Re: [PATCH 1/2] sched/topology: Introduce sched_numa_hop_mask()

[Tariq Toukan <ttoukan.linux@gmail.com>]

On 8/10/2022 1:51 PM, 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. Add in an iteration helper
> to iterate over CPUs at an incremental distance from a given node.
> 
> Link: http://lore.kernel.org/r/20220728191203.4055-1-tariqt@nvidia.com
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> ---
>   include/linux/topology.h | 12 ++++++++++++
>   kernel/sched/topology.c  | 28 ++++++++++++++++++++++++++++
>   2 files changed, 40 insertions(+)
> 
> diff --git a/include/linux/topology.h b/include/linux/topology.h
> index 4564faafd0e1..d66e3cf40823 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)
> +{
> +	return -ENOTSUPP;

missing ERR_PTR()

> +}
> +#endif	/* CONFIG_NUMA */
> +
> +#define for_each_numa_hop_mask(node, hops, mask)			\
> +	for (mask = sched_numa_hop_mask(node, hops); !IS_ERR_OR_NULL(mask); \
> +	     mask = sched_numa_hop_mask(node, ++hops))
>   
>   #endif /* _LINUX_TOPOLOGY_H */
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 8739c2a5a54e..f0236a0ae65c 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2067,6 +2067,34 @@ 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 size; 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 >= nr_node_ids || hops >= sched_domains_numa_levels)
> +		return ERR_PTR(-EINVAL);
> +
> +	if (!masks)
> +		return NULL;
> +
> +	return masks[hops][node];
> +}
> +EXPORT_SYMBOL_GPL(sched_numa_hop_mask);
> +
>   #endif /* CONFIG_NUMA */
>   
>   static int __sdt_alloc(const struct cpumask *cpu_map)