Re: [RFC 2/3] sched/topology: fix sched groups on NUMA machines with mesh topology

[Peter Zijlstra]

On Thu, Apr 13, 2017 at 10:56:08AM -0300, Lauro Ramos Venancio wrote:
> This patch constructs the sched groups from each CPU perspective. So, on
> a 4 nodes machine with ring topology, while nodes 0 and 2 keep the same
> groups as before [(3, 0, 1)(1, 2, 3)], nodes 1 and 3 have new groups
> [(0, 1, 2)(2, 3, 0)]. This allows moving tasks between any node 2-hops
> apart.

Ah,.. so after drawing pictures I see what went wrong; duh :-(

An equivalent patch would be (if for_each_cpu_wrap() were exposed):

@@ -521,11 +588,11 @@ build_overlap_sched_groups(struct sched_domain *sd, int cpu)
 	struct cpumask *covered = sched_domains_tmpmask;
 	struct sd_data *sdd = sd->private;
 	struct sched_domain *sibling;
-	int i;
+	int i, wrap;
 
 	cpumask_clear(covered);
 
-	for_each_cpu(i, span) {
+	for_each_cpu_wrap(i, span, cpu, wrap) {
 		struct cpumask *sg_span;
 
 		if (cpumask_test_cpu(i, covered))


We need to start iterating at @cpu, not start at 0 every time.