Re: [PATCH] psi: get poll_work to run when calling poll syscall next time

[Johannes Weiner <hannes@cmpxchg.org>]

Hi Jason,

On Tue, Jul 23, 2019 at 02:45:39PM +0800, Jason Xing wrote:
> Only when calling the poll syscall the first time can user
> receive POLLPRI correctly. After that, user always fails to
> acquire the event signal.
> 
> Reproduce case:
> 1. Get the monitor code in Documentation/accounting/psi.txt
> 2. Run it, and wait for the event triggered.
> 3. Kill and restart the process.
> 
> If the user doesn't kill the monitor process, it seems the
> poll_work works fine. After killing and restarting the monitor,
> the poll_work in kernel will never run again due to the wrong
> value of poll_scheduled. Therefore, we should reset the value
> as group_init() does after the last trigger is destroyed.
> 
> Signed-off-by: Jason Xing <kerneljasonxing@linux.alibaba.com>

Good catch, and the fix makes sense to me. However, it was a bit hard
to understand how the problem occurs:

> ---
>  kernel/sched/psi.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 7acc632..66f4385 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -1133,6 +1133,12 @@ static void psi_trigger_destroy(struct kref *ref)
>  	if (kworker_to_destroy) {
>  		kthread_cancel_delayed_work_sync(&group->poll_work);
>  		kthread_destroy_worker(kworker_to_destroy);
> +		/*
> +		 * The poll_work should have the chance to be put into the
> +		 * kthread queue when calling poll syscall next time. So
> +		 * reset poll_scheduled to zero as group_init() does
> +		 */
> +		atomic_set(&group->poll_scheduled, 0);

The question is why we can end up with poll_scheduled = 1 but the work
not running (which would reset it to 0). And the answer is because the
scheduling side sees group->poll_kworker under RCU protection and then
schedules it, but here we cancel the work and destroy the worker. The
cancel needs to pair with resetting the poll_scheduled flag:

	if (kworker_to_destroy) {
		/*
		 * After the RCU grace period has expired, the worker
		 * can no longer be found through group->poll_kworker.
		 * But it might have been already scheduled before
		 * that - deschedule it cleanly before destroying it.
		 */
		kthread_cancel_delayed_work_sync(&group->poll_work);
		atomic_set(&group->poll_scheduled, 0);

		kthread_destroy_worker(kworker_to_destroy);
	}

With that change, please add:

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Thanks!