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Date:   Mon, 19 Sep 2022 07:10:33 +0000
From:   Hao Lee <haolee.swjtu@gmail.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Zhaoyang Huang <zhaoyang.huang@unisoc.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] psi: fix possible missing or delayed pending event
Message-ID: <20220919071033.GA26869@haolee.io>
References: <20220914092959.GA20640@haolee.io>
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 <20220918105510.GA22671@haolee.io>
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On Sun, Sep 18, 2022 at 10:16:53PM -0700, Suren Baghdasaryan wrote:
> On Sun, Sep 18, 2022 at 3:55 AM Hao Lee <haolee.swjtu@gmail.com> wrote:
> >
> > On Sat, Sep 17, 2022 at 09:44:12PM -0700, Suren Baghdasaryan wrote:
> > > On Sat, Sep 17, 2022 at 12:31 AM Hao Lee <haolee.swjtu@gmail.com> wrote:
> > > >
> > > > On Fri, Sep 16, 2022 at 11:08:34PM -0700, Suren Baghdasaryan wrote:
> > > > > On Wed, Sep 14, 2022 at 2:30 AM Hao Lee <haolee.swjtu@gmail.com> wrote:
> > > > > >
> > > > > > When a pending event exists and growth is less than the threshold, the
> > > > > > current logic is to skip this trigger without generating event. However,
> > > > > > from e6df4ead85d9 ("psi: fix possible trigger missing in the window"),
> > > > > > our purpose is to generate event as long as pending event exists and the
> > > > > > rate meets the limit. This patch fixes the possible pending-event
> > > > > > missing or delay.
> > > > > >
> > > > > > Fixes: e6df4ead85d9 ("psi: fix possible trigger missing in the window")
> > > > > > Signed-off-by: Hao Lee <haolee.swjtu@gmail.com>
> > > > > > ---
> > > > > >  kernel/sched/psi.c | 2 +-
> > > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > >
> > > > > > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> > > > > > index 9711827e3..0bae4ee2b 100644
> > > > > > --- a/kernel/sched/psi.c
> > > > > > +++ b/kernel/sched/psi.c
> > > > > > @@ -539,7 +539,7 @@ static u64 update_triggers(struct psi_group *group, u64 now)
> > > > > >
> > > > > >                         /* Calculate growth since last update */
> > > > > >                         growth = window_update(&t->win, now, total[t->state]);
> > > > > > -                       if (growth < t->threshold)
> > > > > > +                       if (growth < t->threshold && !t->pending_event)
> > > > >
> > > > > I'm not sure how this additional condition changes things. Current
> > > > > logic is to set t->pending_event=true whenever growth exceeds the
> > > > > t->threshold. This patch will change this logic into setting
> > > > > t->pending_event=true also when t->pending_event=true.
> > > >
> > > > This is right.
> > > >
> > > > > But why would
> > > > > you want to set t->pending_event=true if it's already true? What am I
> > > > > missing?
> > > >
> > > > If I expand this if-else branch and the pending_event statement
> > > > to a more detailed snippet, it will be like this:
> > > >
> > > > if (growth < t->threshold && !t->pending_event) // under threshold && no pending event. Skip.
> > > >         continue;
> > > > else if (growth >= t->threshold) // above threshold. Try to generate event.
> > > >         t->pending_event = true;
> > > > else // under threshold && have pending events. Try to generate event.
> > > >         ; // pending_event is already true. do nothing
> > > >
> > > >
> > > > The original code didn't handle the `else` condition properly.
> > >
> > > The `else` condition in your code does nothing, and that's why the
> > > original code does not implement a handler for that case.
> > >
> > > > It will
> > > > skip the trigger when its growth is under the threshold, even though it
> > > > has a pending event. This patch handles this condition correctly.
> > > >
> > > > But I think assigning true to pending_event when it's already true doesn't
> > > > have other side effects, so I eliminate the `else if` branch. Maybe we'd
> > > > better make it explicit, like the above snippet? Thanks.
> > >
> > > The new code you posted is functionally the same as the original one
> > > while being more verbose and IMO less readable. Unless you can explain
> > > the problem with the original code, I don't see any reason to change
> > > it.
> >
> > Hi, for the original code, let's assume t->pending_event is true:
> >     * if new_stall is false, we will try to check event ratelimit and
> >       generate an event for this psi_trigger. This case is right.
> >     * but if new_stall is true, we will skip this psi_trigger if growth
> >       growth < t->threshold. I think we shouldn't skip this psi_trigger
> >       in this case because it has a pending event.
> 
> Ok, I see the issue now. I think the following fix would be the simplest:
> 
>                        /* Calculate growth since last update */
>                         growth = window_update(&t->win, now, total[t->state]);
> -                       if (growth < t->threshold)
> -                                continue;
> +                       if (!t->pending_event) {
> +                              if (growth < t->threshold)
> +                                        continue;
> 
> -                        t->pending_event = true;
> +                               t->pending_event = true;
> +                       }

Great! I will update in v2.

> 
> 
> >
> > >
> > > >
> > > > >
> > > > > >                                 continue;
> > > > > >
> > > > > >                         t->pending_event = true;
> > > > > > --
> > > > > > 2.21.0
> > > > > >