Re: [PATCH 1/3] sched/psi: Avoid losing wakeups during rtpoll worker replacement

[Suren Baghdasaryan]

On Fri, Jul 17, 2026 at 2:14 AM Guopeng Zhang <guopeng.zhang@xxxxxxxxx> wrote:
>
> From: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
>
> psi_trigger_destroy() clears rtpoll_task while holding the trigger
> lock, but has to drop the lock before stopping the worker because
> psi_rtpoll_work() takes the same lock. A new trigger can therefore
> install a replacement worker before the old one exits.
>
> rtpoll_wakeup is shared by both workers. The wait condition currently

To be clear, "both workers" refer to the old worked being stopped and
the new worked created by psi_trigger_create(). Please spell that out
to avoid confusion.

> consumes it before checking whether the worker should stop. If the
> replacement timer sets the wakeup while the old worker is being stopped,
> the old worker can consume it and then exit. Since the one-shot timer has
> already fired and rtpoll_scheduled remains set, the replacement can stay
> asleep and rtpolling can stall.
>
> Make the wait condition only observe the wakeup. Check for stop before
> consuming it and, once consumed, always process the work.
>
> Fixes: 461daba06bdc ("psi: eliminate kthread_worker from psi trigger scheduling mechanism")
> Signed-off-by: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
> ---
>  kernel/sched/psi.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 4e152410653d..b9e2a93a757b 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -743,10 +743,16 @@ static int psi_rtpoll_worker(void *data)
>
>         while (true) {
>                 wait_event_interruptible(group->rtpoll_wait,
> -                               atomic_cmpxchg(&group->rtpoll_wakeup, 1, 0) ||
> +                               atomic_read(&group->rtpoll_wakeup) ||
>                                 kthread_should_stop());
>                 if (kthread_should_stop())
>                         break;
> +               /*
> +                * Consume the wakeup only after checking for stop. Once consumed,
> +                * always run the work so a replacement worker cannot lose it.
> +                */
> +               if (atomic_cmpxchg(&group->rtpoll_wakeup, 1, 0) != 1)
> +                       continue;

This works but a simpler fix is to reorder conditions in
wait_event_interruptible() call like this:

         while (true) {
                 wait_event_interruptible(group->rtpoll_wait,
                                kthread_should_stop() ||
                                atomic_cmpxchg(&group->rtpoll_wakeup, 1, 0));
                 if (kthread_should_stop())
                         break;

                 psi_rtpoll_work(group);
         }

The old worker would check kthread_should_stop() and stop before it
can consume rtpoll_wakeup. Do you agree?

>
>                 psi_rtpoll_work(group);
>         }
> --
> 2.43.0