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

[Yury Norov <yury.norov@gmail.com>]

On Mon, Nov 14, 2022 at 04:32:09PM +0200, Andy Shevchenko wrote:
> 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.

I think they are named opaque intentionally, so that user (me) would
cast them to proper data structures and give meaningful names. So I did.
 
> > +{
> 
> 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];
> 
> ?

It would work as well. Not better neither worse.

> > +	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?

User may pass negative value. Currently cpumask_local_spread() will
return nr_cpu_ids.

After rework, bsearch() will return hop #0, After that cpumask_nth_and()
will cast negative cpu to unsigned long, and because it's a too big number,
again will return nr_cpu_ids.

> 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]);

Here we still need to compare weight of prev_hop against k->cpu.
Returning -1 unconditionally is wrong.

> > +	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?

No

> > +	rcu_read_unlock();
> > +	return ret;
> > +}
> 
> -- 
> With Best Regards,
> Andy Shevchenko
>