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

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Date:   Tue, 10 Jan 2017 15:30:18 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        John Stultz <john.stultz@...aro.org>,
        Alex Ng <alexng@...rosoft.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH v2 0/4] hv_util: adjust system time smoothly

Thomas Gleixner <tglx@...utronix.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