]> git.openfabrics.org - ~shefty/rdma-dev.git/commitdiff
ntp: Clamp PLL update interval
authorMiroslav Lichvar <mlichvar@redhat.com>
Tue, 7 Sep 2010 14:43:46 +0000 (16:43 +0200)
committerThomas Gleixner <tglx@linutronix.de>
Thu, 9 Sep 2010 18:48:37 +0000 (20:48 +0200)
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>
kernel/time/ntp.c

index c63116863a80294c7ce1545b2089efe1d43258d0..d2321891538f53425430565fe68197b5cde93d12 100644 (file)
@@ -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);