[PATCH v2] sched/topology: Balance cluster static-key updates

[Hui Su]

build_sched_domains() increments sched_cluster_active once per newly
built partition that contains an SD_CLUSTER domain.
detach_destroy_domains() instead tested the global static key before
decrementing it, so destroying a cluster-less partition could drop a
reference owned by another partition and disable cluster-aware
scheduling while SD_CLUSTER domains remained installed.

Check the installed domains before detaching them, only while the key is
enabled, and decrement only when the removed partition contains an
SD_CLUSTER domain.

With two cluster-bearing partitions installed, removing a singleton
partition left the baseline count at 1 where 2 partitions still
carried SD_CLUSTER; the fixed kernel correctly reports 2.  On an
asymmetric cluster topology where the parent partition gains no
cluster domain, the baseline count drops to 0 while a cluster
partition remains installed; the fixed kernel keeps it at 1.

Fixes: 8881e1639f1f ("sched/fair: Scan cluster before scanning LLC in wake-up path")
Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Signed-off-by: Hui Su <sh_def@xxxxxxx>
---

Changes in v2:
- Guard the SD_CLUSTER domain scan with sched_cluster_active to avoid an
  unnecessary loop on systems without cluster domains. (K Prateek Nayak)

v1: https://lore.kernel.org/r/20261021034314.864713-1-sh_def@xxxxxxx

 kernel/sched/topology.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 3dab0253976fb..21e3ab1af4c85 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -3314,13 +3314,25 @@ int __init sched_init_domains(const struct cpumask *cpu_map)
 static void detach_destroy_domains(const struct cpumask *cpu_map)
 {
 	unsigned int cpu = cpumask_any(cpu_map);
+	bool has_cluster = false;
 	int i;
 
 	if (rcu_access_pointer(per_cpu(sd_asym_cpucapacity, cpu)))
 		static_branch_dec_cpuslocked(&sched_asym_cpucapacity);
 
-	if (static_branch_unlikely(&sched_cluster_active))
-		static_branch_dec_cpuslocked(&sched_cluster_active);
+	if (static_branch_unlikely(&sched_cluster_active)) {
+		rcu_read_lock();
+		for_each_cpu(i, cpu_map) {
+			if (lowest_flag_domain(i, SD_CLUSTER)) {
+				has_cluster = true;
+				break;
+			}
+		}
+		rcu_read_unlock();
+
+		if (has_cluster)
+			static_branch_dec_cpuslocked(&sched_cluster_active);
+	}
 
 	rcu_read_lock();
 	for_each_cpu(i, cpu_map)
-- 
2.55.0