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>

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index c63116863a80294c7ce1545b2089efe1d43258d0..d2321891538f53425430565fe68197b5cde93d12 100644 (file)
--- 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);