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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>
This commit is contained in:
Yury Norov 2023-01-20 20:24:36 -08:00 committed by Jakub Kicinski
parent 2acda57736
commit 2ac4980c57
1 changed files with 22 additions and 4 deletions
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26
lib/cpumask.c
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@ -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)
{