[PATCH 3/6] sched/fair: Fix busiest group selection for asym groups

[Tim Chen]

From: Tim C Chen <tim.c.chen@xxxxxxxxxxxxxxx>

For two groups that have spare capacity and are partially busy, the
busier group should be the group with pure CPUs rather than the group
with SMT CPUs.  We do not want to make the SMT group the busiest one to
pull task off the SMT and makes the whole core empty.

Otherwise suppose in the search for busiest group,
we first encounter an SMT group with 1 task and set it as the busiest.
The local group is an atom cluster with 1 task and we then encounter an atom
cluster group with 3 tasks, we will not pick this atom cluster group over the
SMT group, even though we should.  As a result, we do not load balance
the busier Atom cluster (with 3 tasks) towards the local Atom cluster
(with 1 task).  And it doesn't make sense to pick the 1 task SMT group
as the busier group as we also should not pull task off the SMT towards
the 1 task atom cluster and make the SMT core completely empty.

Fix this case.

Reviewed-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bde962aa160a..8a325db34b02 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9548,6 +9548,18 @@ static bool update_sd_pick_busiest(struct lb_env *env,
 		break;
 
 	case group_has_spare:
+		/*
+		 * Do not pick sg with SMT CPUs over sg with pure CPUs,
+		 * as we do not want to pull task off half empty SMT core
+		 * and make the core idle.
+		 */
+		if (asymmetric_groups(sds->busiest, sg)) {
+			if (sds->busiest->flags & SD_SHARE_CPUCAPACITY)
+				return true;
+			else
+				return false;
+		}
+
 		/*
 		 * Select not overloaded group with lowest number of idle cpus
 		 * and highest number of running tasks. We could also compare
-- 
2.32.0