linux-kernel - Setting monotonic time?

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Message-ID: <877ej2xc23.fsf_-_@xmission.com>
Date:   Mon, 01 Oct 2018 11:15:32 +0200
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andrey Vagin <avagin@...tuozzo.com>,
        Dmitry Safonov <dima@...sta.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Adrian Reber <adrian@...as.de>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jeff Dike <jdike@...toit.com>, Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelianov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        "containers\@lists.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>,
        "criu\@openvz.org" <criu@...nvz.org>,
        "linux-api\@vger.kernel.org" <linux-api@...r.kernel.org>,
        "x86\@kernel.org" <x86@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "linux-kselftest\@vger.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Setting monotonic time?

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.

Dmitry would setting CLOCK_MONOTONIC exactly once at boot time solve
your problem that is you are looking at a time namespace to solve?

Eric