[PATCH] sched/topology: Allow multiple domains to claim sched_domain_shared
From: K Prateek Nayak
Date: Tue May 19 2026 - 01:14:23 EST
Recent optimizations of sd->shared assignment moved to allocating a
single instance of per-CPU sched_domain_shared objects per s_data.
Recent optimizations to select_idle_capacity() moved the sd->shared
assignments to "sd_asym" domain when ASYM_CPUCAPACITY is detected but
cache-aware scheduling mandates the presence of "sd_llc_shared" to
compute and cache per-LLC statistics.
Use an "alloc_flags" union in sched_domain_shared to claim a
sched_domain_shared object per sched_domain. Allocation starts searching
for an available / matching sched_domain_shared instance from the first
CPU of sched_domain_span(sd) (sd can be sd_llc, or sd_asym). If the
shared object is claimed by another domain, the instance corresponding
to next CPU in the domain span is explored until a matching / available
instance is found.
In case of a single CPU in sched_domain_span(), the domain will be
degenerated and a temporary overlap of ->shared objects across different
domains is acceptable.
"alloc_flags" forms a union with "nr_idle_scan" and the stale flags are
left as is when the sd->shared is published. The expectation is for the
first load balancing instance to correct the value just like the current
behavior, except the initial value is no longer 0.
Originally-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Tested-by: Andrea Righi <arighi@xxxxxxxxxx>
Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---
include/linux/sched/topology.h | 16 ++++++++-
kernel/sched/topology.c | 63 +++++++++++++++++++++++++++++-----
2 files changed, 69 insertions(+), 10 deletions(-)
diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index fe09d3268bc9..b5d9d7c2b8ad 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -67,7 +67,21 @@ 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.
+ *
+ * Note: between build and the first periodic LB tick, which
+ * rewrites the union via update_idle_cpu_scan(), readers of
+ * nr_idle_scan may observe the transient SD_* flag value as
+ * the scan bound. The flag bits are small positive integers,
+ * so the effect is just a slightly relaxed scan bound for one
+ * window and self-heals on the first tick.
+ */
+ 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..df2ceb54c970 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 +2927,45 @@ 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));
+ struct sched_domain_shared *sds = NULL;
+ 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 balancing.
+ *
+ * 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)
+ *
+ * If the domain only has single CPU, allow temporary overlap
+ * in allocation since the domains will be degenerated later.
+ */
+ for_each_cpu(cpu, sched_domain_span(sd)) {
+ sds = *per_cpu_ptr(d->sds, cpu);
+
+ if (!sds->alloc_flags ||
+ sd->span_weight == 1 ||
+ sds->alloc_flags == flags) {
+ sds->alloc_flags = flags;
+ sd->shared = sds;
+ break;
+ }
+ }
+
+ /*
+ * Use the sd_shared corresponding to the last
+ * CPU in the span if none are avaialable.
+ */
+ if (WARN_ON_ONCE(!sd->shared))
+ sd->shared = sds;
- 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 +3005,7 @@ 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 +3160,7 @@ 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