[PATCH 3/3] sched/fair: reduce cases for active balance

[Vincent Guittot <vincent.guittot@linaro.org>]

Active balance is triggered for a number of voluntary case like misfit or
pinned tasks cases but also after that a number of load balance failed to
migrate a task. Remove the active load balance case for overloaded group
as an overloaded state means that there is at least one waiting tasks. The
threshold on the upper limit of the task's load will decrease with the
number of failed LB until the task has migrated.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 43 +++++++++++++++++++++----------------------
 1 file changed, 21 insertions(+), 22 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 69a455113b10..ee87fd6f7359 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9499,13 +9499,30 @@ asym_active_balance(struct lb_env *env)
 }
 
 static inline bool
-voluntary_active_balance(struct lb_env *env)
+imbalanced_active_balance(struct lb_env *env)
+{
+	struct sched_domain *sd = env->sd;
+
+	/* The imbalanced case includes the case of pinned tasks preventing a fair
+	 * distribution of the load on the system but also the even distribution of the
+	 * threads on a system with spare capacity
+	 */
+	if ((env->migration_type == migrate_task) &&
+		(sd->nr_balance_failed > sd->cache_nice_tries+2))
+		return 1;
+
+	return 0;
+}
+
+static int need_active_balance(struct lb_env *env)
 {
 	struct sched_domain *sd = env->sd;
 
 	if (asym_active_balance(env))
 		return 1;
 
+	if (imbalanced_active_balance(env))
+		return 1;
 	/*
 	 * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
 	 * It's worth migrating the task if the src_cpu's capacity is reduced
@@ -9525,16 +9542,6 @@ voluntary_active_balance(struct lb_env *env)
 	return 0;
 }
 
-static int need_active_balance(struct lb_env *env)
-{
-	struct sched_domain *sd = env->sd;
-
-	if (voluntary_active_balance(env))
-		return 1;
-
-	return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
-}
-
 static int active_load_balance_cpu_stop(void *data);
 
 static int should_we_balance(struct lb_env *env)
@@ -9785,21 +9792,13 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 			/* We've kicked active balancing, force task migration. */
 			sd->nr_balance_failed = sd->cache_nice_tries+1;
 		}
-	} else
+	} else {
 		sd->nr_balance_failed = 0;
+	}
 
-	if (likely(!active_balance) || voluntary_active_balance(&env)) {
+	if (likely(!active_balance) || need_active_balance(&env)) {
 		/* We were unbalanced, so reset the balancing interval */
 		sd->balance_interval = sd->min_interval;
-	} else {
-		/*
-		 * If we've begun active balancing, start to back off. This
-		 * case may not be covered by the all_pinned logic if there
-		 * is only 1 task on the busy runqueue (because we don't call
-		 * detach_tasks).
-		 */
-		if (sd->balance_interval < sd->max_interval)
-			sd->balance_interval *= 2;
 	}
 
 	goto out;
-- 
2.17.1