[PATCH v6 1/3] sched/topology: Introduce sched_numa_hop_mask()

[Valentin Schneider <vschneid@redhat.com>]

Tariq has pointed out that drivers allocating IRQ vectors would benefit
from having smarter NUMA-awareness - cpumask_local_spread() only knows
about the local node and everything outside is in the same bucket.

sched_domains_numa_masks is pretty much what we want to hand out (a cpumask
of CPUs reachable within a given distance budget), introduce
sched_numa_hop_mask() to export those cpumasks.

Link: http://lore.kernel.org/r/20220728191203.4055-1-tariqt@nvidia.com
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Yury Norov <yury.norov@gmail.com>
---
 include/linux/topology.h | 10 ++++++++++
 kernel/sched/topology.c  | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/include/linux/topology.h b/include/linux/topology.h
index 4564faafd0e12..64199545d7cf6 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -245,5 +245,15 @@ static inline const struct cpumask *cpu_cpu_mask(int cpu)
 	return cpumask_of_node(cpu_to_node(cpu));
 }
 
+#ifdef CONFIG_NUMA
+extern const struct cpumask *sched_numa_hop_mask(unsigned int node, unsigned int hops);
+#else
+static inline const struct cpumask *
+sched_numa_hop_mask(unsigned int node, unsigned int hops)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
+#endif	/* CONFIG_NUMA */
+
 
 #endif /* _LINUX_TOPOLOGY_H */
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 8739c2a5a54ea..3bce567241fc4 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2067,6 +2067,38 @@ int sched_numa_find_closest(const struct cpumask *cpus, int cpu)
 	return found;
 }
 
+/**
+ * sched_numa_hop_mask() - Get the cpumask of CPUs at most @hops hops away from
+ *                         @node
+ * @node: The node to count hops from.
+ * @hops: Include CPUs up to that many hops away. 0 means local node.
+ *
+ * Return: On success, a pointer to a cpumask of CPUs at most @hops away from
+ * @node, an error value otherwise.
+ *
+ * Requires rcu_lock to be held. Returned cpumask is only valid within that
+ * read-side section, copy it if required beyond that.
+ *
+ * Note that not all hops are equal in distance; see sched_init_numa() for how
+ * distances and masks are handled.
+ * Also note that this is a reflection of sched_domains_numa_masks, which may change
+ * during the lifetime of the system (offline nodes are taken out of the masks).
+ */
+const struct cpumask *sched_numa_hop_mask(unsigned int node, unsigned int hops)
+{
+	struct cpumask ***masks;
+
+	if (node >= nr_node_ids || hops >= sched_domains_numa_levels)
+		return ERR_PTR(-EINVAL);
+
+	masks = rcu_dereference(sched_domains_numa_masks);
+	if (!masks)
+		return ERR_PTR(-EBUSY);
+
+	return masks[hops][node];
+}
+EXPORT_SYMBOL_GPL(sched_numa_hop_mask);
+
 #endif /* CONFIG_NUMA */
 
 static int __sdt_alloc(const struct cpumask *cpu_map)
-- 
2.31.1