sched/fair: use min capacity when evaluating idle backup cpus

When we are calculating what the impact of placing a task on a specific
cpu is, we should include the information that there might be a minimum
capacity imposed upon that cpu which could change the performance and/or
energy cost decisions.

When choosing an idle backup CPU, favour CPUs that won't end up
running at a high OPP due to a min capacity cap imposed by external
actors.

Change-Id: I566623ffb3a7c5b61a23242dcce1cb4147ef8a4a
Signed-off-by: Ionela Voinescu <[email protected]>
Signed-off-by: Chris Redpath <[email protected]>

kernel/sched/fair.c

@@ -7583,6 +7583,7 @@ static inline int find_best_target(struct task_struct *p, int *backup_cpu,
 	unsigned long min_wake_util = ULONG_MAX;
 	unsigned long target_max_spare_cap = 0;
 	unsigned long best_active_util = ULONG_MAX;
+	unsigned long target_idle_max_spare_cap = 0;
 	int best_idle_cstate = INT_MAX;
 	struct sched_domain *sd;
 	struct sched_group *sg;
@@ -7618,7 +7619,7 @@ static inline int find_best_target(struct task_struct *p, int *backup_cpu,
 		for_each_cpu_and(i, tsk_cpus_allowed(p), sched_group_cpus(sg)) {
 			unsigned long capacity_curr = capacity_curr_of(i);
 			unsigned long capacity_orig = capacity_orig_of(i);
-			unsigned long wake_util, new_util;
+			unsigned long wake_util, new_util, min_capped_util;
 
 			if (!cpu_online(i))
 				continue;
@@ -7640,6 +7641,16 @@ static inline int find_best_target(struct task_struct *p, int *backup_cpu,
 			 * than the one required to boost the task.
 			 */
 			new_util = max(min_util, new_util);
+
+			/*
+			 * Include minimum capacity constraint:
+			 * new_util contains the required utilization including
+			 * boost. min_capped_util also takes into account a
+			 * minimum capacity cap imposed on the CPU by external
+			 * actors.
+			 */
+			min_capped_util = max(new_util, capacity_min_of(i));
+
 			if (new_util > capacity_orig)
 				continue;
 
@@ -7767,6 +7778,12 @@ static inline int find_best_target(struct task_struct *p, int *backup_cpu,
 				/* Select idle CPU with lower cap_orig */
 				if (capacity_orig > best_idle_min_cap_orig)
 					continue;
+				/* Favor CPUs that won't end up running at a
+				 * high OPP.
+				 */
+				if ((capacity_orig - min_capped_util) <
+					target_idle_max_spare_cap)
+					continue;
 
 				/*
 				 * Skip CPUs in deeper idle state, but only
@@ -7780,6 +7797,8 @@ static inline int find_best_target(struct task_struct *p, int *backup_cpu,
 
 				/* Keep track of best idle CPU */
 				best_idle_min_cap_orig = capacity_orig;
+				target_idle_max_spare_cap = capacity_orig -
+							    min_capped_util;
 				best_idle_cstate = idle_idx;
 				best_idle_cpu = i;
 				continue;