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

[Alexander Shishkin <virtuoso@slind.org>]

On Wed, Dec 01, 2010 at 10:43:59AM +0000, Jamie Lokier wrote:
> 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 :-)

Can you please check if my new patch from earlier today [1] seems better?

[1] http://marc.info/?l=linux-kernel&m=129116762414291&w=2

> 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.

Generally, I used CLOCK_MONOTONIC + TFD_NOTIFY_CLOCK_SET to signal
suspend/resume time changes and CLOCK_REALTIME + TFD_NOTIFY_CLOCK_SET
to signal wall clock changes. Without TFD_NOTIFY_CLOCK_SET it works
as it did before.

> 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.

Regards,
--
Alex