[PATCH] ntp: Clamp PLL update interval

[PATCH] ntp: Clamp PLL update interval

[Miroslav Lichvar]

Clamp update interval to reduce PLL gain with low sampling rate (e.g.
intermittent network connection) to avoid instability.

The clamp roughly corresponds to the loop time constant, it's 8 * poll
interval for SHIFT_PLL 2 and 32 * poll interval for SHIFT_PLL 4. This
gives good results without affecting the gain in normal conditions where
ntpd skips only up to seven consecutive samples.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
---
 kernel/time/ntp.c |   14 +++++++++++---
 1 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index c631168..d232189 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -149,10 +149,18 @@ static void ntp_update_offset(long offset)
 	time_reftime = get_seconds();
 
 	offset64    = offset;
-	freq_adj    = (offset64 * secs) <<
-			(NTP_SCALE_SHIFT - 2 * (SHIFT_PLL + 2 + time_constant));
+	freq_adj    = ntp_update_offset_fll(offset64, secs);
 
-	freq_adj    += ntp_update_offset_fll(offset64, secs);
+	/*
+	 * Clamp update interval to reduce PLL gain with low
+	 * sampling rate (e.g. intermittent network connection)
+	 * to avoid instability.
+	 */
+	if (unlikely(secs > 1 << (SHIFT_PLL + 1 + time_constant)))
+		secs = 1 << (SHIFT_PLL + 1 + time_constant);
+
+	freq_adj    += (offset64 * secs) <<
+			(NTP_SCALE_SHIFT - 2 * (SHIFT_PLL + 2 + time_constant));
 
 	freq_adj    = min(freq_adj + time_freq, MAXFREQ_SCALED);
 
-- 
1.7.2

Re: [PATCH] ntp: Clamp PLL update interval

[john stultz]

On Tue, 2010-09-07 at 16:43 +0200, Miroslav Lichvar wrote:
> Clamp update interval to reduce PLL gain with low sampling rate (e.g.
> intermittent network connection) to avoid instability.
> 
> The clamp roughly corresponds to the loop time constant, it's 8 * poll
> interval for SHIFT_PLL 2 and 32 * poll interval for SHIFT_PLL 4. This
> gives good results without affecting the gain in normal conditions where
> ntpd skips only up to seven consecutive samples.
> 
> Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>

Thanks Miroslav!

Acked-by: John Stultz <johnstul@us.ibm.com>


> ---
>  kernel/time/ntp.c |   14 +++++++++++---
>  1 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
> index c631168..d232189 100644
> --- a/kernel/time/ntp.c
> +++ b/kernel/time/ntp.c
> @@ -149,10 +149,18 @@ static void ntp_update_offset(long offset)
>  	time_reftime = get_seconds();
> 
>  	offset64    = offset;
> -	freq_adj    = (offset64 * secs) <<
> -			(NTP_SCALE_SHIFT - 2 * (SHIFT_PLL + 2 + time_constant));
> +	freq_adj    = ntp_update_offset_fll(offset64, secs);
> 
> -	freq_adj    += ntp_update_offset_fll(offset64, secs);
> +	/*
> +	 * Clamp update interval to reduce PLL gain with low
> +	 * sampling rate (e.g. intermittent network connection)
> +	 * to avoid instability.
> +	 */
> +	if (unlikely(secs > 1 << (SHIFT_PLL + 1 + time_constant)))
> +		secs = 1 << (SHIFT_PLL + 1 + time_constant);
> +
> +	freq_adj    += (offset64 * secs) <<
> +			(NTP_SCALE_SHIFT - 2 * (SHIFT_PLL + 2 + time_constant));
> 
>  	freq_adj    = min(freq_adj + time_freq, MAXFREQ_SCALED);
> 

[tip:timers/core] ntp: Clamp PLL update interval

[tip-bot for Miroslav Lichvar]

Commit-ID:  8af3c153baf95374eff20a37f00c59a295b52756
Gitweb:     http://git.kernel.org/tip/8af3c153baf95374eff20a37f00c59a295b52756
Author:     Miroslav Lichvar <mlichvar@redhat.com>
AuthorDate: Tue, 7 Sep 2010 16:43:46 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Thu, 9 Sep 2010 20:48:37 +0200

ntp: Clamp PLL update interval

Clamp update interval to reduce PLL gain with low sampling rate (e.g.
intermittent network connection) to avoid instability.

The clamp roughly corresponds to the loop time constant, it's 8 * poll
interval for SHIFT_PLL 2 and 32 * poll interval for SHIFT_PLL 4. This
gives good results without affecting the gain in normal conditions where
ntpd skips only up to seven consecutive samples.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Acked-by: john stultz <johnstul@us.ibm.com>
LKML-Reference: <1283870626-9472-1-git-send-email-mlichvar@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/time/ntp.c |   14 +++++++++++---
 1 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index c631168..d232189 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -149,10 +149,18 @@ static void ntp_update_offset(long offset)
 	time_reftime = get_seconds();
 
 	offset64    = offset;
-	freq_adj    = (offset64 * secs) <<
-			(NTP_SCALE_SHIFT - 2 * (SHIFT_PLL + 2 + time_constant));
+	freq_adj    = ntp_update_offset_fll(offset64, secs);
 
-	freq_adj    += ntp_update_offset_fll(offset64, secs);
+	/*
+	 * Clamp update interval to reduce PLL gain with low
+	 * sampling rate (e.g. intermittent network connection)
+	 * to avoid instability.
+	 */
+	if (unlikely(secs > 1 << (SHIFT_PLL + 1 + time_constant)))
+		secs = 1 << (SHIFT_PLL + 1 + time_constant);
+
+	freq_adj    += (offset64 * secs) <<
+			(NTP_SCALE_SHIFT - 2 * (SHIFT_PLL + 2 + time_constant));
 
 	freq_adj    = min(freq_adj + time_freq, MAXFREQ_SCALED);