sched/topology: Switch to assigning "sd->shared" from s_data

Use the "sched_domain_shared" object allocated in s_data for
"sd->shared" assignments. Assign "sd->shared" for the topmost
SD_SHARE_LLC domain before degeneration and rely on the degeneration
path to correctly pass down the shared object to "sd_llc".

sd_degenerate_parent() ensures degenerating domains must have the same
sched_domain_span() which ensures 1:1 passing down of the shared object.
If the topmost SD_SHARE_LLC domain degenerates, the shared object is
freed from destroy_sched_domain() when the last reference is dropped.

claim_allocations() NULLs out the objects that have been assigned as
"sd->shared" and the unassigned ones are freed from the __sds_free()
path.

To keep all the claim_allocations() bits in one place,
claim_allocations() has been extended to accept "s_data" and iterate the
domains internally to free both "sched_domain_shared" and the
per-topology-level data for the particular CPU in one place.

Post cpu_attach_domain(), all reclaims of "sd->shared" are handled via
call_rcu() on the sched_domain object via destroy_sched_domains_rcu().

Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://patch.msgid.link/20260312044434.1974-5-kprateek.nayak@amd.com

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 9006586720bf05a2f0b5864698811aaeb40b4148..b19d84f4466948113b17e0d36c7f1b7df91fba0d 100644 (file)
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -685,6 +685,9 @@ static void update_top_cache_domain(int cpu)
        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;
        }
 
@@ -733,6 +736,13 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
                if (sd_parent_degenerate(tmp, parent)) {
                        tmp->parent = parent->parent;
 
+                       /* Pick reference to parent->shared. */
+                       if (parent->shared) {
+                               WARN_ON_ONCE(tmp->shared);
+                               tmp->shared = parent->shared;
+                               parent->shared = NULL;
+                       }
+
                        if (parent->parent) {
                                parent->parent->child = tmp;
                                parent->parent->groups->flags = tmp->flags;
@@ -1586,21 +1596,28 @@ __visit_domain_allocation_hell(struct s_data *d, const struct cpumask *cpu_map)
  * sched_group structure so that the subsequent __free_domain_allocs()
  * will not free the data we're using.
  */
-static void claim_allocations(int cpu, struct sched_domain *sd)
+static void claim_allocations(int cpu, struct s_data *d)
 {
-       struct sd_data *sdd = sd->private;
+       struct sched_domain *sd;
+
+       if (atomic_read(&(*per_cpu_ptr(d->sds, cpu))->ref))
+               *per_cpu_ptr(d->sds, cpu) = NULL;
 
-       WARN_ON_ONCE(*per_cpu_ptr(sdd->sd, cpu) != sd);
-       *per_cpu_ptr(sdd->sd, cpu) = NULL;
+       for (sd = *per_cpu_ptr(d->sd, cpu); sd; sd = sd->parent) {
+               struct sd_data *sdd = sd->private;
 
-       if (atomic_read(&(*per_cpu_ptr(sdd->sds, cpu))->ref))
-               *per_cpu_ptr(sdd->sds, cpu) = NULL;
+               WARN_ON_ONCE(*per_cpu_ptr(sdd->sd, cpu) != sd);
+               *per_cpu_ptr(sdd->sd, cpu) = NULL;
 
-       if (atomic_read(&(*per_cpu_ptr(sdd->sg, cpu))->ref))
-               *per_cpu_ptr(sdd->sg, cpu) = NULL;
+               if (atomic_read(&(*per_cpu_ptr(sdd->sds, cpu))->ref))
+                       *per_cpu_ptr(sdd->sds, cpu) = NULL;
 
-       if (atomic_read(&(*per_cpu_ptr(sdd->sgc, cpu))->ref))
-               *per_cpu_ptr(sdd->sgc, cpu) = NULL;
+               if (atomic_read(&(*per_cpu_ptr(sdd->sg, cpu))->ref))
+                       *per_cpu_ptr(sdd->sg, cpu) = NULL;
+
+               if (atomic_read(&(*per_cpu_ptr(sdd->sgc, cpu))->ref))
+                       *per_cpu_ptr(sdd->sgc, cpu) = NULL;
+       }
 }
 
 #ifdef CONFIG_NUMA
@@ -1738,16 +1755,6 @@ sd_init(struct sched_domain_topology_level *tl,
                sd->cache_nice_tries = 1;
        }
 
-       /*
-        * For all levels sharing cache; connect a sched_domain_shared
-        * instance.
-        */
-       if (sd->flags & SD_SHARE_LLC) {
-               sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
-               atomic_inc(&sd->shared->ref);
-               atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
-       }
-
        sd->private = sdd;
 
        return sd;
@@ -2729,12 +2736,20 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
                while (sd->parent && (sd->parent->flags & SD_SHARE_LLC))
                        sd = sd->parent;
 
-               /*
-                * In presence of higher domains, adjust the
-                * NUMA imbalance stats for the hierarchy.
-                */
-               if (IS_ENABLED(CONFIG_NUMA) && (sd->flags & SD_SHARE_LLC) && sd->parent)
-                       adjust_numa_imbalance(sd);
+               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);
+
+                       /*
+                        * In presence of higher domains, adjust the
+                        * NUMA imbalance stats for the hierarchy.
+                        */
+                       if (IS_ENABLED(CONFIG_NUMA) && sd->parent)
+                               adjust_numa_imbalance(sd);
+               }
        }
 
        /* Calculate CPU capacity for physical packages and nodes */
@@ -2742,10 +2757,10 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
                if (!cpumask_test_cpu(i, cpu_map))
                        continue;
 
-               for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
-                       claim_allocations(i, sd);
+               claim_allocations(i, &d);
+
+               for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent)
                        init_sched_groups_capacity(i, sd);
-               }
        }
 
        /* Attach the domains */