[PATCH] sched/fair: Add SMT4 group_smt_balance handling

[Tim Chen]

For SMT4, any group with more than 2 tasks will be marked as
group_smt_balance. Retain the behaviour of group_has_spare by marking
the busiest group as the group which has the least number of idle_cpus.

Also, handle rounding effect of adding (ncores_local + ncores_busy)
when the local is fully idle and busy group has more than 2 tasks.
Local group should try to pull at least 1 task in this case.

Fixes: fee1759e4f04 ("sched/fair: Determine active load balance for SMT sch=
ed groups")
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0b7445cd5af9..6e7ee2efc1ba 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9575,7 +9575,7 @@ static inline long sibling_imbalance(struct lb_env *e=
nv,
 	imbalance /=3D ncores_local + ncores_busiest;
=20
 	/* Take advantage of resource in an empty sched group */
-	if (imbalance =3D=3D 0 && local->sum_nr_running =3D=3D 0 &&
+	if (imbalance <=3D 1 && local->sum_nr_running =3D=3D 0 &&
 	    busiest->sum_nr_running > 1)
 		imbalance =3D 2;
=20
@@ -9763,6 +9763,19 @@ static bool update_sd_pick_busiest(struct lb_env *en=
v,
 		break;
=20
 	case group_smt_balance:
+		/* no idle cpus on both groups handled by group_fully_busy below */
+		if (sgs->idle_cpus !=3D 0 || busiest->idle_cpus !=3D 0) {
+			if (sgs->idle_cpus > busiest->idle_cpus)
+				return false;
+			if (sgs->idle_cpus < busiest->idle_cpus)
+				return true;
+			if (sgs->sum_nr_running <=3D busiest->sum_nr_running)
+				return false;
+			else
+				return true;
+		}
+		goto fully_busy;
+
 	case group_fully_busy:
 		/*
 		 * Select the fully busy group with highest avg_load. In
@@ -9775,7 +9788,7 @@ static bool update_sd_pick_busiest(struct lb_env *env=
,
 		 * select the 1st one, except if @sg is composed of SMT
 		 * siblings.
 		 */
-
+fully_busy:
 		if (sgs->avg_load < busiest->avg_load)
 			return false;
=20
--=20
2.32.0