Re: Detecting shift of CLOCK_REALTIME with clock_nanosleep (again)

Thomas Gleixner
2012-11-15T21:01:45Z

On Thu, 15 Nov 2012, John Stultz wrote:
> On 11/15/2012 11:28 AM, Scot Salmon wrote:
> > Thomas Gleixner <tglx@linutronix.de> wrote on 10/31/2012 03:08:45 PM:
> > > What you are concerned about is keeping the machines in sync on a
> > > common global timeline. Though your more fundamental requirement is
> > > that you get the wakeup on each machine in the given cycle time. The
> > > global synchronization mechanism just adjusts that local periodic
> > > schedule.
> > [...]
> > > What you really want is an atomic readout facility for CLOCK_MONOTONIC
> > > and CLOCK_REALTIME. That allows you to align the CLOCK_MONOTONIC based
> > > timer with the global CLOCK_REALTIME based time line and in the event
> > > that the CLOCK_REALTIME clock was set and jumped forward/backward you
> > > have full software control over the aligning mechanism including the
> > > ability to do sanity checking.
> > This works for me. We discussed it on IRC and agreed on "something
> > like clock_gettime2(id, id, *t1, *t2) with a sanity check on the ids,
> > that allows us other combinations than MONO/REAL as well".
> Yea, there's been a few times that folks have brought up the need, and this
> sort of interface is probably the best way to go.
>
> I suspect clock_gettime3 isn't far behind though :)

I guess you are talking about MONO/MONORAW/REAL. So if you already
know about such requirements we should go for that, but we should try
to restrict it to combinations which can be easily supported in the
VDSO as well.

Thanks,

tglx