sched/topology: Mark SD_PREFER_SIBLING as SDF_NEEDS_GROUPS

SD_PREFER_SIBLING is currently considered in sd_parent_degenerate() but not
in sd_degenerate(). It too hinges on load balancing, and thus won't have
any effect when set on a domain with a single group. Add it to
SD_DEGENERATE_GROUPS_MASK.
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: https://lore.kernel.org/r/20200817113003.20802-12-valentin.schneider@arm.com

include/linux/sched/sd_flags.h

@@ -131,8 +131,10 @@ SD_FLAG(SD_ASYM_PACKING, SDF_SHARED_CHILD)
  *
  * Set up until domains start spanning NUMA nodes. Close to being a SHARED_CHILD
  * flag, but cleared below domains with SD_ASYM_CPUCAPACITY.
+ *
+ * NEEDS_GROUPS: Load balancing flag.
  */
-SD_FLAG(SD_PREFER_SIBLING, 0)
+SD_FLAG(SD_PREFER_SIBLING, SDF_NEEDS_GROUPS)
 
 /*
  * sched_groups of this level overlap

kernel/sched/topology.c

@@ -184,7 +184,7 @@ sd_parent_degenerate(struct sched_domain *sd, struct sched_domain *parent)
 
 	/* Flags needing groups don't count if only 1 group in parent */
 	if (parent->groups == parent->groups->next)
-		pflags &= ~(SD_DEGENERATE_GROUPS_MASK | SD_PREFER_SIBLING);
+		pflags &= ~SD_DEGENERATE_GROUPS_MASK;
 
 	if (~cflags & pflags)
 		return 0;