Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753048Ab1FAGIC (ORCPT ); Wed, 1 Jun 2011 02:08:02 -0400 Received: from e6.ny.us.ibm.com ([32.97.182.146]:47976 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752508Ab1FAGHz (ORCPT ); Wed, 1 Jun 2011 02:07:55 -0400 From: John Stultz To: linux-kernel@vger.kernel.org Cc: John Stultz , =?UTF-8?q?Arve=20Hj=C3=83=C2=B8nnev=C3=83=C2=A5g?= , Thomas Gleixner Subject: [PATCH 1/2] time: Avoid accumulating time drift in suspend/resume Date: Tue, 31 May 2011 23:07:48 -0700 Message-Id: <1306908469-15275-2-git-send-email-john.stultz@linaro.org> X-Mailer: git-send-email 1.7.3.2.146.gca209 In-Reply-To: <1306908469-15275-1-git-send-email-john.stultz@linaro.org> References: <1306908469-15275-1-git-send-email-john.stultz@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3211 Lines: 82 Because the read_persistent_clock interface is usually backed by only a second granular interface, each time we read from the persistent clock for suspend/resume, we introduce a half second (on average) of error. In order to avoid this error accumulating as the system is suspended over and over, this patch measures the time delta between the persistent clock and the system CLOCK_REALTIME. If the delta is less then 2 seconds from the last suspend, we compensate by using the previous time delta (keeping it close). If it is larger then 2 seconds, we assume the clock was set or has been changed, so we do no correction and update the delta. Note: If NTP is running, ths could seem to "fight" with the NTP corrected time, where as if the system time was off by 1 second, and NTP slewed the value in, a suspend/resume cycle could undo this correction, by trying to restore the previous offset from the persistent clock. However, without this patch, since each read could cause almost a full second worth of error, its possible to get almost 2 seconds of error just from the suspend/resume cycle alone, so this about equal to any offset added by the compensation. Further on systems that suspend/resume frequently, this should keep time closer then NTP could compensate for if the errors were allowed to accumulate. Credits to Arve Hjønnevåg for suggesting this solution. CC: Arve Hjønnevåg CC: Thomas Gleixner Signed-off-by: John Stultz --- kernel/time/timekeeping.c | 22 ++++++++++++++++++++++ 1 files changed, 22 insertions(+), 0 deletions(-) diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 342408c..7665fea 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -686,12 +686,34 @@ static void timekeeping_resume(void) static int timekeeping_suspend(void) { unsigned long flags; + struct timespec delta, delta_delta; + static struct timespec old_delta; read_persistent_clock(&timekeeping_suspend_time); write_seqlock_irqsave(&xtime_lock, flags); timekeeping_forward_now(); timekeeping_suspended = 1; + + /* + * To avoid drift caused by repeated suspend/resumes, + * which each can add ~1 second drift error, + * try to compensate so the difference in system time + * and persistent_clock time stays close to constant. + */ + delta = timespec_sub(xtime, timekeeping_suspend_time); + delta_delta = timespec_sub(delta, old_delta); + if (abs(delta_delta.tv_sec) >= 2) { + /* + * if delta_delta is too large, assume time correction + * has occured and set old_delta to the current delta. + */ + old_delta = delta; + } else { + /* Otherwise try to adjust old_system to compensate */ + timekeeping_suspend_time = + timespec_add(timekeeping_suspend_time, delta_delta); + } write_sequnlock_irqrestore(&xtime_lock, flags); clockevents_notify(CLOCK_EVT_NOTIFY_SUSPEND, NULL); -- 1.7.3.2.146.gca209 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/