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

[Andy Shevchenko]

On Mon, Nov 14, 2022 at 04:32:10PM +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.
> 
> > +{
> 
> 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;
> > +}

Below is a diff I have got on top of your patch, only compile tested:

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 024f1da0e941..e04262578b52 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2070,26 +2070,28 @@ int sched_numa_find_closest(const struct cpumask *cpus, int cpu)
 }
 
 struct __cmp_key {
-	const struct cpumask *cpus;
 	struct cpumask ***masks;
+	const struct cpumask *cpus;
 	int node;
 	int cpu;
 	int w;
 };
 
-static int cmp(const void *a, const void *b)
+static int cmp(const void *key, const void *pivot)
 {
-	struct cpumask **prev_hop = *((struct cpumask ***)b - 1);
-	struct cpumask **cur_hop = *(struct cpumask ***)b;
-	struct __cmp_key *k = (struct __cmp_key *)a;
+	struct __cmp_key *k = container_of(key, struct __cmp_key, masks);
+	const struct cpumask ***masks = (const struct cpumask ***)pivot;
+	const struct cpumask **prev = masks[-1];
+	const struct cpumask **cur = masks[0];
 
-	if (cpumask_weight_and(k->cpus, cur_hop[k->node]) <= k->cpu)
+	if (cpumask_weight_and(k->cpus, cur[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)
+	k->w = 0;
+	if (masks == (const struct cpumask ***)k->masks)
 		return 0;
 
+	k->w = cpumask_weight_and(k->cpus, prev[k->node]);
 	return -1;
 }
 
@@ -2103,17 +2105,17 @@ static int cmp(const void *a, const void *b)
  */
 int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)
 {
-	struct __cmp_key k = { cpus, NULL, node, cpu, 0 };
+	struct __cmp_key k = { .cpus = cpus, .node = node, .cpu = cpu };
 	int hop, ret = nr_cpu_ids;
+	struct cpumask ***masks;
 
 	rcu_read_lock();
 	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;
-
+	masks = bsearch(&k.masks, k.masks, sched_domains_numa_levels, sizeof(*k.masks), cmp);
+	hop = masks - k.masks;
 	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]);

-- 
With Best Regards,
Andy Shevchenko