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>

1 files changed, 11 insertions, 3 deletions

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index c63116863a80..d2321891538f 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);