Re: [PATCH 2/6] sched/topology: Introduce for_each_numa_node() iterator

Yury Norov
2025-02-09T17:50:28Z

On Fri, Feb 07, 2025 at 09:40:49PM +0100, Andrea Righi wrote:
> Introduce the new helper for_each_numa_node() to iterate over node IDs
> in order of increasing NUMA distance from a given starting node.
>
> This iterator is similar to for_each_numa_hop_mask(), but instead of
> providing a cpumask at each iteration, it provides a node ID.
>
> Example usage:
>
> nodemask_t unvisited = NODE_MASK_ALL;
> int node, start = cpu_to_node(smp_processor_id());
>
> node = start;
> for_each_numa_node(node, unvisited, N_ONLINE)
> pr_info("node (%d, %d) -> %d\n",
> start, node, node_distance(start, node));
>
> On a system with equidistant nodes:
>
> $ numactl -H
> ...
> node distances:
> node 0 1 2 3
> 0: 10 20 20 20
> 1: 20 10 20 20
> 2: 20 20 10 20
> 3: 20 20 20 10
>
> Output of the example above (on node 0):
>
> [ 7.367022] node (0, 0) -> 10
> [ 7.367151] node (0, 1) -> 20
> [ 7.367186] node (0, 2) -> 20
> [ 7.367247] node (0, 3) -> 20
>
> On a system with non-equidistant nodes (simulated using virtme-ng):

Great to see virtme-ng maturing!

> $ numactl -H
> ...
> node distances:
> node 0 1 2 3
> 0: 10 51 31 41
> 1: 51 10 21 61
> 2: 31 21 10 11
> 3: 41 61 11 10
>
> Output of the example above (on node 0):
>
> [ 8.953644] node (0, 0) -> 10
> [ 8.953712] node (0, 2) -> 31
> [ 8.953764] node (0, 3) -> 41
> [ 8.953817] node (0, 1) -> 51
>
> Cc: Yury Norov <yury.norov@gmail.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
> include/linux/topology.h | 28 ++++++++++++++++++++++++++++
> 1 file changed, 28 insertions(+)
>
> diff --git a/include/linux/topology.h b/include/linux/topology.h
> index 52f5850730b3e..09c18ee8be0eb 100644
> --- a/include/linux/topology.h
> +++ b/include/linux/topology.h
> @@ -261,6 +261,34 @@ sched_numa_hop_mask(unsigned int node, unsigned int hops)
> }
> #endif /* CONFIG_NUMA */
>
> +/**
> + * for_each_numa_node - iterate over nodes at increasing distances from a
> + * given starting node.

Nit: in increasing distance order, starting from a given node

> + * @node: the iteration variable and the starting node.
> + * @unvisited: a nodemask to keep track of the unvisited nodes.
> + * @state: state of NUMA nodes to iterate.
> + *
> + * This macro iterates over NUMA node IDs in increasing distance from the
> + * starting @node and yields MAX_NUMNODES when all the nodes have been
> + * visited.

Please also mention that the unvisited nodemask will be empty when it finish.

> + *
> + * The difference between for_each_node() and for_each_numa_node() is that
> + * the former allows to iterate over nodes in numerical order, whereas the
> + * latter iterates over nodes in increasing order of distance.
> + *
> + * This complexity of this iterator is O(N^2), where N represents the
> + * number of nodes, as each iteration involves scanning all nodes to
> + * find the one with the shortest distance.
> + *
> + * Requires rcu_lock to be held.
> + */
> +#define for_each_numa_node(node, unvisited, state) \
> + for (int start = (node), \
> + node = numa_nearest_nodemask((start), (state), &(unvisited)); \
> + node < MAX_NUMNODES; \
> + node_clear(node, (unvisited)), \
> + node = numa_nearest_nodemask((start), (state), &(unvisited)))
> +
> /**
> * for_each_numa_hop_mask - iterate over cpumasks of increasing NUMA distance
> * from a given node.
> --
> 2.48.1