Re: [PATCH v4 5/7] sched/topology: Introduce sched_numa_hop_mask()

[Yury Norov <yury.norov@xxxxxxxxx>]

On Fri, Sep 23, 2022 at 04:55:40PM +0100, Valentin Schneider wrote:
> Tariq has pointed out that drivers allocating IRQ vectors would benefit
> from having smarter NUMA-awareness - cpumask_local_spread() only knows
> about the local node and everything outside is in the same bucket.
> 
> sched_domains_numa_masks is pretty much what we want to hand out (a cpumask
> of CPUs reachable within a given distance budget), introduce
> sched_numa_hop_mask() to export those cpumasks.
> 
> Link: http://lore.kernel.org/r/20220728191203.4055-1-tariqt@xxxxxxxxxx
> Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
> ---
>  include/linux/topology.h | 12 ++++++++++++
>  kernel/sched/topology.c  | 31 +++++++++++++++++++++++++++++++
>  2 files changed, 43 insertions(+)
> 
> diff --git a/include/linux/topology.h b/include/linux/topology.h
> index 4564faafd0e1..3e91ae6d0ad5 100644
> --- a/include/linux/topology.h
> +++ b/include/linux/topology.h
> @@ -245,5 +245,17 @@ static inline const struct cpumask *cpu_cpu_mask(int cpu)
>  	return cpumask_of_node(cpu_to_node(cpu));
>  }
>  
> +#ifdef CONFIG_NUMA
> +extern const struct cpumask *sched_numa_hop_mask(int node, int hops);
> +#else
> +static inline const struct cpumask *sched_numa_hop_mask(int node, int hops)
> +{
> +	if (node == NUMA_NO_NODE && !hops)
> +		return cpu_online_mask;
> +
> +	return ERR_PTR(-EOPNOTSUPP);
> +}
> +#endif	/* CONFIG_NUMA */
> +
>  
>  #endif /* _LINUX_TOPOLOGY_H */
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 8739c2a5a54e..ee77706603c0 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2067,6 +2067,37 @@ int sched_numa_find_closest(const struct cpumask *cpus, int cpu)
>  	return found;
>  }
>  
> +/**
> + * sched_numa_hop_mask() - Get the cpumask of CPUs at most @hops hops away.
> + * @node: The node to count hops from.
> + * @hops: Include CPUs up to that many hops away. 0 means local node.
> + *
> + * Requires rcu_lock to be held. Returned cpumask is only valid within that
> + * read-side section, copy it if required beyond that.
> + *
> + * Note that not all hops are equal in distance; see sched_init_numa() for how
> + * distances and masks are handled.
> + *
> + * Also note that this is a reflection of sched_domains_numa_masks, which may change
> + * during the lifetime of the system (offline nodes are taken out of the masks).
> + */
> +const struct cpumask *sched_numa_hop_mask(int node, int hops)
> +{
> +	struct cpumask ***masks = rcu_dereference(sched_domains_numa_masks);
> +
> +	if (node == NUMA_NO_NODE && !hops)
> +		return cpu_online_mask;
> +
> +	if (node >= nr_node_ids || hops >= sched_domains_numa_levels)
> +		return ERR_PTR(-EINVAL);

This looks like a sanity check. If so, it should go before the snippet
above, so that client code would behave consistently.

> +
> +	if (!masks)
> +		return NULL;

In (node == NUMA_NO_NODE && !hops) case you return online cpus. Here
you return NULL just to convert it to cpu_online_mask in the caller.
This looks inconsistent. So, together with the above comment, this
makes me feel that you'd do it like this:

 const struct cpumask *sched_numa_hop_mask(int node, int hops)
 {
	struct cpumask ***masks;

	if (node >= nr_node_ids || hops >= sched_domains_numa_levels)
        {
 #ifdef CONFIG_SCHED_DEBUG
                pr_err(...);
 #endif
		return ERR_PTR(-EINVAL);
        }

	if (node == NUMA_NO_NODE && !hops)
		return cpu_online_mask; /* or NULL */

        masks = rcu_dereference(sched_domains_numa_masks);
	if (!masks)
		return cpu_online_mask; /* or NULL */

	return masks[hops][node];
 }