Message-ID: <CAJuCfpFn_TJbOp69wR26jsqDquM+5EASr7giJWSHVQ2gvRcCHw@mail.gmail.com>
Date:   Mon, 20 Dec 2021 22:37:15 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] psi: fix possible trigger missing in the window

On Mon, Dec 20, 2021 at 7:08 PM Zhaoyang Huang <huangzhaoyang@...il.com> wrote:
>
> On Tue, Dec 21, 2021 at 11:00 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
> >
> > On Mon, Dec 20, 2021 at 6:51 PM Zhaoyang Huang <huangzhaoyang@...il.com> wrote:
> > >
> > > On Tue, Dec 21, 2021 at 10:30 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
> > > >
> > > > On Mon, Dec 20, 2021 at 5:57 PM Zhaoyang Huang <huangzhaoyang@...il.com> wrote:
> > > > >
> > > > > On Tue, Dec 21, 2021 at 3:58 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
> > > > > >
> > > > > > On Fri, Dec 17, 2021 at 10:03 PM Zhaoyang Huang <huangzhaoyang@...il.com> wrote:
> > > > > > >
> > > > > > > loop Suren
> > > > > >
> > > > > > Thanks.
> > > > > >
> > > > > >
> > > > > > >
> > > > > > > On Fri, Dec 17, 2021 at 2:08 PM Huangzhaoyang <huangzhaoyang@...il.com> wrote:
> > > > > > > >
> > > > > > > > From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> > > > > > > >
> > > > > > > > There could be missing wake up if the rest of the window remain the
> > > > > > > > same stall states as the polling_total updates for every polling_min_period.
> > > > > >
> > > > > > Could you please expand on this description? I'm unclear what the
> > > > > > problem is. I assume "polling_min_period" in this description refers
> > > > > > to the group->poll_min_period.
> > > > > >
> > > > > > From the code, looks like the change results in update_triggers()
> > > > > > calling window_update() once there was a new stall recorded for the
> > > > > > trigger state and until the tracking window is complete. I don't see
> > > > > > the point of calling window_update() if there was no stall change
> > > > > > since the last call to window_update(). The resulting growth will not
> > > > > > increase if there is no new stall.
> > > > > > Maybe what you want to achieve here is more than one trigger per
> > > > > > window if the stall limit was breached? If so, then this goes against
> > > > > > the design for psi triggers in which we want to rate-limit the number
> > > > > > of generated triggers per tracking window (see:
> > > > > > https://elixir.bootlin.com/linux/latest/source/kernel/sched/psi.c#L545).
> > > > > > Please clarify the issue and the intentions here.
> > > > > > Thanks!
> > > > > Please correct me if I am wrong. Imagine that there is a new stall
> > > > > during the 1st polling_min_period among 10 of them in the window and
> > > > > group->polling_total will be updated to total without trigger. If the
> > > > > rest of 9 polling_min_periods remain the same states, the trigger will
> > > > > be missed when window timing is reached.
> > > >
> > > > I don't see why updating group->polling_total after the first
> > > > registered stall is an issue here. window_update() calculates growth
> > > > using current group->total[] and win->start_value (see:
> > > > https://elixir.bootlin.com/linux/latest/source/kernel/sched/psi.c#L483)
> > > > which is set at the beginning of the window (see:
> > > > https://elixir.bootlin.com/linux/latest/source/kernel/sched/psi.c#L462).
> > > > If the calculated growth did not reach t->threshold then the trigger
> > > > should not be fired (see:
> > > > https://elixir.bootlin.com/linux/latest/source/kernel/sched/psi.c#L542).
> > > > We fire the trigger only if growth within a given window is higher
> > > > than the threshold.
> > > >
> > > > In your scenario if the stall recorded in the 1st polling_min_period
> > > > was less than the threshold and in the other 9 polling_min_periods no
> > > > new stalls were registered then there should be no triggers fired in
> > > > that window. This is intended behavior. Trigger is fired only when the
> > > The stall in the 1st polling_min_period was *LARGE* then the threshold
> > > will also be ignored here.
> >
> > Ok, in that case is it ignored due to this condition:
> > https://elixir.bootlin.com/linux/latest/source/kernel/sched/psi.c#L546
> > ?
> > If so then I see what the problem might be. Please confirm.
> Yes.

Ok, I understand the issue now. Couple problems with your approach, see inline:

