Now that we have an iterator-based alternative for a very common case
of using cpumask_local_spread for all cpus in a row, it's worth to
mention that in comment to cpumask_local_spread().
Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
Reviewed-by: Valentin Schneider <vschneid@xxxxxxxxxx>
Reviewed-by: Tariq Toukan <tariqt@xxxxxxxxxx>
---
lib/cpumask.c | 26 ++++++++++++++++++++++----
1 file changed, 22 insertions(+), 4 deletions(-)
diff --git a/lib/cpumask.c b/lib/cpumask.c
index 10aa15715c0d..98291b07c756 100644
--- a/lib/cpumask.c
+++ b/lib/cpumask.c
@@ -114,11 +114,29 @@ void __init free_bootmem_cpumask_var(cpumask_var_t mask)
* @i: index number
* @node: local numa_node
*
- * This function selects an online CPU according to a numa aware policy;
- * local cpus are returned first, followed by non-local ones, then it
- * wraps around.
+ * Returns online CPU according to a numa aware policy; local cpus are returned
+ * first, followed by non-local ones, then it wraps around.
*
- * It's not very efficient, but useful for setup.
+ * For those who wants to enumerate all CPUs based on their NUMA distances,
+ * i.e. call this function in a loop, like:
+ *
+ * for (i = 0; i < num_online_cpus(); i++) {
+ * cpu = cpumask_local_spread(i, node);
+ * do_something(cpu);
+ * }
+ *
+ * There's a better alternative based on for_each()-like iterators:
+ *
+ * for_each_numa_hop_mask(mask, node) {
+ * for_each_cpu_andnot(cpu, mask, prev)
+ * do_something(cpu);
+ * prev = mask;
+ * }
+ *
+ * It's simpler and more verbose than above. Complexity of iterator-based
+ * enumeration is O(sched_domains_numa_levels * nr_cpu_ids), while
+ * cpumask_local_spread() when called for each cpu is
+ * O(sched_domains_numa_levels * nr_cpu_ids * log(nr_cpu_ids)).
*/
unsigned int cpumask_local_spread(unsigned int i, int node)
{
--
2.34.1
