void set_cpu_sd_state_idle(int cpu)
 		goto unlock;
 	sd->nohz_idle = 1;
 
-	atomic_dec(&sd->shared->nr_busy_cpus);
+	if (sd->shared)
+		atomic_dec(&sd->shared->nr_busy_cpus);
 unlock:
 	rcu_read_unlock();
 }
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 5847b83d9d55..45b919b39c7d 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -680,19 +680,38 @@ static void update_top_cache_domain(int cpu)
 	int id = cpu;
 	int size = 1;
 
+	sd = lowest_flag_domain(cpu, SD_ASYM_CPUCAPACITY_FULL);
+	if (sd) {
+		/*
+		 * If sd_asym_cpucapacity exists,
+		 * the shared object should exist too.
+		 */
+		WARN_ON_ONCE(!sd->shared);
+		sds = sd->shared;
+	}
+
+	rcu_assign_pointer(per_cpu(sd_asym_cpucapacity, cpu), sd);
+
 	sd = highest_flag_domain(cpu, SD_SHARE_LLC);
 	if (sd) {
 		id = cpumask_first(sched_domain_span(sd));
 		size = cpumask_weight(sched_domain_span(sd));
 
-		/* If sd_llc exists, sd_llc_shared should exist too. */
-		WARN_ON_ONCE(!sd->shared);
-		sds = sd->shared;
+		/*
+		 * If sd_asym_cpucapacity doesn't exist,
+		 * sd_llc_shared must have a sd->shared linked.
+		 */
+		if (!sds) {
+			WARN_ON_ONCE(!sd->shared);
+			sds = sd->shared;
+		}
 	}
 
 	rcu_assign_pointer(per_cpu(sd_llc, cpu), sd);
 	per_cpu(sd_llc_size, cpu) = size;
 	per_cpu(sd_llc_id, cpu) = id;
+
+	/* TODO: Rename sd_llc_shared to fit the new role. */
 	rcu_assign_pointer(per_cpu(sd_llc_shared, cpu), sds);
 
 	sd = lowest_flag_domain(cpu, SD_CLUSTER);
@@ -711,9 +730,6 @@ static void update_top_cache_domain(int cpu)
 
 	sd = highest_flag_domain(cpu, SD_ASYM_PACKING);
 	rcu_assign_pointer(per_cpu(sd_asym_packing, cpu), sd);
-
-	sd = lowest_flag_domain(cpu, SD_ASYM_CPUCAPACITY_FULL);
-	rcu_assign_pointer(per_cpu(sd_asym_cpucapacity, cpu), sd);
 }
 
 /*
@@ -2650,6 +2666,15 @@ static void adjust_numa_imbalance(struct sched_domain *sd_llc)
 	}
 }
 
+static void init_sched_domain_shared(struct s_data *d, struct sched_domain *sd)
+{
+	int sd_id = cpumask_first(sched_domain_span(sd));
+
+	sd->shared = *per_cpu_ptr(d->sds, sd_id);
+	atomic_set(&sd->shared->nr_busy_cpus, sd->span_weight);
+	atomic_inc(&sd->shared->ref);
+}
+
 /*
  * Build sched domains for a given set of CPUs and attach the sched domains
  * to the individual CPUs
@@ -2712,16 +2737,33 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 		if (!sd)
 			continue;
 
+		/*
+		 * In case of ASYM_CPUCAPACITY, attach sd->shared to
+		 * sd_asym_cpucapacity for wakeup stat tracking.
+		 *
+		 * XXX: This assumes SD_ASYM_CPUCAPACITY_FULL domain
+		 * always has more than one group else it is prone to
+		 * degeneration.
+		 */
+		if (has_asym) {
+			while (sd && !(sd->flags & SD_ASYM_CPUCAPACITY_FULL))
+				sd = sd->parent;
+
+			init_sched_domain_shared(&d, sd);
+		}
+
 		/* First, find the topmost SD_SHARE_LLC domain */
+		sd = *per_cpu_ptr(d.sd, i);
 		while (sd->parent && (sd->parent->flags & SD_SHARE_LLC))
 			sd = sd->parent;
 
 		if (sd->flags & SD_SHARE_LLC) {
-			int sd_id = cpumask_first(sched_domain_span(sd));
-
-			sd->shared = *per_cpu_ptr(d.sds, sd_id);
-			atomic_set(&sd->shared->nr_busy_cpus, sd->span_weight);
-			atomic_inc(&sd->shared->ref);
+			/*
+			 * Initialize the sd->shared for SD_SHARE_LLC if
+			 * SD_ASYM_CPUCAPACITY_FULL hasn't claimed it already.
+			 */
+			if (!has_asym)
+				init_sched_domain_shared(&d, sd);
 
 			/*
 			 * In presence of higher domains, adjust the
---

I still have one question: Can first SD_ASYM_CPUCAPACITY_FULL be set at
a SD_NUMA?

We'll need to deal with overlapping domains then but seems like it could
be possible with weird cpusets :-(

But in that case, do we even want to search CPUs outside the NUMA in
select_idle_capacity()? I don't think anything stops this currently but
I might be wrong.

-- 
Thanks and Regards,
Prateek