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Date:   Mon, 1 Oct 2018 20:52:41 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
cc:     Andrey Vagin <avagin@virtuozzo.com>,
        Dmitry Safonov <dima@arista.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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        Adrian Reber <adrian@lisas.de>,
        Andy Lutomirski <luto@kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Jeff Dike <jdike@addtoit.com>, Oleg Nesterov <oleg@redhat.com>,
        Pavel Emelianov <xemul@virtuozzo.com>,
        Shuah Khan <shuah@kernel.org>,
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Subject: Re: Setting monotonic time?
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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