linux-kernel - Re: Setting monotonic time?

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Message-ID: <20181001205124.GA23620@outlook.office365.com>
Date:   Mon, 1 Oct 2018 20:51:36 +0000
From:   Andrey Vagin <avagin@...tuozzo.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <dima@...sta.com>,
        "linux-kernel@...r.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@...ts.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>,
        "criu@...nvz.org" <criu@...nvz.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: Setting monotonic time?

On Mon, Oct 01, 2018 at 11:15:32AM +0200, 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?

Here is a question about how to synchronize clocks between nodes. It
looks like we will need to have a working network for this, but a
network configuration may be non-trivial and it can require to run a few
processes which can use CLOCK_MONOTNIC...

> 
> 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?

Process migration is only one of use-cases. Another use-case is
restoring from snapshots. It may be even more popular than process
migration. We can't guarantee that all snapshots will be done in one
cluster. For example, a user meets a bug, does a container snapshot and
attaches it to a bug report.

> 
> Eric