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

[Suren Baghdasaryan]

On Wed, Dec 22, 2021 at 4:50 PM Zhaoyang Huang <huangzhaoyang@xxxxxxxxx> wrote:
>
> @Peter Zijlstra @Johannes Weiner
> Would you please review this patch? We would like to have it be merged
> ASAP. Thanks.

It's the holiday season, so I would not expect a quick turnaround.

>
> 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
> > >