[tip: sched/urgent] sched/fair: Fix negative imbalance in imbalance calculation

[tip-bot2 for Aubrey Li]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     111688ca1c4a43a7e482f5401f82c46326b8ed49
Gitweb:        https://git.kernel.org/tip/111688ca1c4a43a7e482f5401f82c46326b8ed49
Author:        Aubrey Li <aubrey.li@xxxxxxxxx>
AuthorDate:    Thu, 26 Mar 2020 13:42:29 +08:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Wed, 08 Apr 2020 11:35:20 +02:00

sched/fair: Fix negative imbalance in imbalance calculation

A negative imbalance value was observed after imbalance calculation,
this happens when the local sched group type is group_fully_busy,
and the average load of local group is greater than the selected
busiest group. Fix this problem by comparing the average load of the
local and busiest group before imbalance calculation formula.

Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Reviewed-by: Phil Auld <pauld@xxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Signed-off-by: Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/1585201349-70192-1-git-send-email-aubrey.li@xxxxxxxxx
---
 kernel/sched/fair.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 95cbd9e..02f323b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9036,6 +9036,14 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 
 		sds->avg_load = (sds->total_load * SCHED_CAPACITY_SCALE) /
 				sds->total_capacity;
+		/*
+		 * If the local group is more loaded than the selected
+		 * busiest group don't try to pull any tasks.
+		 */
+		if (local->avg_load >= busiest->avg_load) {
+			env->imbalance = 0;
+			return;
+		}
 	}
 
 	/*