linux-kernel - Re: [PATCH v4 2/2] hv_utils: implement Hyper-V PTP source

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Message-ID: <874m0g7vm0.fsf@vitty.brq.redhat.com>
Date:   Mon, 30 Jan 2017 10:19:51 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     KY Srinivasan <kys@...rosoft.com>
Cc:     "devel\@linuxdriverproject.org" <devel@...uxdriverproject.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "John Stultz" <john.stultz@...aro.org>,
        "Alex Ng \(LIS\)" <alexng@...rosoft.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Olaf Hering <olaf@...fle.de>,
        Richard Cochran <richardcochran@...il.com>,
        Radim Krcmar <rkrcmar@...hat.com>
Subject: Re: [PATCH v4 2/2] hv_utils: implement Hyper-V PTP source

KY Srinivasan <kys@...rosoft.com> writes:

>> -----Original Message-----
>> From: Vitaly Kuznetsov [mailto:vkuznets@...hat.com]
>> Sent: Thursday, January 19, 2017 6:17 AM
>> To: devel@...uxdriverproject.org
>> Cc: Thomas Gleixner <tglx@...utronix.de>; linux-kernel@...r.kernel.org;
>> Haiyang Zhang <haiyangz@...rosoft.com>; KY Srinivasan
>> <kys@...rosoft.com>; John Stultz <john.stultz@...aro.org>; Alex Ng (LIS)
>> <alexng@...rosoft.com>; Stephen Hemminger
>> <stephen@...workplumber.org>; Olaf Hering <olaf@...fle.de>; Richard
>> Cochran <richardcochran@...il.com>; Radim Krcmar
>> <rkrcmar@...hat.com>
>> Subject: [PATCH v4 2/2] hv_utils: implement Hyper-V PTP source
>> 
>> 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.
>> 
>> I tested the solution with chrony, the config was:
>> 
>>  refclock PHC /dev/ptp0 poll 3 precision 1e-9
>> 
>> The result I'm seeing is accurate enough, the time delta between the guest
>> and the host is almost always within [-10us, +10us], the in-kernel solution
>> was giving us comparable results.
>> 
>> I also tried implementing PPS device instead of PTP by using not currently
>> used Hyper-V synthetic timers (we use only one of four for clockevent) but
>> with PPS source only chrony wasn't able to give me the required accuracy,
>> the delta often more that 100us.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>
> Vitaly,
>
> This patch does not apply. Please rebase and send.
>

Sure, I'm currently testing v5 which includes the .getcrosststamp()
callback support. Will be sending it out shortly.

-- 
  Vitaly