sched/fair: Fix imbalance overflow

When local group is fully busy but its average load is above system load,
computing the imbalance will overflow and local group is not the best
target for pulling this load.

Fixes: 0b0695f2b34a ("sched/fair: Rework load_balance()")
Reported-by: Tingjia Cao <[e-mail]>
Signed-off-by: Vincent Guittot <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Tested-by: Tingjia Cao <[e-mail]>
Link: https://lore.kernel.org/lkml/CABcWv9_DAhVBOq2=W=[e-mail]/T/

kernel/sched/fair.c

···
 
 		sds->avg_load = (sds->total_load * SCHED_CAPACITY_SCALE) /
 				sds->total_capacity;
+
+		/*
+		 * If the local group is more loaded than the average system
+		 * load, don't try to pull any tasks.
+		 */
+		if (local->avg_load >= sds->avg_load) {
+			env->imbalance = 0;
+			return;
+		}
+
 	}
 
 	/*