sched/tune: Introducing a new schedtune attribute prefer_idle

Hint to enable biasing of tasks towards idle cpus, even when a given
task is negatively boosted. The mechanism allows upto 20% reduction in
camera power without hurting performance.

bug: 28312446
Change-Id: I97ea5671aa1e6bcb165408b41e17bc82e41c2c9e

kernel/sched/fair.c

@@ -5589,7 +5589,7 @@ done:
 	return target;
 }
 
-static inline int find_best_target(struct task_struct *p, bool boosted)
+static inline int find_best_target(struct task_struct *p, bool prefer_idle)
 {
 	int iter_cpu;
 	int target_cpu = -1;
@@ -5607,9 +5607,9 @@ static inline int find_best_target(struct task_struct *p, bool boosted)
 		int idle_idx;
 
 		/*
-		 * favor higher cpus for boosted tasks
+		 * favor higher cpus for tasks that prefer idle cores
 		 */
-		int i = boosted ? NR_CPUS-iter_cpu-1 : iter_cpu;
+		int i = prefer_idle ? NR_CPUS-iter_cpu-1 : iter_cpu;
 
 		if (!cpu_online(i) || !cpumask_test_cpu(i, tsk_cpus_allowed(p)))
 			continue;
@@ -5634,10 +5634,10 @@ static inline int find_best_target(struct task_struct *p, bool boosted)
 			continue;
 #endif
 		/*
-		 * For boosted tasks we favor idle cpus unconditionally to
+		 * Unconditionally favoring tasks that prefer idle cpus to
 		 * improve latency.
 		 */
-		if (idle_cpu(i) && boosted) {
+		if (idle_cpu(i) && prefer_idle) {
 			if (best_idle_cpu < 0)
 				best_idle_cpu = i;
 			continue;
@@ -5654,7 +5654,7 @@ static inline int find_best_target(struct task_struct *p, bool boosted)
 					target_cpu = i;
 					target_util = new_util;
 				}
-			} else if (!boosted) {
+			} else if (!prefer_idle) {
 				if (best_idle_cpu < 0 ||
 					(sysctl_sched_cstate_aware &&
 						best_idle_cstate > idle_idx)) {
@@ -5669,7 +5669,7 @@ static inline int find_best_target(struct task_struct *p, bool boosted)
 		}
 	}
 
-	if (boosted && best_idle_cpu >= 0)
+	if (prefer_idle && best_idle_cpu >= 0)
 		target_cpu = best_idle_cpu;
 	else if (target_cpu < 0)
 		target_cpu = best_idle_cpu >= 0 ? best_idle_cpu : backup_cpu;
@@ -5761,14 +5761,17 @@ static int energy_aware_wake_cpu(struct task_struct *p, int target, int sync)
 		 */
 #ifdef CONFIG_CGROUP_SCHEDTUNE
 		bool boosted = schedtune_task_boost(p) > 0;
+		bool prefer_idle = schedtune_prefer_idle(p) > 0;
 #else
 		bool boosted = 0;
+		bool prefer_idle = 0;
 #endif
-		int tmp_target = find_best_target(p, boosted);
-		if (tmp_target >= 0)
+		int tmp_target = find_best_target(p, boosted || prefer_idle);
+		if (tmp_target >= 0) {
 			target_cpu = tmp_target;
-			if (boosted && idle_cpu(target_cpu))
+			if ((boosted || prefer_idle) && idle_cpu(target_cpu))
 				return target_cpu;
+		}
 	}
 
 	if (target_cpu != task_cpu(p)) {

kernel/sched/tune.c

@@ -125,6 +125,10 @@ struct schedtune {
 
 	/* Performance Constraint (C) region threshold params */
 	int perf_constrain_idx;
+
+	/* Hint to bias scheduling of tasks on that SchedTune CGroup
+	 * towards idle CPUs */
+	int prefer_idle;
 };
 
 static inline struct schedtune *css_st(struct cgroup_subsys_state *css)
@@ -156,6 +160,7 @@ root_schedtune = {
 	.boost	= 0,
 	.perf_boost_idx = 0,
 	.perf_constrain_idx = 0,
+	.prefer_idle = 0,
 };
 
 int
@@ -536,6 +541,38 @@ int schedtune_task_boost(struct task_struct *p)
 	return task_boost;
 }
 
+int schedtune_prefer_idle(struct task_struct *p)
+{
+	struct schedtune *st;
+	int prefer_idle;
+
+	/* Get prefer_idle value */
+	rcu_read_lock();
+	st = task_schedtune(p);
+	prefer_idle = st->prefer_idle;
+	rcu_read_unlock();
+
+	return prefer_idle;
+}
+
+static u64
+prefer_idle_read(struct cgroup_subsys_state *css, struct cftype *cft)
+{
+	struct schedtune *st = css_st(css);
+
+	return st->prefer_idle;
+}
+
+static int
+prefer_idle_write(struct cgroup_subsys_state *css, struct cftype *cft,
+	    u64 prefer_idle)
+{
+	struct schedtune *st = css_st(css);
+	st->prefer_idle = prefer_idle;
+
+	return 0;
+}
+
 static s64
 boost_read(struct cgroup_subsys_state *css, struct cftype *cft)
 {
@@ -587,6 +624,11 @@ static struct cftype files[] = {
 		.read_s64 = boost_read,
 		.write_s64 = boost_write,
 	},
+	{
+		.name = "prefer_idle",
+		.read_u64 = prefer_idle_read,
+		.write_u64 = prefer_idle_write,
+	},
 	{ }	/* terminate */
 };
 

kernel/sched/tune.h

@@ -17,6 +17,8 @@ struct target_nrg {
 int schedtune_cpu_boost(int cpu);
 int schedtune_task_boost(struct task_struct *tsk);
 
+int schedtune_prefer_idle(struct task_struct *tsk);
+
 void schedtune_exit_task(struct task_struct *tsk);
 
 void schedtune_enqueue_task(struct task_struct *p, int cpu);