Re: [RFC PATCH 6/7] sched/fair: fix tasks_rcu with task based throttle

[K Prateek Nayak]

Hello Aaron,

On 3/13/2025 12:52 PM, Aaron Lu wrote:
> Taskes throttled on exit to user path are scheduled by cond_resched() in
> task_work_run() but that is a preempt schedule and doesn't mark a task
> rcu quiescent state.
>
> Fix this by directly calling schedule() in throttle_cfs_rq_work().

Perhaps that can be gotten around by just using set_ti_thread_flag()
resched_curr() will also call set_preempt_need_resched() which allows
cond_resched() to resched the task.

Since exit_to_user_mode_loop() will run once again seeing that
TIF_NEED_RESCHED is set, schedule() should follow soon. Thoughts?

> Signed-off-by: Aaron Lu <ziqianlu@xxxxxxxxxxxxx>
> ---
>   kernel/sched/fair.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f26d53ac143fe..be96f7d32998c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5847,6 +5847,7 @@ static void throttle_cfs_rq_work(struct
> callback_head *work)
>   	struct cfs_rq *cfs_rq;
>   	struct rq *rq;
>   	struct rq_flags rf;
> +	bool sched = false;
>
>   	WARN_ON_ONCE(p != current);
>   	p->sched_throttle_work.next = &p->sched_throttle_work;
> @@ -5879,9 +5880,13 @@ static void throttle_cfs_rq_work(struct
> callback_head *work)
>   	dequeue_task_fair(rq, p, DEQUEUE_SLEEP | DEQUEUE_SPECIAL);
>   	list_add(&p->throttle_node, &cfs_rq->throttled_limbo_list);
>   	resched_curr(rq);
> +	sched = true;
>
>   out_unlock:
>   	task_rq_unlock(rq, p, &rf);
> +
> +	if (sched)
> +		schedule();
>   }
>
>   void init_cfs_throttle_work(struct task_struct *p)

--
Thanks and Regards,
Prateek