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

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Date:   Wed, 4 Jan 2017 11:09:53 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
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

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.