On 8/25/2022 9:12 PM, Valentin Schneider wrote:
The recently introduced sched_numa_hop_mask() exposes cpumasks of CPUs
reachable within a given distance budget, but this means each successive
cpumask is a superset of the previous one.
Code wanting to allocate one item per CPU (e.g. IRQs) at increasing
distances would thus need to allocate a temporary cpumask to note which
CPUs have already been visited. This can be prevented by leveraging
for_each_cpu_andnot() - package all that logic into one ugl^D fancy macro.
Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
---
include/linux/topology.h | 37 +++++++++++++++++++++++++++++++++++++
1 file changed, 37 insertions(+)
diff --git a/include/linux/topology.h b/include/linux/topology.h
index 13b82b83e547..6c671dc3252c 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -254,5 +254,42 @@ static inline const struct cpumask *sched_numa_hop_mask(int node, int hops)
}
#endif /* CONFIG_NUMA */
+/**
+ * for_each_numa_hop_cpu - iterate over CPUs by increasing NUMA distance,
+ * starting from a given node.
+ * @cpu: the iteration variable.
+ * @node: the NUMA node to start the search from.
+ *
+ * Requires rcu_lock to be held.
+ * Careful: this is a double loop, 'break' won't work as expected.
+ *
+ *
+ * Implementation notes:
+ *
+ * Providing it is valid, the mask returned by
+ * sched_numa_hop_mask(node, hops+1)
+ * is a superset of the one returned by
+ * sched_numa_hop_mask(node, hops)
+ * which may not be that useful for drivers that try to spread things out and
+ * want to visit a CPU not more than once.
+ *
+ * To accommodate for that, we use for_each_cpu_andnot() to iterate over the cpus
+ * of sched_numa_hop_mask(node, hops+1) with the CPUs of
+ * sched_numa_hop_mask(node, hops) removed, IOW we only iterate over CPUs
+ * a given distance away (rather than *up to* a given distance).
+ *
+ * hops=0 forces us to play silly games: we pass cpu_none_mask to
+ * for_each_cpu_andnot(), which turns it into for_each_cpu().
+ */
+#define for_each_numa_hop_cpu(cpu, node) \
+ for (struct { const struct cpumask *curr, *prev; int hops; } __v = \
+ { sched_numa_hop_mask(node, 0), NULL, 0 }; \
+ !IS_ERR_OR_NULL(__v.curr); \
+ __v.hops++, \
+ __v.prev = __v.curr, \
+ __v.curr = sched_numa_hop_mask(node, __v.hops)) \
+ for_each_cpu_andnot(cpu, \
+ __v.curr, \
+ __v.hops ? __v.prev : cpu_none_mask)
Hiding two nested loops together in one for_each_* macro leads to
unexpected behavior for the standard usage of 'break/continue'.
for_each_numa_hop_cpu(cpu, node) {
if (condition)
break; <== will terminate the inner loop only, but it's
invisible to the human developer/reviewer.
}
These bugs will not be easy to spot in code review.
#endif /* _LINUX_TOPOLOGY_H */
