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

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Date:   Wed, 15 Mar 2023 21:12:14 +0100
From:   Michael Nazzareno Trimarchi <michael@...rulasolutions.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>
Subject: Re: [RFC][PATCH 2/2] time: alarmtimer: Use TASK_FREEZABLE to cleanup
 freezer handling

Hi Rafael

On Thu, Mar 2, 2023 at 4:01 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> 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?

Can we add a function like pm_get_state()?

So can we mark the suspend window using some state?

Michael