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

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Thomas Gleixner <tglx@linutronix.de> writes:

> 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.
>

Thank you for your comments, Thomas,

> 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.

True, as Stephen suggested we'll probably need a way to inform (or
block) the second NTP client about the ongoing timesync.

>
> 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?

ICTIMESYNCFLAG_SYNC flag, demanding us to do so, is only set on the
first packet (when our VM boots) and after suspend/resume/migration
events. In these cases guest's time can be off by minutes/hours and
do_settimeofday() is probably justified.

>
> 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).
>

My understanding is that we have no guarantees from the host that these
messages are sent every 5 seconds and even when they are the interval is
not very precise. We can probably create a 'fake' pps signal out of
these messages (e.g. these messages will just be adjusting the frequency
of the signal. I can play with such approach if you think this is the
way to go.

-- 
  Vitaly

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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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