linux-kernel - Re: [PATCH v2 3/4] hv_util: use do_adjtimex() to update system time

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Date:   Thu, 05 Jan 2017 13:37:38 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alex Ng <alexng@...rosoft.com>
Subject: Re: [PATCH v2 3/4] hv_util: use do_adjtimex() to update system time

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

> On Wed,  4 Jan 2017 18:24:38 +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 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.
>> 
>> Instead of calling do_settimeofday64() we can pretend being an NTP client
>> and use do_adjtimex(). Do do_settimeofday64() in case the difference is too
>> big or ICTIMESYNCFLAG_SYNC flag was set in the request.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>> 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)
>> ---
>>  drivers/hv/hv_util.c | 32 +++++++++++++++++++++++++++++---
>>  1 file changed, 29 insertions(+), 3 deletions(-)
>> 
>> diff --git a/drivers/hv/hv_util.c b/drivers/hv/hv_util.c
>> index 94719eb..7e97231 100644
>> --- a/drivers/hv/hv_util.c
>> +++ b/drivers/hv/hv_util.c
>> @@ -182,9 +182,11 @@ 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};
>> +	int ret;
>>  
>>  	wrk = container_of(work, struct adj_time_work, work);
>>  
>> @@ -205,7 +207,31 @@ 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;
>
> This looks correct to me.
> Did you consider using ktime? It provides a cleaner abstraction for handling
> nanosecond time resolution.

I see. While s64 should work ktime seems preferable. I'll give it a try.

-- 
  Vitaly