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authorYury Norov <yury.norov@gmail.com>2023-01-20 20:24:36 -0800
committerJakub Kicinski <kuba@kernel.org>2023-02-07 18:20:00 -0800
commit2ac4980c57f54db7c5b416f7946d2921fc16d9d2 (patch)
treee446248b006239a53bc21ef49dc7c75262be8870 /lib/cpumask.c
parent2acda57736de1e486036b90a648e67a3599080a1 (diff)
lib/cpumask: update comment for cpumask_local_spread()
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@gmail.com> Reviewed-by: Valentin Schneider <vschneid@redhat.com> Reviewed-by: Tariq Toukan <tariqt@nvidia.com> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Diffstat (limited to 'lib/cpumask.c')
-rw-r--r--lib/cpumask.c26
1 files changed, 22 insertions, 4 deletions
diff --git a/lib/cpumask.c b/lib/cpumask.c
index 10aa15715c0d..e7258836b60b 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)
{