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

[Suren Baghdasaryan]

On Tue, Jun 22, 2021 at 8:08 AM Johannes Weiner <hannes at cmpxchg.org> wrote:
>
> On Tue, Jun 22, 2021 at 03:56:03PM +0200, Peter Zijlstra wrote:
> > On Thu, Jun 17, 2021 at 02:26:54PM -0700, Suren Baghdasaryan wrote:
> > > Fixes: 461daba06bdc ("psi: eliminate kthread_worker from psi trigger scheduling mechanism")
> > > Reported-by: Kathleen Chang <yt.chang at mediatek.com>
> > > Reported-by: Wenju Xu <wenju.xu at mediatek.com>
> > > Reported-by: Jonathan Chen <jonathan.jmchen at mediatek.com>
> > > Signed-off-by: Suren Baghdasaryan <surenb at google.com>
> >
> > Johannes?
>
> Looks generally good to me, I'd just want to get to the bottom of the
> memory ordering before acking...
>
> > > -/* 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))
> > > +   /* cmpxchg should be called even when !force to set poll_scheduled */
> > > +   if (atomic_cmpxchg(&group->poll_scheduled, 0, 1) != 0 && !force)

Will change to Peter's suggested "if
(atomic_cmpxchg(&group->poll_scheduled, 0, 1) && !force)" once the
ordering question is finalized.

> >
> > Do you care about memory ordering here? Afaict the whole thing is
> > supposed to be ordered by ->trigger_lock, so you don't.
>
> It's not always held when we get here.
>
> The worker holds it when it reschedules itself, to serialize against
> userspace destroying the trigger itself. But the scheduler doesn't
> hold it when it kicks the worker on an actionable task change.
>
> That said, I think the ordering we care about there is that when the
> scheduler side sees the worker still queued, the worker must see the
> scheduler's updates to the percpu states and process them correctly.
> But that should be ensured already by the ordering implied by the
> seqcount sections around both the writer and the reader side.

Thanks Johannes! I have nothing to add here really.

>
> Is there another possible race that I'm missing?