From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754799Ab0IISRY (ORCPT ); Thu, 9 Sep 2010 14:17:24 -0400 Received: from e38.co.us.ibm.com ([32.97.110.159]:44981 "EHLO e38.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751521Ab0IISRV (ORCPT ); Thu, 9 Sep 2010 14:17:21 -0400 Subject: Re: [PATCH] ntp: Clamp PLL update interval From: john stultz To: Miroslav Lichvar Cc: linux-kernel@vger.kernel.org, Thomas Gleixner In-Reply-To: <1283870626-9472-1-git-send-email-mlichvar@redhat.com> References: <1283870626-9472-1-git-send-email-mlichvar@redhat.com> Content-Type: text/plain; charset="UTF-8" Date: Thu, 09 Sep 2010 11:16:54 -0700 Message-ID: <1284056214.2762.55.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2010-09-07 at 16:43 +0200, Miroslav Lichvar wrote: > 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 Thanks Miroslav! Acked-by: John Stultz > --- > kernel/time/ntp.c | 14 +++++++++++--- > 1 files changed, 11 insertions(+), 3 deletions(-) > > diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c > index c631168..d232189 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); >