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

Andy Shevchenko
2022-11-14T15:08:05Z

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