Re: [PATCH] PM / suspend: Count suspend-to-idle loop as sleep time

["Rafael J. Wysocki" <rafael@kernel.org>]

On Fri, Sep 14, 2018 at 9:41 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Sep 14, 2018 at 08:59:03AM +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> >
> > There is a difference in behavior between suspend-to-idle and
> > suspend-to-RAM in the timekeeping handling that leads to functional
> > issues.  Namely, every iteration of the loop in s2idle_loop()
> > increases the monotinic clock somewhat, even if timekeeping_suspend()
> > and timekeeping_resume() are invoked from s2idle_enter(), and if
> > many of them are carried out in a row, the monotonic clock can grow
> > significantly while the system is regarded as suspended, which
> > doesn't happen during suspend-to-RAM and so it is unexpected and
> > leads to confusion and misbehavior in user space (similar to what
> > ensued when we tried to combine the boottime and monotonic clocks).
> >
> > To avoid that, count all iterations of the loop in s2idle_loop()
> > as "sleep time" and adjust the clock for that on exit from
> > suspend-to-idle.
> >
> > [That also covers systems on which timekeeping is not suspended
> >  by by s2idle_enter().]
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> Do we want a 'warning' of sorts when the delta becomes significant (for
> whatever that is) ? That might be an indication that there are frequent
> wakeups which we might not be expecting. Of keep the number of spurious
> wakeups in a stat counter somewhere -- something to look at if the
> battery drains faster than expected.

If you echo 1 to /sys/power/pm_debug_messages, dmesg will tell you
that (with gory details). :-)

> Otherwise:
>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>
> One minor nit below:
>
> > ---
> >  kernel/power/suspend.c |   18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> >
> > Index: linux-pm/kernel/power/suspend.c
> > ===================================================================
> > --- linux-pm.orig/kernel/power/suspend.c
> > +++ linux-pm/kernel/power/suspend.c
> > @@ -109,8 +109,12 @@ static void s2idle_enter(void)
> >
> >  static void s2idle_loop(void)
> >  {
> > +     ktime_t start, delta;
> > +
> >       pm_pr_dbg("suspend-to-idle\n");
> >
> > +     start = ktime_get();
> > +
> >       for (;;) {
> >               int error;
> >
> > @@ -150,6 +154,20 @@ static void s2idle_loop(void)
> >               pm_wakeup_clear(false);
> >       }
> >
> > +     /*
> > +      * If the monotonic clock difference between the start of the loop and
> > +      * this point is too large, user space may get confused about whether or
> > +      * not the system has been suspended and tasks may get killed by
> > +      * watchdogs etc., so count the loop as "sleep time" to compensate for
> > +      * that.
> > +      */
> > +     delta = ktime_sub(ktime_get(), start);
> > +     if (ktime_to_ns(delta) > 0) {
> > +             struct timespec64 timespec64_delta = ktime_to_timespec64(delta);
> > +
> > +             timekeeping_inject_sleeptime64(&timespec64_delta);
> > +     }
> > +
> >       pm_pr_dbg("resume from suspend-to-idle\n");
> >  }
>
> Like I mentioned yesterday; I myself prefer the form:
>
>
>         u64 stamp = ktimer_get_ns();
>
>         for (;;) {
>                 /* ... */
>         }
>
>         stamp = ktime_get_ns() - stamp;
>         if (stamp)
>                 timekeeping_inject_sleeptime64(ns_to_timespec64(ns));
>
>
> Esp. since ktime_t _is_ s64 these days, there is no point in keep using
> all the weird ktime helpers that make the code harder to read.

Looks like a good idea, let me try that.