[PATCH 3/3] sched/fair: make tick based schedule always happen

[Aaron Lu]

When a hyperthread is forced idle and the other hyperthread has a single
CPU intensive task running, the running task can occupy the hyperthread
for a long time with no scheduling point and starve the other
hyperthread.

Fix this temporarily by always checking if the task has exceed its
timeslice and if so, for root cfs_rq, do a schedule.

Signed-off-by: Aaron Lu <ziqian.lzq@xxxxxxxxxx>
---
 kernel/sched/fair.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 26d29126d6a5..b1f0defdad91 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4011,6 +4011,9 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 		return;
 	}
 
+	if (cfs_rq->nr_running <= 1)
+		return;
+
 	/*
 	 * Ensure that a task that missed wakeup preemption by a
 	 * narrow margin doesn't have to wait for a full slice.
@@ -4179,7 +4182,7 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
 		return;
 #endif
 
-	if (cfs_rq->nr_running > 1)
+	if (cfs_rq->nr_running > 1 || cfs_rq->tg == &root_task_group)
 		check_preempt_tick(cfs_rq, curr);
 }
 
-- 
2.19.1.3.ge56e4f7