Re: [PATCH v3 1/1] psi: stop relying on timer_pending for poll_work rescheduling

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jul 7, 2021 at 6:39 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> This looks good to me now code wise. Just a comment on the comments:
>
> On Tue, Jul 06, 2021 at 07:39:33PM -0700, Suren Baghdasaryan wrote:
> > @@ -559,18 +560,14 @@ static u64 update_triggers(struct psi_group *group, u64 now)
> >       return now + group->poll_min_period;
> >  }
> >
> > -/* 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.
> > -      */
> > -     if (timer_pending(&group->poll_timer))
> > +     /* xchg should be called even when !force to set poll_scheduled */
> > +     if (atomic_xchg(&group->poll_scheduled, 1) && !force)
> >               return;
>
> This explains what the code does, but not why. It would be good to
> explain the ordering with poll_work, here or there. But both sides
> should mention each other.

How about this:

/*
 * atomic_xchg should be called even when !force to always set poll_scheduled
 * and to provide a memory barrier (see the comment inside psi_poll_work).
 */

>
> > @@ -595,6 +595,28 @@ static void psi_poll_work(struct psi_group *group)
> >
> >       now = sched_clock();
> >
> > +     if (now > group->polling_until) {
> > +             /*
> > +              * We are either about to start or might stop polling if no
> > +              * state change was recorded. Resetting poll_scheduled leaves
> > +              * a small window for psi_group_change to sneak in and schedule
> > +              * an immegiate poll_work before we get to rescheduling. One
> > +              * potential extra wakeup at the end of the polling window
> > +              * should be negligible and polling_next_update still keeps
> > +              * updates correctly on schedule.
> > +              */
> > +             atomic_set(&group->poll_scheduled, 0);
> > +             /*
> > +              * Ensure that operations of clearing group->poll_scheduled and
> > +              * obtaining changed_states are not reordered.
> > +              */
> > +             smp_mb();
>
> Same here, it would be good to explain that this is ordering the
> scheduler with the timer such that no events are missed. Feel free to
> reuse my race diagram from the other thread - those are better at
> conveying the situation than freeform text.

I tried to make your diagram a bit less abstract by using the actual
names. How about this?

/*
 * We need to enforce ordering between poll_scheduled and psi_group_cpu.times
 * reads and writes in psi_poll_work and psi_group_change functions.
Otherwise we
 * might fail to reschedule the timer when monitored states change:
 *
 * psi_poll_work:
 *     poll_scheduled = 0
 *     smp_mb()
 *     changed_states = collect_percpu_times()
 *     if changed_states && xchg(poll_scheduled, 1) == 0
 *         mod_timer()
 *
 * psi_group_change:
 *     record_times()
 *     smp_mb()
 *     if xchg(poll_scheduled, 1) == 0
 *         mod_timer()
 *
 * atomic_xchg in psi_schedule_poll_work implements an implicit memory
barrier but
 * we need an explicit one here.
 */

If we remove smp_mb barriers then there are the following possible
reordering cases:

Case1: reordering in psi_poll_work
psi_poll_work                    psi_group_change
  changed_states = collect_percpu_times()
                                              record_times()
                                              if xchg(poll_scheduled,
1) == 0 <-- false
                                                  mod_timer()
  poll_scheduled = 0
  if changed_states && xchg(poll_scheduled, 1) == 0 <-- changed_states is false
      mod_timer()

Case2: reordering in psi_group_change
psi_poll_work                    psi_group_change
                                              if xchg(poll_scheduled,
1) == 0 <-- false
                                                  mod_timer()
  poll_scheduled = 0
  changed_states = collect_percpu_times()
                                                  record_times()
  if changed_states && xchg(poll_scheduled, 1) == 0 <-- changed_states is false
      mod_timer()

In both cases mod_timer() is not called, poll update is missed. But
describing this all in the comments would be an overkill IMHO.
WDYT?

>
> Thanks