linux-kernel - Re: [PATCH 3/4] hv_util: use do_adjtimex() to update system time

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Date:   Tue, 03 Jan 2017 13:32:22 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     "Alex Ng \(LIS\)" <alexng@...rosoft.com>
Cc:     "devel\@linuxdriverproject.org" <devel@...uxdriverproject.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "KY Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "John Stultz" <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/4] hv_util: use do_adjtimex() to update system time

"Alex Ng (LIS)" <alexng@...rosoft.com> writes:

>> -----Original Message-----
>> From: Vitaly Kuznetsov [mailto:vkuznets@...hat.com]
>> Sent: Monday, January 2, 2017 11:41 AM
>> To: devel@...uxdriverproject.org
>> Cc: linux-kernel@...r.kernel.org; KY Srinivasan <kys@...rosoft.com>;
>> Haiyang Zhang <haiyangz@...rosoft.com>; John Stultz
>> <john.stultz@...aro.org>; Thomas Gleixner <tglx@...utronix.de>; Alex Ng
>> (LIS) <alexng@...rosoft.com>
>> Subject: [PATCH 3/4] hv_util: use do_adjtimex() to update system time
>> 
>> 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.
>
> These are all good points. I am working on a patch to address point 2.
> It will allow new TimeSync behavior to be disabled even if the TimeSync IC is
> enabled from the host. This can be set to prevent TimeSync IC from interfering
> with NTP client.
>

Good, this can happen in parallel to my series, right?

>> 
>> Instead of calling do_settimeofday64() we can pretend being an NTP client
>> and use do_adjtimex().
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  drivers/hv/hv_util.c | 25 ++++++++++++++++++++++---
>>  1 file changed, 22 insertions(+), 3 deletions(-)
>> 
>> diff --git a/drivers/hv/hv_util.c b/drivers/hv/hv_util.c index
>> 94719eb..4c0fbb0 100644
>> --- a/drivers/hv/hv_util.c
>> +++ b/drivers/hv/hv_util.c
>> @@ -182,9 +182,10 @@ struct adj_time_work {  static void
>> hv_set_host_time(struct work_struct *work)  {
>>  	struct adj_time_work	*wrk;
>> -	s64 host_tns;
>> +	s64 host_tns, our_tns, delta;
>>  	u64 newtime;
>> -	struct timespec64 host_ts;
>> +	struct timespec64 host_ts, our_ts;
>> +	struct timex txc = {0};
>> 
>>  	wrk = container_of(work, struct adj_time_work, work);
>> 
>> @@ -205,7 +206,25 @@ static void hv_set_host_time(struct work_struct
>> *work)
>>  	host_tns = (newtime - WLTIMEDELTA) * 100;
>>  	host_ts = ns_to_timespec64(host_tns);
>> 
>> -	do_settimeofday64(&host_ts);
>> +	getnstimeofday64(&our_ts);
>> +	our_tns = timespec64_to_ns(&our_ts);
>> +
>> +	/* Difference between our time and host time */
>> +	delta = host_tns - our_tns;
>> +
>> +	/* Try adjusting time by using phase adjustment if possible */
>> +	if (abs(delta) > MAXPHASE) {
>> +		do_settimeofday64(&host_ts);
>> +		return;
>> +	}
>
> We should also call do_settimeofday64() if the host sends flag
> ICTIMESYNCFLAG_SYNC. This is a signal from host that the guest
> shall sync with host time immediately (often when the guest has
> just booted).

Ok, point taken, will do in v2. We don't get ICTIMESYNCFLAG_SYNC very
often, right?

>
>> +
>> +	txc.modes = ADJ_TICK | ADJ_FREQUENCY | ADJ_OFFSET |
>> ADJ_NANO |
>> +		ADJ_STATUS;
>> +	txc.tick = TICK_USEC;
>> +	txc.freq = 0;
>
> I'm not familiar with the ADJ_FREQUENCY flag. What does setting this to 'zero' achieve?
> Are there any side-effects from doing this?

Zero means no frequency adjustment required (we reset it in case it was
previously made by an NTP client).

>
>> +	txc.status = STA_PLL;
>> +	txc.offset = delta;
>> +	do_adjtimex(&txc);
>
> Might be a good idea to handle the return code from do_adjtimex() and log something
> in case of error.

I can add a debug message here but as this is a regular action we don't
want to get a flood of messages in case this fails permanently. I'd
avoid printing info messages here.

>
>>  }
>> 
>>  /*
>> --
>> 2.9.3

-- 
  Vitaly