sched/fair: enforce EAS mode

For non latency sensitive tasks the goal is to optimize for energy efficiency.
Thus, we should try our best to avoid moving a task on a CPU which is then
going to be marked as overutilized.

Let's use the capacity_margin metric to verify if a candidate target CPU
should be considered without risking to bail out of EAS mode.

Change-Id: Ib3697106f4073aedf4a6c6ce42bd5d000fa8c007
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>

Author: derkling

kernel/sched/fair.c

@@ -6469,6 +6469,19 @@ static inline int find_best_target(struct task_struct *p, int *backup_cpu,
 				continue;
 			}
 
+			/*
+			 * Enforce EAS mode
+			 *
+			 * For non latency sensitive tasks, skip CPUs that
+			 * will be overutilized by moving the task there.
+			 *
+			 * The goal here is to remain in EAS mode as long as
+			 * possible at least for !prefer_idle tasks.
+			 */
+			if ((new_util * capacity_margin) >
+			    (capacity_orig * SCHED_CAPACITY_SCALE))
+				continue;
+
 			/*
 			 * Case B) Non latency sensitive tasks on IDLE CPUs.
 			 *