On Thu, Nov 10, 2022 at 08:00:26PM -0800, Yury Norov wrote:
> 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.
...
> +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;
*** ?
Hmm... Do we really need such deep indirection?
> + 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]);
See below.
> + if (w <= cpu)
> + break;
> +
> + last = mid - 1;
> + }
We have lib/bsearch.h. I haven't really looked deeply into the above, but my
gut feelings that that might be useful here. Can you check that?
> + 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]);
You can also shorten this by inversing the conditional:
ret = mid ? ...not 0... : ...for 0...;
> +out:
out_unlock: ?
> + rcu_read_unlock();
> + return ret;
> +}
--
With Best Regards,
Andy Shevchenko
