[PATCH 4/8] sched/topology: Introduce for_each_node_numadist() iterator

[Andrea Righi <arighi@nvidia.com>]

Introduce the new helper for_each_node_numadist() to iterate over node
IDs in order of increasing NUMA distance from a given starting node.

This iterator is somehow 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_node_numadist(node, unvisited)
  	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):

 $ 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

Suggested-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 include/linux/topology.h | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/include/linux/topology.h b/include/linux/topology.h
index 52f5850730b3e..a1815f4395ab6 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -261,6 +261,36 @@ sched_numa_hop_mask(unsigned int node, unsigned int hops)
 }
 #endif	/* CONFIG_NUMA */
 
+/**
+ * for_each_node_numadist() - iterate over nodes 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.
+ *
+ * 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.
+ *
+ * Note that by the time the loop completes, the @unvisited nodemask will
+ * be fully cleared, unless the loop exits early.
+ *
+ * The difference between for_each_node() and for_each_node_numadist() 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_node_numadist(node, unvisited)					\
+	for (int __start = (node),						\
+	     (node) = nearest_node_nodemask((__start), &(unvisited));		\
+	     (node) < MAX_NUMNODES;						\
+	     node_clear((node), (unvisited)),					\
+	     (node) = nearest_node_nodemask((__start), &(unvisited)))
+
 /**
  * for_each_numa_hop_mask - iterate over cpumasks of increasing NUMA distance
  *                          from a given node.
-- 
2.48.1