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

From: Thomas Gleixner
Date: Mon Jan 09 2017 - 16:28:04 EST


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