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Subject[PATCH] sched/fair : prevent unlimited runtime on throttled group
Date
When a running task is moved on a throttled task group and there is no
other task enqueued on the CPU, the task can keep running using 100% CPU
whatever the allocated bandwidth for the group and although its cfs rq is
throttled. Furthermore, the group entity of the cfs_rq and its parents are
not enqueued but only set as curr on their respective cfs_rqs.

We have the following sequence:

sched_move_task
-dequeue_task: dequeue task and group_entities.
-put_prev_task: put task and group entities.
-sched_change_group: move task to new group.
-enqueue_task: enqueue only task but not group entities because cfs_rq is
throttled.
-set_next_task : set task and group_entities as current sched_entity of
their cfs_rq.

Another impact is that the root cfs_rq runnable_load_avg at root rq stays
null because the group_entities are not enqueued. This situation will stay
the same until an "external" event triggers a reschedule. Let trigger it
immediately instead.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
kernel/sched/core.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e7b08d52db93..d0acc67336c0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7062,8 +7062,15 @@ void sched_move_task(struct task_struct *tsk)

if (queued)
enqueue_task(rq, tsk, queue_flags);
- if (running)
+ if (running) {
set_next_task(rq, tsk);
+ /*
+ * After changing group, the running task may have joined a
+ * throttled one but it's still the running task. Trigger a
+ * resched to make sure that task can still run.
+ */
+ resched_curr(rq);
+ }

task_rq_unlock(rq, tsk, &rf);
}
--
2.7.4
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