linux-kernel - Re: [RFC][PATCH 2/2] time: alarmtimer: Use TASK

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Message-ID: <CAJZ5v0jEZ6dgy5TjKVZ7dE24WX6HHNOYKR1BG5wM1gS2_ONXeA@mail.gmail.com>
Date:   Thu, 2 Mar 2023 16:00:52 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     John Stultz <jstultz@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Michael Nazzareno Trimarchi <michael@...rulasolutions.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephen Boyd <sboyd@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Michael <michael@...isi.de>, kernel-team@...roid.com,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [RFC][PATCH 2/2] time: alarmtimer: Use TASK_FREEZABLE to cleanup
 freezer handling

On Thu, Mar 2, 2023 at 1:48 AM John Stultz <jstultz@...gle.com> wrote:
>
> On Wed, Mar 1, 2023 at 2:11 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > On Mon, Feb 27 2023 at 20:06, John Stultz wrote:
> > > On Mon, Feb 27, 2023 at 4:03 PM John Stultz <jstultz@...gle.com> wrote:
> > >> > On Mon, Feb 20 2023 at 19:11, Michael Nazzareno Trimarchi wrote:
> > >> > +static int alarmtimer_pm_notifier_fn(struct notifier_block *bl, unsigned long state,
> > >> > +                                    void *unused)
> > >> > +{
> > >> > +       switch (state) {
> > >> > +       case PM_HIBERNATION_PREPARE:
> > >> > +       case PM_POST_HIBERNATION:
> > >> > +               atomic_set(&alarmtimer_wakeup, 0);
> > >> > +               break;
> > >> > +       }
> > >> > +       return NOTIFY_DONE;
> > >>
> > >> But here, we're setting the alarmtimer_wakeup count to zero if we get
> > >> PM_HIBERNATION_PREPARE or  PM_POST_HIBERNATION notifications?
> > >> And Michael noted we need to add  PM_SUSPEND_PREPARE and
> > >> PM_POST_SUSPEND there for this to seemingly work.
> >
> > Yup. I missed those when sending out that hack.
> >
> > > So Thomas's notifier method of zeroing at the begining of suspend and
> > > tracking any wakeups after that point makes more sense now. It still
> > > feels a bit messy, but I'm not sure there's something better.
> >
> > I'm not enthused about it either.
>
> That said, it does work. :) In my testing, your approach has been
> reliable, so it has that going for it.
>
> > > My only thought is this feels a little bit like its mirroring what the
> > > pm_wakeup_event() logic is supposed to do. Should we be adding a
> > > pm_wakeup_event() to alarmtimer_fired() to try to prevent suspend from
> > > occuring for 500ms or so after an alarmtimer has fired so there is
> > > enough time for it to be re-armed if needed?
> >
> > The question is whether this can be called unconditionally and how that
> > interacts with the suspend logic. Rafael?
>
> I took a brief stab at this, and one thing is the test needs to use
> the /sys/power/wakeup_count dance before suspending.

That's correct.

> However, I still had some cases where the recurring alarmtimer got
> lost, so I need to dig a bit more to understand what was going wrong
> there.

I'm interested in that too, so if you have any conclusions, please let me know.

> In the meantime, I'm ok with Thomas' approach, but we probably need
> some comment documentation that suggests it might be reworked in a
> cleaner way?

Well, in theory, the PM notifier can run in parallel with
alarmtimer_fired() right after it has incremented the atomic var,
can't it?