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Commit 8af3c153 authored by Miroslav Lichvar's avatar Miroslav Lichvar Committed by Thomas Gleixner
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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: default avatarMiroslav Lichvar <mlichvar@redhat.com>
Acked-by: default avatarjohn stultz <johnstul@us.ibm.com>
LKML-Reference: <1283870626-9472-1-git-send-email-mlichvar@redhat.com>
Signed-off-by: default avatarThomas 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);