linux-kernel - Re: Setting monotonic time?

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Date:   Tue, 2 Oct 2018 22:06:28 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Arnd Bergmann <arnd@...db.de>
cc:     "Eric W . Biederman" <ebiederm@...ssion.com>, avagin@...tuozzo.com,
        dima@...sta.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        0x7f454c46@...il.com, adrian@...as.de,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        gorcunov@...nvz.org, "H. Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...hat.com>, Jeff Dike <jdike@...toit.com>,
        Oleg Nesterov <oleg@...hat.com>, xemul@...tuozzo.com,
        Shuah Khan <shuah@...nel.org>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        Linux API <linux-api@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        linux-kselftest@...r.kernel.org
Subject: Re: Setting monotonic time?

On Tue, 2 Oct 2018, Arnd Bergmann wrote:
> On Mon, Oct 1, 2018 at 8:53 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > 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.
> 
> If we make CLOCK_MONOTONIC settable this way in a namespace,
> do you think that should include device drivers that report timestamps
> in CLOCK_MONOTONIC base, or only the timekeeping clock and timer
> interfaces?

Uurgh. That gets messy very fast.

> Examples for drivers that can report timestamps are input, sound, v4l,
> and drm. I think most of these can report stamps in either monotonic
> or realtime base, while socket timestamps notably are always in
> realtime.
> 
> We can probably get away with not setting the timebase for those
> device drivers as long as the checkpoint/restart and migration features
> are not expected to restore the state of an open character device
> in that way. I don't know if that is a reasonable assumption to make
> for the examples I listed.

No idea. I'm not a container migration wizard.

Thanks,

	tglx