linux-kernel - Re: [PATCH] ntp: Clamp PLL update interval

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Date:	Thu, 09 Sep 2010 11:16:54 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Miroslav Lichvar <mlichvar@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] ntp: Clamp PLL update interval

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@...hat.com>

Thanks Miroslav!

Acked-by: John Stultz <johnstul@...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);
> 


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