[PATCH 3/4] sched: add sched_numa_find_nth_cpu()

[Yury Norov <yury.norov@gmail.com>]

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 <yury.norov@gmail.com>
---
 include/linux/topology.h |  8 ++++++++
 kernel/sched/topology.c  | 42 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)

diff --git a/include/linux/topology.h b/include/linux/topology.h
index 4564faafd0e1..63048ac3207c 100644
--- a/include/linux/topology.h
+++ b/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
+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 */
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 8739c2a5a54e..c8f56287de46 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2067,6 +2067,48 @@ int sched_numa_find_closest(const struct cpumask *cpus, int cpu)
 	return found;
 }
 
+/*
+ * 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)
+{
+	unsigned int first = 0, mid, last = sched_domains_numa_levels;
+	struct cpumask ***masks;
+	int w, ret = nr_cpu_ids;
+
+	rcu_read_lock();
+	masks = rcu_dereference(sched_domains_numa_masks);
+	if (!masks)
+		goto out;
+
+	while (last >= first) {
+		mid = (last + first) / 2;
+
+		if (cpumask_weight_and(cpus, masks[mid][node]) <= cpu) {
+			first = mid + 1;
+			continue;
+		}
+
+		w = (mid == 0) ? 0 : cpumask_weight_and(cpus, masks[mid - 1][node]);
+		if (w <= cpu)
+			break;
+
+		last = mid - 1;
+	}
+
+	ret = (mid == 0) ?
+		cpumask_nth_and(cpu - w, cpus, masks[mid][node]) :
+		cpumask_nth_and_andnot(cpu - w, cpus, masks[mid][node], masks[mid - 1][node]);
+out:
+	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)
-- 
2.34.1