[RFC PATCH 3/2] sched/topology: Alternative diameter >= 2 fixup

[Valentin Schneider]

This follows Barry's approach, which yields fewer groups in the higher domains.

Signed-off-by: Valentin Schneider <valentin.schnei...@arm.com>
---
 kernel/sched/topology.c | 42 +++++++++++++++++++----------------------
 1 file changed, 19 insertions(+), 23 deletions(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 0fa41aab74e0..e1bb97c06f53 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -949,20 +949,21 @@ static void init_overlap_sched_group(struct sched_domain 
*sd,
        sg->sgc->max_capacity = SCHED_CAPACITY_SCALE;
 }
 
-static struct sched_domain *find_node_domain(struct sched_domain *sd)
+static struct sched_domain *
+find_sibling_domain(struct sched_domain *sd, struct sched_domain *sibling)
 {
-       struct sched_domain *parent;
-
        BUG_ON(!(sd->flags & SD_NUMA));
 
-       /* Get to the level above NODE */
-       while (sd && sd->child) {
-               parent = sd;
-               sd = sd->child;
+       /* Find a subset sibling */
+       while (sibling->child &&
+              !cpumask_subset(sched_domain_span(sibling->child),
+                              sched_domain_span(sd)))
+               sibling = sibling->child;
+
+       /* If the above loop was a no-op, we're done */
+       if (sd->private == sibling->private)
+               return sibling;
 
-               if (!(sd->flags & SD_NUMA))
-                       break;
-       }
        /*
         * We're going to create cross topology level sched_group_capacity
         * references. This can only work if the domains resulting from said
@@ -972,16 +973,14 @@ static struct sched_domain *find_node_domain(struct 
sched_domain *sd)
         *
         * Of course, groups aren't available yet, so we can't call the usual
         * sd_degenerate(). Checking domain spans is the closest we get.
-        * Start from NODE's parent, and keep going up until we get a domain
-        * we're sure won't be degenerated.
+        * We can't go up as per the above subset check, so keep going down
+        * until we get a domain we're sure won't be degenerated.
         */
-       while (sd->parent &&
-              cpumask_equal(sched_domain_span(sd), sched_domain_span(parent))) 
{
-               sd = parent;
-               parent = sd->parent;
-       }
+       while (sibling->child &&
+              cpumask_equal(sched_domain_span(sibling->child), 
sched_domain_span(sibling)))
+               sibling = sibling->child;
 
-       return parent;
+       return sibling;
 }
 
 static int
@@ -1017,12 +1016,9 @@ build_overlap_sched_groups(struct sched_domain *sd, int 
cpu)
                if (!cpumask_test_cpu(i, sched_domain_span(sibling)))
                        continue;
 
-               /*
-                * Local group is child domain's span, as is tradition.
-                * Non-local groups will only span remote nodes.
-                */
+               /* Ensure sibling domain's span is a subset of this domain */
                if (first)
-                       sibling = find_node_domain(sibling);
+                       sibling = find_sibling_domain(sd, sibling);
 
                sg = build_group_from_child_sched_domain(sibling, cpu);
                if (!sg)
-- 
2.27.0