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SubjectRe: [PATCH] sched/fair : prevent unlimited runtime on throttled group
Date
Vincent Guittot <vincent.guittot@linaro.org> writes:

> 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.

Sounds reasonable to me, "moved group" being an explicit resched check
doesn't sound like a problem in general.

>
> 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);
> }

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