Re: Setting monotonic time?

[Thomas Gleixner]

On Tue, 2 Oct 2018, Arnd Bergmann wrote:
> On Mon, Oct 1, 2018 at 8:53 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >
> > On Mon, 1 Oct 2018, Eric W. Biederman wrote:
> > > In the context of process migration there is a simpler subproblem that I
> > > think it is worth exploring if we can do something about.
> > >
> > > For a cluster of machines all running with synchronized
> > > clocks. CLOCK_REALTIME matches. CLOCK_MONOTNIC does not match between
> > > machines.   Not having a matching CLOCK_MONOTONIC prevents successful
> > > process migration between nodes in that cluster.
> > >
> > > Would it be possible to allow setting CLOCK_MONOTONIC at the very
> > > beginning of time?  So that all of the nodes in a cluster can be in
> > > sync?
> > >
> > > No change in skew just in offset for CLOCK_MONOTONIC.
> > >
> > > There are also dragons involved in coordinating things so that
> > > CLOCK_MONOTONIC gets set before CLOCK_MONOTONIC gets used.  So I don't
> > > know if allowing CLOCK_MONOTONIC to be set would be practical but it
> > > seems work exploring all on it's own.
> >
> > It's used very early on in the kernel, so that would be a major surprise
> > for many things including user space which has expectations on clock
> > monotonic.
> >
> > It would be reasonably easy to add CLOCK_MONONOTIC_SYNC which can be set in
> > the way you described and then in name spaces make it possible to magically
> > map CLOCK_MONOTONIC to CLOCK_MONOTONIC_SYNC.
> >
> > It still wouldn't allow to have different NTP/PTP time domains, but might
> > be a good start to address the main migration headaches.
> 
> If we make CLOCK_MONOTONIC settable this way in a namespace,
> do you think that should include device drivers that report timestamps
> in CLOCK_MONOTONIC base, or only the timekeeping clock and timer
> interfaces?

Uurgh. That gets messy very fast.

> Examples for drivers that can report timestamps are input, sound, v4l,
> and drm. I think most of these can report stamps in either monotonic
> or realtime base, while socket timestamps notably are always in
> realtime.
> 
> We can probably get away with not setting the timebase for those
> device drivers as long as the checkpoint/restart and migration features
> are not expected to restore the state of an open character device
> in that way. I don't know if that is a reasonable assumption to make
> for the examples I listed.

No idea. I'm not a container migration wizard.

Thanks,

	tglx