linux-kernel - Re: [PATCH v3 0/2] hv_util: adjust system time smoothly

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Message-ID: <87k29tzjyc.fsf@vitty.brq.redhat.com>
Date:   Tue, 17 Jan 2017 18:15:39 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     devel@...uxdriverproject.org, Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        John Stultz <john.stultz@...aro.org>,
        Alex Ng <alexng@...rosoft.com>, Olaf Hering <olaf@...fle.de>,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH v3 0/2] hv_util: adjust system time smoothly

Stephen Hemminger <stephen@...workplumber.org> writes:

> On Tue, 17 Jan 2017 16:27:17 +0100
> Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
>> 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 server runs in parallel and uses something else for time
>>   sync (network, PTP,...) system time will never converge.
>> - Systemd starts annoying you by printing "Time has been changed" every 5
>>   seconds to the system log.
>> 
>> Instead of doing in-kernel time adjustments offload the work to an
>> NTP client by exposing TimeSync messages as a PTP device. Users may now
>> decide what they want to use as a source.
>> 
>> Changes since v2:
>> - Implement Hyper-V PTP device instead of doint in-kernel time sync.
>> 
>> Changes since "[PATCH RFC] hv_utils: implement Hyper-V PTP source":
>> - Richard Cochran: implement .adjfreq, .adjtime, .settime64 returning
>>   -EOPNOTSUPP.
>> - Olaf Hering: change IS_ERR->IS_ERR_OR_NULL as CONFIG_PTP_1588_CLOCK
>>   can be disabled.
>> - Thomas Gleixner: formatting fixes, comments added.
>> 
>> Vitaly Kuznetsov (2):
>>   hv_util: switch to using timespec64
>>   hv_utils: implement Hyper-V PTP source
>> 
>>  drivers/hv/hv_util.c | 142 +++++++++++++++++++++++++++++++++++++++++----------
>>  1 file changed, 116 insertions(+), 26 deletions(-)
>> 
>
> It would be good to update Documentation files to describe any configuration needed.

This is just a PTP device, not any different for other PTP devices so
users will be reading their NTP server docs to figure out how to add a
PTP reference clock.

Or do you have any particular idea where to put an example?

-- 
  Vitaly