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
