[RFC,v2 3/3] sched: ignore task_h_load for CPU_NEWLY_IDLE

[Uladzislau Rezki]

From: Uladzislau 2 Rezki <uladzislau2.rezki@xxxxxxxxxxxxxx>

A load balancer calculates imbalance factor for particular shed
domain and tries to steal up the prescribed amount of weighted load.
However, a small imbalance factor would sometimes prevent us from
stealing any tasks at all. When a CPU is newly idle, it should
steal first task which passes a migration criteria.

Signed-off-by: Uladzislau 2 Rezki <uladzislau2.rezki@xxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 232ef3c..29e0d7f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6802,6 +6802,14 @@ static int detach_tasks(struct lb_env *env)
 		if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1)
 			break;
 
+		/*
+		 * Another CPU can place tasks, since we do not hold dst_rq lock
+		 * while doing balancing. If newly idle CPU already got something,
+		 * give up to reduce latency.
+		 */
+		if (env->idle == CPU_NEWLY_IDLE && env->dst_rq->nr_running > 0)
+			break;
+
 		p = list_first_entry(tasks, struct task_struct, se.group_node);
 
 		env->loop++;
@@ -6824,8 +6832,9 @@ static int detach_tasks(struct lb_env *env)
 		if (sched_feat(LB_MIN) && load < 16 && !env->sd->nr_balance_failed)
 			goto next;
 
-		if ((load / 2) > env->imbalance)
-			goto next;
+		if (env->idle != CPU_NEWLY_IDLE)
+			if ((load / 2) > env->imbalance)
+				goto next;
 
 		detach_task(p, env);
 		list_add(&p->se.group_node, &env->tasks);
-- 
2.1.4