Re: [PATCH v2 2/5] sched/fair: Attach sched_domain_shared to sd_asym_cpucapacity

[K Prateek Nayak]

Hello Peter, Andrea,

On 5/19/2026 2:28 AM, Peter Zijlstra wrote:
> @@@ -2775,20 -3049,16 +3107,15 @@@ build_sched_domains(const struct cpumas
>   		if (!sd)
>   			continue;
>   
>  +		if (has_asym)
> - 			asym_claimed = claim_asym_sched_domain_shared(&d, i);
> ++			claim_asym_sched_domain_shared(&d, i);
>  +
>   		/* First, find the topmost SD_SHARE_LLC domain */
>   		while (sd->parent && (sd->parent->flags & SD_SHARE_LLC))
>   			sd = sd->parent;
>   
>   		if (sd->flags & SD_SHARE_LLC) {
> - 			/*
> - 			 * Initialize the sd->shared for SD_SHARE_LLC unless
> - 			 * the asym path above already claimed it.
> - 			 */
> - 			if (!asym_claimed)
> - 				init_sched_domain_shared(&d, 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);
> ++			init_sched_domain_shared(&d, sd);

This will run into a small problem with "nr_idle_scan" if
cpumask_first(sched_domain_span(sd)) is the same for both sd_asym and
sd_llc.

Load balancer at different domains will populate "nr_idle_scan" with
different values and they alias to same ->shared if one isn't
degenerated and I believe there is at least one way to hit the WARN_ON()
from cpu_attach_domain() if the SD_ASYM_CPUCAPACITY_FULL comes before
the last SD_SHARE_LLC domain and the latter is degenerated.

How about this:

  (On top of queue:sched/core; Lightly tested on !ASYM_CPUCAPACITY system)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index fe09d3268bc9..1d2c98dca211 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -67,7 +67,15 @@ struct sched_domain_shared {
 	atomic_t	ref;
 	atomic_t	nr_busy_cpus;
 	int		has_idle_cores;
-	int		nr_idle_scan;
+	union {
+		int	nr_idle_scan;
+		/*
+		 * Used during allocation to claim the
+		 * sched_domain_shared object at
+		 * multiple levels.
+		 */
+		int	alloc_flags;
+	};
 #ifdef CONFIG_SCHED_CACHE
 	unsigned long	util_avg;
 	unsigned long	capacity;
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index dbfd9657f897..9ebd14652e9d 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -623,6 +623,12 @@ static void free_sched_groups(struct sched_group *sg, int free_sgc)
 	} while (sg != first);
 }
 
+static void free_sched_domain_shared(struct sched_domain_shared *sds)
+{
+	if (sds && atomic_dec_and_test(&sds->ref))
+		kfree(sds);
+}
+
 static void destroy_sched_domain(struct sched_domain *sd)
 {
 	/*
@@ -631,9 +637,7 @@ static void destroy_sched_domain(struct sched_domain *sd)
 	 * dropping group/capacity references, freeing where none remain.
 	 */
 	free_sched_groups(sd->groups, 1);
-
-	if (sd->shared && atomic_dec_and_test(&sd->shared->ref))
-		kfree(sd->shared);
+	free_sched_domain_shared(sd->shared);
 
 #ifdef CONFIG_SCHED_CACHE
 	/* only the bottom sd has llc_counts array */
@@ -755,7 +759,14 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
 
 			/* Pick reference to parent->shared. */
 			if (parent->shared) {
-				WARN_ON_ONCE(tmp->shared);
+				/*
+				 * It is safe to free a sd->shared that
+				 * has not been published yet. If a
+				 * sd->shared was published, the refcount
+				 * will end up being non-zero and it will
+				 * not be freed here.
+				 */
+				free_sched_domain_shared(tmp->shared);
 				tmp->shared = parent->shared;
 				parent->shared = NULL;
 			}
@@ -2916,11 +2927,40 @@ static void adjust_numa_imbalance(struct sched_domain *sd_llc)
 	}
 }
 
-static void init_sched_domain_shared(struct s_data *d, struct sched_domain *sd)
+static void
+init_sched_domain_shared(struct s_data *d, struct sched_domain *sd, int flags)
 {
-	int sd_id = cpumask_first(sched_domain_span(sd));
+	int cpu;
+
+	/*
+	 * Multiple domains can try to claim a shared object like
+	 * SD_ASYM_CPUCAPACITY and SD_SHARE_LLC which can alias to
+	 * same cpumask_first(sched_domain_span(sd)) CPU and can
+	 * cause "nr_idle_scan" to be populated incorrectly during
+	 * load balncing.
+	 *
+	 * Find the first CPU in sched_domain_span(sd) with an
+	 * unclaimed domain (!alloc_flags) or where the alloc_flag
+	 * matches the requested flag (SD_* flag)
+	 */
+	for_each_cpu(cpu, sched_domain_span(sd)) {
+		struct sched_domain_shared *sds = *per_cpu_ptr(d->sds, cpu);
+
+		/*
+		 * If the domain only has single CPU, allow temporary overlap
+		 * in allocation since the domains will be degenerated anyways.
+		 */
+		if (!sds->alloc_flags ||
+		    sd->span_weight == 1 ||
+		    sds->alloc_flags == flags) {
+			sds->alloc_flags = flags;
+			sd->shared = sds;
+			break;
+		}
+	}
+
+	BUG_ON(!sd->shared);
 
-	sd->shared = *per_cpu_ptr(d->sds, sd_id);
 	/*
 	 * nr_busy_cpus is consumed only by the NOHZ kick path via
 	 * sd_balance_shared; on the asym-capacity path it is initialized but
@@ -2960,7 +3000,7 @@ static bool claim_asym_sched_domain_shared(struct s_data *d, int cpu)
 	if (!sd_asym || (sd_asym->flags & SD_NUMA))
 		return false;
 
-	init_sched_domain_shared(d, sd_asym);
+	init_sched_domain_shared(d, sd_asym, SD_ASYM_CPUCAPACITY);
 	return true;
 }
 
@@ -3115,7 +3155,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 			sd = sd->parent;
 
 		if (sd->flags & SD_SHARE_LLC) {
-			init_sched_domain_shared(&d, sd);
+			init_sched_domain_shared(&d, sd, SD_SHARE_LLC);
 
 			/*
 			 * In presence of higher domains, adjust the
-- 
Thanks and Regards,
Prateek