[PATCH] ntp: Make periodic RTC update more reliable

[PATCH] ntp: Make periodic RTC update more reliable

[Miroslav Lichvar]

The current code requires that the scheduled update of the RTC happens
in the closest tick to the half of the second. This seems to be
difficult to achieve reliably. The scheduled work may be missing the
target time by a tick or two and be constantly rescheduled every second.

Relax the limit to 10 ticks. As a typical RTC drifts in the 11-minute
update interval by several milliseconds, this shouldn't affect the
overall accuracy of the RTC much.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
---
 kernel/time/ntp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 8f5b3b9..ab1fa7c 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -475,6 +475,7 @@ static void sync_cmos_clock(struct work_struct *work)
 	 * called as close as possible to 500 ms before the new second starts.
 	 * This code is run on a timer.  If the clock is set, that timer
 	 * may not expire at the correct time.  Thus, we adjust...
+	 * We want the clock to be within a couple of ticks from the target.
 	 */
 	if (!ntp_synced()) {
 		/*
@@ -485,7 +486,7 @@ static void sync_cmos_clock(struct work_struct *work)
 	}
 
 	getnstimeofday(&now);
-	if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec / 2) {
+	if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec * 5) {
 		struct timespec adjust = now;
 
 		fail = -ENODEV;
-- 
1.8.3.1

Re: [PATCH] ntp: Make periodic RTC update more reliable

[John Stultz]

On 08/01/2013 10:31 AM, Miroslav Lichvar wrote:
> The current code requires that the scheduled update of the RTC happens
> in the closest tick to the half of the second. This seems to be
> difficult to achieve reliably. The scheduled work may be missing the
> target time by a tick or two and be constantly rescheduled every second.
>
> Relax the limit to 10 ticks. As a typical RTC drifts in the 11-minute
> update interval by several milliseconds, this shouldn't affect the
> overall accuracy of the RTC much.
>
> Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
> ---
>  kernel/time/ntp.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
> index 8f5b3b9..ab1fa7c 100644
> --- a/kernel/time/ntp.c
> +++ b/kernel/time/ntp.c
> @@ -475,6 +475,7 @@ static void sync_cmos_clock(struct work_struct *work)
>  	 * called as close as possible to 500 ms before the new second starts.
>  	 * This code is run on a timer.  If the clock is set, that timer
>  	 * may not expire at the correct time.  Thus, we adjust...
> +	 * We want the clock to be within a couple of ticks from the target.
>  	 */
>  	if (!ntp_synced()) {
>  		/*
> @@ -485,7 +486,7 @@ static void sync_cmos_clock(struct work_struct *work)
>  	}
>  
>  	getnstimeofday(&now);
> -	if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec / 2) {
> +	if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec * 5) {
>  		struct timespec adjust = now;
>  
>  		fail = -ENODEV;

Applied to my 3.12 branch.

thanks
-john