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

[PATCH] 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 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")
Signed-off-by: Hui Su <sh_def@163.com>
---
 kernel/sched/topology.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 3dab0253976fb..c9e10bfade6d0 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -3314,12 +3314,22 @@ 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))
+	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();
-- 
2.55.0

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

[K Prateek Nayak]

Hello Hui,

On 10/21/2026 9:13 AM, Hui Su wrote:
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 3dab0253976fb..c9e10bfade6d0 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -3314,12 +3314,22 @@ 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))
> +	rcu_read_lock();
> +	for_each_cpu(i, cpu_map) {
> +		if (lowest_flag_domain(i, SD_CLUSTER)) {
> +			has_cluster = true;
> +			break;
> +		}
> +	}
> +	rcu_read_unlock();

nit.

This should be done only when sched_cluster_active is enabled to
avoid the extra loop on systems that don't have any cluster domain.

Apart from that, feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>

> +
> +	if (has_cluster)
>  		static_branch_dec_cpuslocked(&sched_cluster_active);
>  
>  	rcu_read_lock();

-- 
Thanks and Regards,
Prateek

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

[Hui Su]

> nit.
>
> This should be done only when sched_cluster_active is enabled to
> avoid the extra loop on systems that don't have any cluster domain.

Agreed. v2 guards the SD_CLUSTER scan with
static_branch_unlikely(&sched_cluster_active), so systems without cluster
domains skip the loop entirely.

Thanks for the review!

Hui