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Re: [PATCH v2 0/4] hv_util: adjust system time smoothly

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On Wed, 4 Jan 2017, Vitaly Kuznetsov wrote:

> Changes since v1:
> - do do_settimeofday64() when ICTIMESYNCFLAG_SYNC flag is present in the
>   request (Alex Ng)
> - add pr_debug() for the case when do_adjtimex() fails (Alex Ng)
> 
> Original description:
> 
> With TimeSync version 4 protocol support we started updating system time
> continuously through the whole lifetime of Hyper-V guests. Every 5 seconds
> there is a time sample from the host which triggers do_settimeofday[64]().
> While the time from the host is very accurate such adjustments may cause
> issues:
> - Time is jumping forward and backward, some applications may misbehave.
> - In case an NTP client is run in parallel things may go south, e.g. when
>   an NTP client tries to adjust tick/frequency with ADJ_TICK/ADJ_FREQUENCY
>   the Hyper-V module will not see this changes and time will oscillate and
>   never converge.
> - Systemd starts annoying you by printing "Time has been changed" every 5
>   seconds to the system log.
> 
> With this series I suggest to use do_adjtimex() to adjust time. My tests
> show that such method gives equally good time convergence but avoids all
> the drawbacks described above.

To be honest, I think all of this is just tinkering.

1) do_adjtimex() is assuming that there is a single client connected which
   is responsible for the updates. So I seriously doubt that a NTP client
   running in the guest will cooperate nicely with that timesync magic
   under all circumstances.

2) There is still the possibility to force do_settimeofday() calls which
   will upset NTP clients and have other side effects.

   Why is this call necessary at all? Just because it's in some spec?

3) What happens if you have a PTP capable network card mapped into your
   guest and the guest uses PTP for time synchronization? The outcome is
   predictible: CRAP.

I can see the value for a host wide time synchronization, but please use
mechanisms which do not interfere with the rest of the time eco system in
Linux.

The timesync thing happens periodically every 5 seconds, which you can feed
nicely into the PPS subsystem and then the guest side NTP daemon can
utilize it (or not).

Thanks,

	tglx

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[PATCH v2 0/4] hv_util: adjust system time smoothly Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-01-04 18:40 +0100
  [PATCH v2 4/4] hv_util: improve time adjustment accuracy by disabling interrupts Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-01-04 18:50 +0100
    Re: [PATCH v2 4/4] hv_util: improve time adjustment accuracy by  disabling interrupts Stephen Hemminger <stephen@networkplumber.org> - 2017-01-04 20:20 +0100
      Re: [PATCH v2 4/4] hv_util: improve time adjustment accuracy by disabling interrupts Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-01-05 13:40 +0100
        Re: [PATCH v2 4/4] hv_util: improve time adjustment accuracy by  disabling interrupts Stephen Hemminger <stephen@networkplumber.org> - 2017-01-05 18:50 +0100
    Re: [PATCH v2 4/4] hv_util: improve time adjustment accuracy by  disabling interrupts John Stultz <john.stultz@linaro.org> - 2017-01-07 02:10 +0100
      Re: [PATCH v2 4/4] hv_util: improve time adjustment accuracy by disabling interrupts Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-01-09 14:10 +0100
  Re: [PATCH v2 0/4] hv_util: adjust system time smoothly Thomas Gleixner <tglx@linutronix.de> - 2017-01-09 22:30 +0100
    Re: [PATCH v2 0/4] hv_util: adjust system time smoothly Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-01-10 15:40 +0100
      Re: [PATCH v2 0/4] hv_util: adjust system time smoothly Thomas Gleixner <tglx@linutronix.de> - 2017-01-10 16:00 +0100
        [PATCH RFC] hv_utils: implement Hyper-V PTP source Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-01-13 14:10 +0100
          Re: [PATCH RFC] hv_utils: implement Hyper-V PTP source Richard Cochran <richardcochran@gmail.com> - 2017-01-13 16:00 +0100
            Re: [PATCH RFC] hv_utils: implement Hyper-V PTP source Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-01-13 16:40 +0100
          Re: [PATCH RFC] hv_utils: implement Hyper-V PTP source Olaf Hering <olaf@aepfle.de> - 2017-01-13 16:30 +0100
            Re: [PATCH RFC] hv_utils: implement Hyper-V PTP source Vitaly Kuznetsov <vkuznets@redhat.com> - 2017-01-13 16:40 +0100

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