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

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Date:   Thu, 2 Mar 2023 15:56:43 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     John Stultz <jstultz@...gle.com>,
        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 3:54 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Wed, Mar 1, 2023 at 11:11 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.
> >
> > > 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?
>
> The pm_wakeup_event() doesn't need the timeout, but it is conditional
> on using /sys/power/wakeup_count.
>
> However, in any case it doesn't guarantee that the target user of the
> alarm timer will be able to handle the signal on time AFAICS.  To my
> eyes, it is entirely possible for alarmtimer_fired() to run before
> /sys/power/wakeup_count is written to while the signal will be sent to

I actually should have said "read from" instead of "written to" here, sorry.

> the given task later, in which case there is no guarantee that the
> task will not be frozen in the meantime.
>
> Moreover, I'm wondering if all alarm timers should always wake up the
> system from sleep or abort suspends in progress?  If the answer is
> "no" here, it changes the problem at hand quite a bit.