Re: [PATCH v3] psi: fix possible trigger missing in the window

[Zhaoyang Huang]

@Peter Zijlstra @Johannes Weiner
Would you please review this patch? We would like to have it be merged
ASAP. Thanks.

On Wed, Dec 22, 2021 at 8:31 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Tue, Dec 21, 2021 at 4:24 PM Huangzhaoyang <huangzhaoyang@xxxxxxxxx> wrote:
> >
> > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
>
> Please remember to CC Peter.
>
> >
> > When a new threshold breaching stall happens after a psi event was
> > generated and within the window duration, the new event is not
> > generated because the events are rate-limited to one per window. If
> > after that no new stall is recorded then the event will not be
> > generated even after rate-limiting duration has passed. This is
> > happening because with no new stall, window_update will not be called
> > even though threshold was previously breached. To fix this, record
> > threshold breaching occurrence and generate the event once window
> > duration is passed.
> >
> > Suggested-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > ---
> > v2: modify the logic according to Suren's suggestion
> > v3: update commit message
> > ---
> > ---
> >  include/linux/psi_types.h |  2 ++
> >  kernel/sched/psi.c        | 38 +++++++++++++++++++++++---------------
> >  2 files changed, 25 insertions(+), 15 deletions(-)
> >
> > diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
> > index 0a23300..87b694a 100644
> > --- a/include/linux/psi_types.h
> > +++ b/include/linux/psi_types.h
> > @@ -132,6 +132,8 @@ struct psi_trigger {
> >
> >         /* Refcounting to prevent premature destruction */
> >         struct kref refcount;
> > +
> > +       bool threshold_breach;
> >  };
> >
> >  struct psi_group {
> > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> > index 1652f2b..5c67ab9 100644
> > --- a/kernel/sched/psi.c
> > +++ b/kernel/sched/psi.c
> > @@ -524,24 +524,29 @@ static u64 update_triggers(struct psi_group *group, u64 now)
> >          */
> >         list_for_each_entry(t, &group->triggers, node) {
> >                 u64 growth;
> > +               bool trigger_stalled =
> > +                       group->polling_total[t->state] != total[t->state];
> >
> > -               /* Check for stall activity */
> > -               if (group->polling_total[t->state] == total[t->state])
> > -                       continue;
> > -
> > -               /*
> > -                * Multiple triggers might be looking at the same state,
> > -                * remember to update group->polling_total[] once we've
> > -                * been through all of them. Also remember to extend the
> > -                * polling time if we see new stall activity.
> > -                */
> > -               new_stall = true;
> > -
> > -               /* Calculate growth since last update */
> > -               growth = window_update(&t->win, now, total[t->state]);
> > -               if (growth < t->threshold)
> > +               /* Check for stall activity or a previous threshold breach */
> > +               if (!trigger_stalled && !t->threshold_breach)
> >                         continue;
> >
> > +               if (trigger_stalled) {
> > +                       /*
> > +                        * Multiple triggers might be looking at the same state,
> > +                        * remember to update group->polling_total[] once we've
> > +                        * been through all of them. Also remember to extend the
> > +                        * polling time if we see new stall activity.
> > +                        */
> > +                       new_stall = true;
> > +
> > +                       /* Calculate growth since last update */
> > +                       growth = window_update(&t->win, now, total[t->state]);
> > +                       if (growth < t->threshold)
> > +                               continue;
> > +
> > +                       t->threshold_breach = true;
> > +               }
> >                 /* Limit event signaling to once per window */
> >                 if (now < t->last_event_time + t->win.size)
> >                         continue;
> > @@ -550,6 +555,8 @@ static u64 update_triggers(struct psi_group *group, u64 now)
> >                 if (cmpxchg(&t->event, 0, 1) == 0)
> >                         wake_up_interruptible(&t->event_wait);
> >                 t->last_event_time = now;
> > +               /* Reset threshold breach flag once event got generated */
> > +               t->threshold_breach = false;
> >         }
> >
> >         if (new_stall)
> > @@ -1152,6 +1159,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
> >         t->last_event_time = 0;
> >         init_waitqueue_head(&t->event_wait);
> >         kref_init(&t->refcount);
> > +       t->threshold_breach = false;
> >
> >         mutex_lock(&group->trigger_lock);
> >
> > --
> > 1.9.1
> >