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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: 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 safely processes the single available node while still honoring the unvisited nodemask. Also extend this optimization to the case where MAX_NUMNODES == 1, as suggested by Yury. Fixes: f09177ca5f242 ("sched/topology: Introduce for_each_node_numadist() iterator") Acked-by: Yury Norov [NVIDIA] Signed-off-by: Andrea Righi --- include/linux/topology.h | 5 +++++ 1 file changed, 5 insertions(+) Changes in v3: - Extend optimization to MAX_NUMNODES == 1, as per Yury - Link to v2: https://lore.kernel.org/all/20250609113536.29743-1-arighi@nvidia.com/ 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..64b81a0aaf449 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. */ +#if defined(CONFIG_NUMA) && (MAX_NUMNODES > 1) #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