[PATCH v2] sched/topology: Fix for_each_node_numadist() lockup with !CONFIG_NUMA

[Andrea Righi <arighi@nvidia.com>]

for_each_node_numadist() can lead to hard lockups on kernels built
without CONFIG_NUMA. For instance, the following was triggered by
sched_ext:

  watchdog: CPU5: Watchdog detected hard LOCKUP on cpu 5
  ...
  RIP: 0010:_find_first_and_bit+0x8/0x60
  ...
  Call Trace:
  <TASK>
   cpumask_any_and_distribute+0x49/0x80
   pick_idle_cpu_in_node+0xcf/0x140
   scx_bpf_pick_idle_cpu_node+0xaa/0x110
   bpf_prog_16ee5b1f077af006_pick_idle_cpu+0x57f/0x5de
   bpf_prog_df2ce5cfac58ce09_bpfland_select_cpu+0x37/0xf4
   bpf__sched_ext_ops_select_cpu+0x4b/0xb3

This happens because nearest_node_nodemask() always returns NUMA_NO_NODE
(-1) when CONFIG_NUMA is disabled, causing the loop to never terminate,
as the condition node >= MAX_NUMNODES is never satisfied.

Prevent this by providing a stub implementation based on
for_each_node_mask() when CONFIG_NUMA is disabled, which can safely
processes the single available node while still honoring the unvisited
nodemask.

Fixes: f09177ca5f242 ("sched/topology: Introduce for_each_node_numadist() iterator")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 include/linux/topology.h | 5 +++++
 1 file changed, 5 insertions(+)

Changes in v2:
 - Provide a stub implementation for the !CONFIG_NUMA case
 - Link to v1: https://lore.kernel.org/all/20250603080402.170601-1-arighi@nvidia.com/

diff --git a/include/linux/topology.h b/include/linux/topology.h
index 33b7fda97d390..97c4f5fc75038 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -304,12 +304,17 @@ sched_numa_hop_mask(unsigned int node, unsigned int hops)
  *
  * Requires rcu_lock to be held.
  */
+#ifdef CONFIG_NUMA
 #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)))
+#else
+#define for_each_node_numadist(node, unvisited)					\
+	for_each_node_mask((node), (unvisited))
+#endif
 
 /**
  * for_each_numa_hop_mask - iterate over cpumasks of increasing NUMA distance
-- 
2.49.0