Re: [PATCH] [RFC] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for clock changes

[Jamie Lokier <jamie@shareable.org>]

Lennart Poettering wrote:
> On Tue, 23.11.10 19:22, Alexander Shishkin (virtuoso@slind.org) wrote:
> 
> > Certain userspace applications (like "clock" desktop applets or cron or
> > systemd) might want to be notified when some other application changes
> > the system time. There are several known to me reasons for this:
> >  - avoiding periodic wakeups to poll time changes;
> >  - rearming CLOCK_REALTIME timers when said changes happen;
> >  - changing system timekeeping policy for system-wide time management
> >    programs;
> >  - keeping guest applications/operating systems running in emulators
> >    up to date.
> > 
> > This is another attempt to approach notifying userspace about system
> > clock changes. The other one is using an eventfd and a syscall [1]. In
> > the course of discussing the necessity of a syscall for this kind of
> > notifications, it was suggested that this functionality can be achieved
> > via timers [2] (and timerfd in particular [3]). This idea got quite
> > some support [4], [5], [6] and some vague criticism [7], so I decided
> > to try and go a bit further with it.
> 
> I agree with Kay, this is pretty much exactly what we want for
> systemd. (Assuming that the time jump due to system suspend is
> propagated to userspace like any other time jump with this path).

I hope the time jump due to suspend is *not* propagated in the same
way to userspace :-)

What I'd like to see:

 1. Time jump due to the system clock being stepped: Notification.

    This is *not* a change in real time.  It means the clock was
    corrected/changed.  No physical time passed.

 2. Time jump due to suspend/resume: Different notification.

    This *is* a change in real time.  Physical time passed.

 3. Time drift corrections: As now, no notification, it's just
    the clock being regulated.

To signal the difference between 1 and 2, there ought to be some way
for userspace to determine how much of the clock delta corresponds
with physical time, by reading some sort of "monotonic" clock :-)

CLOCK_MONOTONIC is unsuitable because it stops at suspend.  Maybe it
should stay that way.  But maybe not - programs using CLOCK_MONOTONIC
usually want to trigger timeouts etc. based on real elapsed time, and
after suspend/resume, it's quite reasonable to want to trigger all of
a program's short timeouts immediately.  Indeed some network protocol
userspace may currently behave *incorrectly* over suspend/resume,
especially those using clock times to validate their caches,
*because* CLOCK_MONOTONIC doesn't count it.

So maybe CLOCK_MONOTONIC should be changed to include elapsed time
during suspend/resume, and CLOCK_MONOTONIC_RAW could remain as it is,
for programs that want that?

That, plus this proposed patch, would signal the difference between 1
and 2 above nicely.

-- Jamie