[PATCH v2 4/5] sched/topology: Introduce for_each_numa_hop_cpu()

[Valentin Schneider <vschneid@redhat.com>]

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@redhat.com>
---
 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)
 
 #endif /* _LINUX_TOPOLOGY_H */
-- 
2.31.1