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

[Andy Shevchenko]

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