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

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Message-ID: <CAJuCfpFdb7VAZExs=ejqG0FDJqRi4LwOEu5E1EM8rtyv1HzbaQ@mail.gmail.com>
Date:   Tue, 21 Dec 2021 16:31:12 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Huangzhaoyang <huangzhaoyang@...il.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3] psi: fix possible trigger missing in the window

On Tue, Dec 21, 2021 at 4:24 PM Huangzhaoyang <huangzhaoyang@...il.com> wrote:
>
> From: Zhaoyang Huang <zhaoyang.huang@...soc.com>

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@...gle.com>
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> ---
> 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
>