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

[john stultz <johnstul@us.ibm.com>]

On Thu, 2010-12-02 at 01:18 +0000, Jamie Lokier wrote:
> Valdis.Kletnieks@vt.edu wrote:
> > On Wed, 01 Dec 2010 10:43:59 GMT, Jamie Lokier said:
> > 
> > > 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?
> > 
> > Wouldn't that be an API break for programs that are expecting the current
> > behavior of CLOCK_MONOTONIC?  Yes, there should be a way to request either of
> > them - but if there's only one way now, it should continue to act the current
> > way, and the added way is the second option.
> 
> I don't know.  Can you think of any program which would break if
> suspend/resume's clocks behaved like ordinary task scheduling - when a
> task doesn't run for a long time because of scheduling decisions?
> Hmm, I guess some realtime apps might like to know.

Like I mentioned earlier, CLOCK_MONOTONIC_RAW and CLOCK_MONOTONIC are
tightly tied, so anything using CLOCK_MONOTONIC_RAW would break.

It might be possible to change both, but I still think such a change
would be bad.

> Currently CLOCK_MONOTONIC jumps forwards by 4 seconds on
> suspend/resume anyway (as seen by userspace), on my x86 laptop running
> 2.6.37-rc3.  So it does already jump a bit...

So just to clarify here, by this do you mean that there's ~4 seconds
delay between the resume event and when userland apps start to run (or
possibly some of that accumulating between the app freeze and the
timekeeping suspend) ?

Or are you seeing CLOCK_MONOTONIC jump 4 seconds out of sync with
CLOCK_REALTIME? 

It should be the delta between CLOCK_MONOTONIC and CLOCK_REALTIME prior
to suspend should be that same delta + suspend time after resume. If
that's not the case, something may be broken.

thanks
-john