sched: add sched_numa_find_nth_cpu()

The function finds Nth set CPU in a given cpumask starting from a given
node.

Leveraging the fact that each hop in sched_domains_numa_masks includes the
same or greater number of CPUs than the previous one, we can use binary
search on hops instead of linear walk, which makes the overall complexity
of O(log n) in terms of number of cpumask_weight() calls.

Signed-off-by: Yury Norov <[email protected]>
Acked-by: Tariq Toukan <[email protected]>
Reviewed-by: Jacob Keller <[email protected]>
Reviewed-by: Peter Lafreniere <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

include/linux/topology.h

@@ -245,5 +245,13 @@ static inline const struct cpumask *cpu_cpu_mask(int cpu)
 	return cpumask_of_node(cpu_to_node(cpu));
 }
 
+#ifdef CONFIG_NUMA
+int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node);
+#else
+static __always_inline int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)
+{
+	return cpumask_nth(cpu, cpus);
+}
+#endif	/* CONFIG_NUMA */
 
 #endif /* _LINUX_TOPOLOGY_H */

kernel/sched/topology.c

@@ -3,6 +3,8 @@
  * Scheduler topology setup/handling methods
  */
 
+#include <linux/bsearch.h>
+
 DEFINE_MUTEX(sched_domains_mutex);
 
 /* Protected by sched_domains_mutex: */
@@ -2067,6 +2069,61 @@ unlock:
 	return found;
 }
 
+struct __cmp_key {
+	const struct cpumask *cpus;
+	struct cpumask ***masks;
+	int node;
+	int cpu;
+	int w;
+};
+
+static int hop_cmp(const void *a, const void *b)
+{
+	struct cpumask **prev_hop = *((struct cpumask ***)b - 1);
+	struct cpumask **cur_hop = *(struct cpumask ***)b;
+	struct __cmp_key *k = (struct __cmp_key *)a;
+
+	if (cpumask_weight_and(k->cpus, cur_hop[k->node]) <= k->cpu)
+		return 1;
+
+	k->w = (b == k->masks) ? 0 : cpumask_weight_and(k->cpus, prev_hop[k->node]);
+	if (k->w <= k->cpu)
+		return 0;
+
+	return -1;
+}
+
+/*
+ * sched_numa_find_nth_cpu() - given the NUMA topology, find the Nth next cpu
+ *                             closest to @cpu from @cpumask.
+ * cpumask: cpumask to find a cpu from
+ * cpu: Nth cpu to find
+ *
+ * returns: cpu, or nr_cpu_ids when nothing found.
+ */
+int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)
+{
+	struct __cmp_key k = { .cpus = cpus, .node = node, .cpu = cpu };
+	struct cpumask ***hop_masks;
+	int hop, ret = nr_cpu_ids;
+
+	rcu_read_lock();
+
+	k.masks = rcu_dereference(sched_domains_numa_masks);
+	if (!k.masks)
+		goto unlock;
+
+	hop_masks = bsearch(&k, k.masks, sched_domains_numa_levels, sizeof(k.masks[0]), hop_cmp);
+	hop = hop_masks	- k.masks;
+
+	ret = hop ?
+		cpumask_nth_and_andnot(cpu - k.w, cpus, k.masks[hop][node], k.masks[hop-1][node]) :
+		cpumask_nth_and(cpu, cpus, k.masks[0][node]);
+unlock:
+	rcu_read_unlock();
+	return ret;
+}
+EXPORT_SYMBOL_GPL(sched_numa_find_nth_cpu);
 #endif /* CONFIG_NUMA */
 
 static int __sdt_alloc(const struct cpumask *cpu_map)