[RFC][PATCH]: sched/fair: search a task from the tail of the queue

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

From: Uladzislau Rezki <urezki@gmail.com>

When a task is enqueued back from a physical CPU to the running
list it is placed in the beginning of the queue. Thus, the cfs_tasks
list is more or less sorted (except woken tasks) starting from recently
given CPU time tasks toward tasks with max wait time in a run-queue.

As part of the load balance operation, this approach starts
detach_tasks()/detach_one_task() from the tail of the queue
instead of the head, giving some advantages:

- tends to pick a task with highest wait time;
- tasks located in the tail are less likely cache-hot,
  therefore the can_migrate_task() decision is higher.

hackbench illustrates slightly better performance. For example
doing 1000 samples and 40 groups on i5-3320M CPU, it shows below
figures:

default: 0.672 avg / 0.674 median
patched: 0.665 avg / 0.667 median

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 kernel/sched/fair.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c77e4b1d51c0..028f5e8bc830 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6806,11 +6806,12 @@ static void detach_task(struct task_struct *p, struct lb_env *env)
  */
 static struct task_struct *detach_one_task(struct lb_env *env)
 {
-	struct task_struct *p, *n;
+	struct task_struct *p;
 
 	lockdep_assert_held(&env->src_rq->lock);
 
-	list_for_each_entry_safe(p, n, &env->src_rq->cfs_tasks, se.group_node) {
+	list_for_each_entry_reverse(p,
+			&env->src_rq->cfs_tasks, se.group_node) {
 		if (!can_migrate_task(p, env))
 			continue;
 
@@ -6856,7 +6857,7 @@ static int detach_tasks(struct lb_env *env)
 		if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1)
 			break;
 
-		p = list_first_entry(tasks, struct task_struct, se.group_node);
+		p = list_last_entry(tasks, struct task_struct, se.group_node);
 
 		env->loop++;
 		/* We've more or less seen every task there is, call it quits */
@@ -6906,7 +6907,7 @@ static int detach_tasks(struct lb_env *env)
 
 		continue;
 next:
-		list_move_tail(&p->se.group_node, tasks);
+		list_move(&p->se.group_node, tasks);
 	}
 
 	/*
-- 
2.11.0