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authorMiroslav Lichvar <mlichvar@redhat.com>2010-09-07 16:43:46 +0200
committerThomas Gleixner <tglx@linutronix.de>2010-09-09 20:48:37 +0200
commit8af3c153baf95374eff20a37f00c59a295b52756 (patch)
treedb80f770b668f7067c3e95af92cc92933edce4e0 /kernel/time
parentd56557af19867edb8c0e96f8e26399698a08857f (diff)
downloadlinux-8af3c153baf95374eff20a37f00c59a295b52756.tar.bz2
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>
Diffstat (limited to 'kernel/time')
-rw-r--r--kernel/time/ntp.c14
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);