[PATCH] cpumask: Fall back to online CPUs when NUMA lookup fails

[Hui Su]

cpumask_local_spread() wraps the requested index by the number of
online CPUs, but sched_numa_find_nth_cpu() can return nr_cpu_ids
when NUMA ordering masks are unavailable.

This can happen after a sched_init_numa() allocation failure or
while the NUMA topology is being rebuilt.

Since b1beed72b8b75, cpumask_local_spread() warns about the
invalid result but still returns it to callers expecting a
valid CPU number.

When the NUMA lookup fails, warn once and fall back to the
corresponding entry in cpu_online_mask. This preserves a valid
CPU result when the online mask is stable, at the expense of
NUMA ordering while the topology information is unavailable.

Fixes: b1beed72b8b75 ("lib/cpumask: reorganize cpumask_local_spread() logic")
Signed-off-by: Hui Su <sh_def@xxxxxxx>
---
 lib/cpumask.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/cpumask.c b/lib/cpumask.c
index 5adb9874fbd0f..f09632219b267 100644
--- a/lib/cpumask.c
+++ b/lib/cpumask.c
@@ -114,7 +114,9 @@ unsigned int cpumask_local_spread(unsigned int i, int node)
 
 	cpu = sched_numa_find_nth_cpu(cpu_online_mask, i, node);
 
-	WARN_ON(cpu >= nr_cpu_ids);
+	if (WARN_ON_ONCE(cpu >= nr_cpu_ids))
+		cpu = cpumask_nth(i, cpu_online_mask);
+
 	return cpu;
 }
 EXPORT_SYMBOL(cpumask_local_spread);

base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222
-- 
2.55.0