linux-kernel - Re: [RESEND PATCH v4 1/1] psi: stop relying on timer_pending for poll

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Date:   Fri, 21 Oct 2022 12:54:16 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     peterz@...radead.org, hannes@...xchg.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v4 1/1] psi: stop relying on timer_pending for
 poll_work rescheduling

On Thu, Oct 20, 2022 at 7:11 PM Hillf Danton <hdanton@...a.com> wrote:
>
> On 10 Oct 2022 15:57:44 -0700 Suren Baghdasaryan <surenb@...gle.com>
> > Psi polling mechanism is trying to minimize the number of wakeups to
> > run psi_poll_work and is currently relying on timer_pending() to detect
> > when this work is already scheduled. This provides a window of opportunity
> > for psi_group_change to schedule an immediate psi_poll_work after
> > poll_timer_fn got called but before psi_poll_work could reschedule itself.
> > Below is the depiction of this entire window:
> >
> > poll_timer_fn
> >   wake_up_interruptible(&group->poll_wait);
> >
> > psi_poll_worker
> >   wait_event_interruptible(group->poll_wait, ...)
> >   psi_poll_work
> >     psi_schedule_poll_work
> >       if (timer_pending(&group->poll_timer)) return;
> >       ...
> >       mod_timer(&group->poll_timer, jiffies + delay);
>
> [...]
>
> >
> > -/* Schedule polling if it's not already scheduled. */
> > -static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay)
> > +/* Schedule polling if it's not already scheduled or forced. */
> > +static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay,
> > +                                bool force)
> >  {
> >       struct task_struct *task;
> >
> >       /*
> > -      * Do not reschedule if already scheduled.
> > -      * Possible race with a timer scheduled after this check but before
> > -      * mod_timer below can be tolerated because group->polling_next_update
> > -      * will keep updates on schedule.
> > +      * atomic_xchg should be called even when !force to provide a
> > +      * full memory barrier (see the comment inside psi_poll_work).
> >        */
> > -     if (timer_pending(&group->poll_timer))
> > +     if (atomic_xchg(&group->poll_scheduled, 1) && !force)
> >               return;
>
> If poll_scheduled works, turning poll_timer, which only wakes up poll
> worker, to a delayed work also works because schedule_delayed_work()
> takes care of pending work, with the bonus of cutting poll worker.

Thanks for the suggestion, Hillf.
psi_poll_worker runs at a low FIFO priority to prevent normal tasks
from preempting PSI signal generation (see sched_set_fifo_low() call
inside psi_poll_worker()), so schedule_delayed_work() would not be
usable as is I think, since it uses normal priority system_wq. I would
probably need to use queue_delayed_work() with a dedicated workqueue
that uses a worker with worker->task set to the same FIFO priority.
However I'm not sure it's worth creating a workqueue for only one task
that might be scheduled in it...
Thanks,
Suren.

>
> Hillf