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From: arnd@arndb.de (Arnd Bergmann)
Subject: Setting monotonic time?
Date: Tue, 2 Oct 2018 22:00:57 +0200	[thread overview]
Message-ID: <CAK8P3a2zTqrCGURE3XVpn3_w6C4R5O+cq8TdtbyKCMGyRk9_jw@mail.gmail.com> (raw)
Message-ID: <20181002200057._kU20m9dNRlcLm_Us6qP4IFEJ7PzpAwdqNaTLIu26Qc@z> (raw)
In-Reply-To: <alpine.DEB.2.21.1810012047520.32062@nanos.tec.linutronix.de>

On Mon, Oct 1, 2018@8:53 PM Thomas Gleixner <tglx@linutronix.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?

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.

      Arnd

  parent reply	other threads:[~2018-10-02 20:00 UTC|newest]

Thread overview: 78+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-19 20:50 [RFC 00/20] ns: Introduce Time Namespace dima
2018-09-19 20:50 ` Dmitry Safonov
2018-09-19 20:50 ` [RFC 16/20] selftest: Add Time Namespace test for supported clocks dima
2018-09-19 20:50   ` Dmitry Safonov
2018-09-24 21:36   ` shuah
2018-09-24 21:36     ` Shuah Khan
2018-09-19 20:50 ` [RFC 17/20] selftest/timens: Add test for timerfd dima
2018-09-19 20:50   ` Dmitry Safonov
2018-09-19 20:50 ` [RFC 18/20] selftest/timens: Add test for clock_nanosleep dima
2018-09-19 20:50   ` Dmitry Safonov
2018-09-19 20:50 ` [RFC 19/20] timens/selftest: Add procfs selftest dima
2018-09-19 20:50   ` Dmitry Safonov
2018-09-19 20:50 ` [RFC 20/20] timens/selftest: Add timer offsets test dima
2018-09-19 20:50   ` Dmitry Safonov
2018-09-21 12:27 ` [RFC 00/20] ns: Introduce Time Namespace ebiederm
2018-09-21 12:27   ` Eric W. Biederman
2018-09-24 20:51   ` avagin
2018-09-24 20:51     ` Andrey Vagin
2018-09-24 22:02     ` ebiederm
2018-09-24 22:02       ` Eric W. Biederman
2018-09-25  1:42       ` avagin
2018-09-25  1:42         ` Andrey Vagin
2018-09-26 17:36         ` ebiederm
2018-09-26 17:36           ` Eric W. Biederman
2018-09-26 17:59           ` 0x7f454c46
2018-09-26 17:59             ` Dmitry Safonov
2018-09-27 21:30           ` tglx
2018-09-27 21:30             ` Thomas Gleixner
2018-09-27 21:41             ` tglx
2018-09-27 21:41               ` Thomas Gleixner
2018-10-01 23:20               ` avagin
2018-10-01 23:20                 ` Andrey Vagin
2018-10-02  6:15                 ` tglx
2018-10-02  6:15                   ` Thomas Gleixner
2018-10-02 21:05                   ` 0x7f454c46
2018-10-02 21:05                     ` Dmitry Safonov
2018-10-02 21:26                     ` tglx
2018-10-02 21:26                       ` Thomas Gleixner
2018-09-28 17:03             ` ebiederm
2018-09-28 17:03               ` Eric W. Biederman
2018-09-28 19:32               ` tglx
2018-09-28 19:32                 ` Thomas Gleixner
2018-10-01  9:05                 ` ebiederm
2018-10-01  9:05                   ` Eric W. Biederman
2018-10-01  9:15                 ` Setting monotonic time? ebiederm
2018-10-01  9:15                   ` Eric W. Biederman
2018-10-01 18:52                   ` tglx
2018-10-01 18:52                     ` Thomas Gleixner
2018-10-02 20:00                     ` arnd [this message]
2018-10-02 20:00                       ` Arnd Bergmann
2018-10-02 20:06                       ` tglx
2018-10-02 20:06                         ` Thomas Gleixner
2018-10-03  4:50                         ` ebiederm
2018-10-03  4:50                           ` Eric W. Biederman
2018-10-03  5:25                           ` tglx
2018-10-03  5:25                             ` Thomas Gleixner
2018-10-03  6:14                             ` ebiederm
2018-10-03  6:14                               ` Eric W. Biederman
2018-10-03  7:02                               ` arnd
2018-10-03  7:02                                 ` Arnd Bergmann
2018-10-03  6:14                             ` tglx
2018-10-03  6:14                               ` Thomas Gleixner
2018-10-01 20:51                   ` avagin
2018-10-01 20:51                     ` Andrey Vagin
2018-10-02  6:16                     ` tglx
2018-10-02  6:16                       ` Thomas Gleixner
2018-10-21  1:41               ` [RFC 00/20] ns: Introduce Time Namespace avagin
2018-10-21  1:41                 ` Andrei Vagin
2018-10-21  3:54                 ` avagin
2018-10-21  3:54                   ` Andrei Vagin
2018-10-29 20:33                 ` tglx
2018-10-29 20:33                   ` Thomas Gleixner
2018-10-29 21:21                   ` ebiederm
2018-10-29 21:21                     ` Eric W. Biederman
2018-10-29 21:36                     ` tglx
2018-10-29 21:36                       ` Thomas Gleixner
2018-10-31 16:26                   ` avagin
2018-10-31 16:26                     ` Andrei Vagin

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