Re: [PATCH v2 3/4] sched: add sched_numa_find_nth_cpu()

Andy Shevchenko
2022-11-14T14:32:26Z

On Sat, Nov 12, 2022 at 11:09:45AM -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.

...

> +struct __cmp_key {
> + const struct cpumask *cpus;
> + struct cpumask ***masks;
> + int node;
> + int cpu;
> + int w;
> +};
> +
> +static int cmp(const void *a, const void *b)

Calling them key and pivot (as in the caller), would make more sense.

> +{

What about

const (?) struct cpumask ***masks = (...)pivot;

> + struct cpumask **prev_hop = *((struct cpumask ***)b - 1);

= masks[-1];

> + struct cpumask **cur_hop = *(struct cpumask ***)b;

= masks[0];

?

> + struct __cmp_key *k = (struct __cmp_key *)a;

> + if (cpumask_weight_and(k->cpus, cur_hop[k->node]) <= k->cpu)
> + return 1;

> + k->w = (b == k->masks) ? 0 : cpumask_weight_and(k->cpus, prev_hop[k->node]);
> + if (k->w <= k->cpu)
> + return 0;

Can k->cpu be negative? If no, we can rewrite above as

k->w = 0;
if (b == k->masks)
return 0;

k->w = cpumask_weight_and(k->cpus, prev_hop[k->node]);

> + return -1;
> +}

...

> +int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)
> +{
> + struct __cmp_key k = { cpus, NULL, node, cpu, 0 };

You can drop NULL and 0 while using C99 assignments.

> + int hop, ret = nr_cpu_ids;

> + rcu_read_lock();

+ Blank line?

> + k.masks = rcu_dereference(sched_domains_numa_masks);
> + if (!k.masks)
> + goto unlock;

> + hop = (struct cpumask ***)
> + bsearch(&k, k.masks, sched_domains_numa_levels, sizeof(k.masks[0]), cmp) - k.masks;

Strange indentation. I would rather see the split on parameters and
maybe '-' operator.

sizeof(*k.masks) is a bit shorter, right?

Also we may go with

struct cpumask ***masks;
struct __cmp_key k = { .cpus = cpus, .node = node, .cpu = cpu };

> + ret = hop ?
> + cpumask_nth_and_andnot(cpu - k.w, cpus, k.masks[hop][node], k.masks[hop-1][node]) :
> + cpumask_nth_and(cpu - k.w, cpus, k.masks[0][node]);

> +unlock:

out_unlock: shows the intention more clearly, no?

> + rcu_read_unlock();
> + return ret;
> +}

--
With Best Regards,
Andy Shevchenko