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.
Thanks,
tglx