[RFC PATCH] sched/fair: update the vruntime to be max vruntime when yield

[Xuewen Yan]

When task call the sched_yield, cfs would set the cfs's skip buddy.
If there is no other task call the sched_yield syscall, the task would
always be skiped when there are tasks in rq. As a result, the task's
vruntime would not be updated for long time, and the cfs's min_vruntime
is almost not updated.
When this scenario happens, when the yield task had wait for a long time,
and other tasks run a long time, once there is other task call the sched_yield,
the cfs's skip_buddy is covered, at this time, the first task can run normally,
but the task's vruntime is small, as a result, the task would always run,
because other task's vruntime is big. This would lead to other tasks can not
run for a long time.
In order to mitigate this, update the yield_task's vruntime to be cfs's max vruntime.
This way, the cfs's min vruntime can be updated as the process running.

Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
---
 kernel/sched/fair.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ff4dbbae3b10..a9ff1921fc07 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -670,7 +670,6 @@ static struct sched_entity *__pick_next_entity(struct sched_entity *se)
 	return __node_2_se(next);
 }
 
-#ifdef CONFIG_SCHED_DEBUG
 struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
 {
 	struct rb_node *last = rb_last(&cfs_rq->tasks_timeline.rb_root);
@@ -681,6 +680,7 @@ struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq)
 	return __node_2_se(last);
 }
 
+#ifdef CONFIG_SCHED_DEBUG
 /**************************************************************
  * Scheduling class statistics methods:
  */
@@ -7751,8 +7751,18 @@ static void set_next_buddy(struct sched_entity *se)
 
 static void set_skip_buddy(struct sched_entity *se)
 {
-	for_each_sched_entity(se)
-		cfs_rq_of(se)->skip = se;
+	for_each_sched_entity(se) {
+		struct sched_entity *last;
+		struct cfs_rq *cfs_rq = cfs_rq_of(se);
+
+		last = __pick_last_entity(cfs_rq);
+		if (last) {
+			se->vruntime = last->vruntime;
+			update_min_vruntime(cfs_rq);
+		}
+
+		cfs_rq->skip = se;
+	}
 }
 
 /*
-- 
2.25.1