linux-kernel - Re: Setting monotonic time?

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Message-ID: <alpine.DEB.2.21.1810020815420.32062@nanos.tec.linutronix.de>
Date:   Tue, 2 Oct 2018 08:16:38 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andrey Vagin <avagin@...tuozzo.com>
cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        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, 1 Oct 2018, Andrey Vagin wrote:
> 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.

Sure, but see my reply to Eric. That could be solved with that extra clock
id, which then gets mapped to monotonic for name spaces.

Thanks,

	tglx