HMP: Avoid using the cpu stopper to stop runnable tasks

When migrating a runnable task, we use the CPU stopper on
the source CPU to ensure that the task to be moved is not
currently running. Before this patch, all forced migrations
(up, offload, idle pull) use the stopper for every migration.

Using the CPU stopper is mandatory only when a task is currently
running on a CPU.  Otherwise tasks can be moved by locking the
source and destination run queues.

This patch checks to see if the task to be moved are currently
running.  If not the task is moved directly without using the
stopper thread.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Jon Medhurst <tixy@linaro.org>
[k.kozlowski: rebased on 4.1, no signed-off-by of previous committer]
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>

1 files changed, 98 insertions, 9 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fa47df3f2eda..38544ed37958 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8989,6 +8989,69 @@ out_unlock:
 	return 0;
 }
 
+/*
+ * Move task in a runnable state to another CPU.
+ *
+ * Tailored on 'active_load_balance_stop_cpu' with slight
+ * modification to locking and pre-transfer checks.  Note
+ * rq->lock must be held before calling.
+ */
+static void hmp_migrate_runnable_task(struct rq *rq)
+{
+	struct sched_domain *sd;
+	int src_cpu = cpu_of(rq);
+	struct rq *src_rq = rq;
+	int dst_cpu = rq->push_cpu;
+	struct rq *dst_rq = cpu_rq(dst_cpu);
+	struct task_struct *p = rq->migrate_task;
+	/*
+	 * One last check to make sure nobody else is playing
+	 * with the source rq.
+	 */
+	if (src_rq->active_balance)
+		return;
+
+	if (src_rq->nr_running <= 1)
+		return;
+
+	if (task_rq(p) != src_rq)
+		return;
+	/*
+	 * Not sure if this applies here but one can never
+	 * be too cautious
+	 */
+	BUG_ON(src_rq == dst_rq);
+
+	double_lock_balance(src_rq, dst_rq);
+
+	rcu_read_lock();
+	for_each_domain(dst_cpu, sd) {
+		if (cpumask_test_cpu(src_cpu, sched_domain_span(sd)))
+			break;
+	}
+
+	if (likely(sd)) {
+		struct lb_env env = {
+			.sd             = sd,
+			.dst_cpu        = dst_cpu,
+			.dst_rq         = dst_rq,
+			.src_cpu        = src_cpu,
+			.src_rq         = src_rq,
+			.idle           = CPU_IDLE,
+		};
+
+		schedstat_inc(sd, alb_count);
+
+		if (move_specific_task(&env, p))
+			schedstat_inc(sd, alb_pushed);
+		else
+			schedstat_inc(sd, alb_failed);
+	}
+
+	rcu_read_unlock();
+	double_unlock_balance(src_rq, dst_rq);
+}
+
 static DEFINE_SPINLOCK(hmp_force_migration);
 
 /*
@@ -9001,13 +9064,14 @@ static void hmp_force_up_migration(int this_cpu)
 	struct sched_entity *curr, *orig;
 	struct rq *target;
 	unsigned long flags;
-	unsigned int force;
+	unsigned int force, got_target;
 	struct task_struct *p;
 
 	if (!spin_trylock(&hmp_force_migration))
 		return;
 	for_each_online_cpu(cpu) {
 		force = 0;
+		got_target = 0;
 		target = cpu_rq(cpu);
 		raw_spin_lock_irqsave(&target->lock, flags);
 		curr = target->cfs.curr;
@@ -9030,15 +9094,14 @@ static void hmp_force_up_migration(int this_cpu)
 		if (hmp_up_migration(cpu, &target_cpu, curr)) {
 			if (!target->active_balance) {
 				get_task_struct(p);
-				target->active_balance = 1;
 				target->push_cpu = target_cpu;
 				target->migrate_task = p;
-				force = 1;
+				got_target = 1;
 				trace_sched_hmp_migrate(p, target->push_cpu, HMP_MIGRATE_FORCE);
 				hmp_next_up_delay(&p->se, target->push_cpu);
 			}
 		}
-		if (!force && !target->active_balance) {
+		if (!got_target && !target->active_balance) {
 			/*
 			 * For now we just check the currently running task.
 			 * Selecting the lightest task for offloading will
@@ -9049,14 +9112,29 @@ static void hmp_force_up_migration(int this_cpu)
 			target->push_cpu = hmp_offload_down(cpu, curr);
 			if (target->push_cpu < NR_CPUS) {
 				get_task_struct(p);
-				target->active_balance = 1;
 				target->migrate_task = p;
-				force = 1;
+				got_target = 1;
 				trace_sched_hmp_migrate(p, target->push_cpu, HMP_MIGRATE_FORCE);
 				hmp_next_down_delay(&p->se, target->push_cpu);
 			}
 		}
+		/*
+		 * We have a target with no active_balance.  If the task
+		 * is not currently running move it, otherwise let the
+		 * CPU stopper take care of it.
+		 */
+		if (got_target && !target->active_balance) {
+			if (!task_running(target, p)) {
+				trace_sched_hmp_migrate_force_running(p, 0);
+				hmp_migrate_runnable_task(target);
+			} else {
+				target->active_balance = 1;
+				force = 1;
+			}
+		}
+
 		raw_spin_unlock_irqrestore(&target->lock, flags);
+
 		if (force)
 			stop_one_cpu_nowait(cpu_of(target),
 				hmp_active_task_migration_cpu_stop,
@@ -9076,7 +9154,7 @@ static unsigned int hmp_idle_pull(int this_cpu)
 	int cpu;
 	struct sched_entity *curr, *orig;
 	struct hmp_domain *hmp_domain = NULL;
-	struct rq *target, *rq;
+	struct rq *target = NULL, *rq;
 	unsigned long flags, ratio = 0;
 	unsigned int force = 0;
 	struct task_struct *p = NULL;
@@ -9128,14 +9206,25 @@ static unsigned int hmp_idle_pull(int this_cpu)
 	raw_spin_lock_irqsave(&target->lock, flags);
 	if (!target->active_balance && task_rq(p) == target) {
 		get_task_struct(p);
-		target->active_balance = 1;
 		target->push_cpu = this_cpu;
 		target->migrate_task = p;
-		force = 1;
 		trace_sched_hmp_migrate(p, target->push_cpu, HMP_MIGRATE_IDLE_PULL);
 		hmp_next_up_delay(&p->se, target->push_cpu);
+		/*
+		 * if the task isn't running move it right away.
+		 * Otherwise setup the active_balance mechanic and let
+		 * the CPU stopper do its job.
+		 */
+		if (!task_running(target, p)) {
+			trace_sched_hmp_migrate_idle_running(p, 0);
+			hmp_migrate_runnable_task(target);
+		} else {
+			target->active_balance = 1;
+			force = 1;
+		}
 	}
 	raw_spin_unlock_irqrestore(&target->lock, flags);
+
 	if (force) {
 		stop_one_cpu_nowait(cpu_of(target),
 			hmp_idle_pull_cpu_stop,