> Actually, we found LMKD is driven by the inside polling_intervals
> which lose the trigger from PSI.
> >
> > >
> > > > recorded stall within the window breaches the threshold. And there
> > > > will be only one trigger generated per window, no matter how much
> > > > stall is being recorded after the threshold was breached.
> > > > Hopefully this clarifies the behavior?
> > >
> > > I don't think so. According to your opinion, if the total keeps no
> > > change in the last polling_min_period, then the growth during the 1-9
> > > min_periods which is much larger than the threshold will also be
> > > ignored. It does not make sense.
> > >
> > > https://elixir.bootlin.com/linux/latest/source/kernel/sched/psi.c#L529
> > > >
> > > > > >
> > > > > > > >
> > > > > > > > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> > > > > > > > ---
> > > > > > > >  include/linux/psi_types.h |  2 ++
> > > > > > > >  kernel/sched/psi.c        | 30 ++++++++++++++++++------------
> > > > > > > >  2 files changed, 20 insertions(+), 12 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
> > > > > > > > index 0a23300..9533d2e 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 new_stall;
> > > > > > > >  };
> > > > > > > >
> > > > > > > >  struct psi_group {
> > > > > > > > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> > > > > > > > index 1652f2b..402718c 100644
> > > > > > > > --- a/kernel/sched/psi.c
> > > > > > > > +++ b/kernel/sched/psi.c
> > > > > > > > @@ -458,9 +458,12 @@ static void psi_avgs_work(struct work_struct *work)
> > > > > > > >  static void window_reset(struct psi_window *win, u64 now, u64 value,
> > > > > > > >                          u64 prev_growth)
> > > > > > > >  {
> > > > > > > > +       struct psi_trigger *t = container_of(win, struct psi_trigger, win);
> > > > > > > > +
> > > > > > > >         win->start_time = now;
> > > > > > > >         win->start_value = value;
> > > > > > > >         win->prev_growth = prev_growth;
> > > > > > > > +       t->new_stall = false;
> > > > > > > >  }
> > > > > > > >
> > > > > > > >  /*
> > > > > > > > @@ -515,7 +518,6 @@ static void init_triggers(struct psi_group *group, u64 now)
> > > > > > > >  static u64 update_triggers(struct psi_group *group, u64 now)
> > > > > > > >  {
> > > > > > > >         struct psi_trigger *t;
> > > > > > > > -       bool new_stall = false;
> > > > > > > >         u64 *total = group->total[PSI_POLL];
> > > > > > > >
> > > > > > > >         /*
> > > > > > > > @@ -523,19 +525,26 @@ static u64 update_triggers(struct psi_group *group, u64 now)
> > > > > > > >          * watchers know when their specified thresholds are exceeded.
> > > > > > > >          */
> > > > > > > >         list_for_each_entry(t, &group->triggers, node) {
> > > > > > > > -               u64 growth;
> > > > > > > > -
> > > > > > > >                 /* 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.
> > > > > > > > +                * update the trigger if there is new stall which will be
> > > > > > > > +                * reset when run out of the window
> > > > > > > >                  */
> > > > > > > > -               new_stall = true;
> > > > > > > > +               t->new_stall = true;
> > > > > > > > +
> > > > > > > > +               memcpy(&group->polling_total[t->state], &total [t->state],
> > > > > > > > +                               sizeof(group->polling_total[t->state]));

If you reset group->polling_total[t->state] here then if there is
another trigger t2 in the group->triggers so that t->state ==
t2->state then t2->new_stall will never be set. That's what the
"Multiple triggers..." comment which you deleted was warning about.
BTW, the above memcpy() can be replaced with a simple assignment:
               group->polling_total[t->state] = total[t->state];


> > > > > > > > +       }
> > > > > > > > +
> > > > > > > > +       list_for_each_entry(t, &group->triggers, node) {
> > > > > > > > +               u64 growth;
> > > > > > > > +
> > > > > > > > +               /* check if new stall happened during this window*/
> > > > > > > > +               if (!t->new_stall)
> > > > > > > > +                       continue;

With this check, once t->new_stall was set to true, window_update()
will be called every time update_triggers() is called, even if there
is no new stall for multiple periods. This is suboptimal. I would
rather remember that we skipped event generation here
https://elixir.bootlin.com/linux/latest/source/kernel/sched/psi.c#L546
and catch up later. Something like this (untested):

--- a/include/linux/psi_types.h
+++ b/include/linux/psi_types.h
@@ -130,6 +130,9 @@ struct psi_trigger {
  */
  u64 last_event_time;

+ /* Flag set when threshold is breached but event was rate-limited */
+ bool threshold_breach;
+
  /* Refcounting to prevent premature destruction */
  struct kref refcount;
 };
diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 1652f2bb54b7..301b31e860ef 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -524,23 +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])
+ /* Check for stall activity or a previous threshold breach */
+ if (!trigger_stalled && !t->threshold_breach)
  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)
- 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)
@@ -550,6 +556,8 @@ 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)
@@ -1150,6 +1158,7 @@ struct psi_trigger *psi_trigger_create(struct
psi_group *group,

  t->event = 0;
  t->last_event_time = 0;
+ t->threshold_breach = false;
  init_waitqueue_head(&t->event_wait);
  kref_init(&t->refcount);


> > > > > > > >
> > > > > > > >                 /* Calculate growth since last update */
> > > > > > > >                 growth = window_update(&t->win, now, total[t->state]);
> > > > > > > > @@ -552,10 +561,6 @@ static u64 update_triggers(struct psi_group *group, u64 now)
> > > > > > > >                 t->last_event_time = now;
> > > > > > > >         }
> > > > > > > >
> > > > > > > > -       if (new_stall)
> > > > > > > > -               memcpy(group->polling_total, total,
> > > > > > > > -                               sizeof(group->polling_total));
> > > > > > > > -
> > > > > > > >         return now + group->poll_min_period;
> > > > > > > >  }
> > > > > > > >
> > > > > > > > @@ -1152,6 +1157,7 @@ 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->new_stall = false;
> > > > > > > >
> > > > > > > >         mutex_lock(&group->trigger_lock);
> > > > > > > >
> > > > > > > > --
> > > > > > > > 1.9.1
> > > > > > > >

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