linux-kernel - Re: [PATCH] sched/fair : prevent unlimited runtime on throttled group

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Date:   Tue, 14 Jan 2020 10:29:43 -0800
From:   bsegall@...gle.com
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair : prevent unlimited runtime on throttled group

Vincent Guittot <vincent.guittot@...aro.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@...aro.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);
>  }