Commit 8af3c153 authored Sep 07, 2010 by Miroslav Lichvar Committed by Thomas Gleixner Sep 09, 2010

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>

parent d56557af

kernel/time/ntp.c

+11 −3

Original line number	Diff line number	Diff line
		@@ -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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