Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757374AbYJFX2q (ORCPT ); Mon, 6 Oct 2008 19:28:46 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757033AbYJFXWk (ORCPT ); Mon, 6 Oct 2008 19:22:40 -0400 Received: from cantor2.suse.de ([195.135.220.15]:55247 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757021AbYJFXWj (ORCPT ); Mon, 6 Oct 2008 19:22:39 -0400 Date: Mon, 6 Oct 2008 16:17:43 -0700 From: Greg KH To: linux-kernel@vger.kernel.org, stable@kernel.org Cc: Justin Forbes , Zwane Mwaikambo , "Theodore Ts'o" , Randy Dunlap , Dave Jones , Chuck Wolber , Chris Wedgwood , Michael Krufky , Chuck Ebbert , Domenico Andreoli , Willy Tarreau , Rodrigo Rubira Branco , Jake Edge , Eugene Teo , torvalds@linux-foundation.org, akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, "Maciej W. Rozycki" , Ingo Molnar Subject: [patch 20/28] ntp: fix calculation of the next jiffie to trigger RTC sync Message-ID: <20081006231743.GU20567@suse.de> References: <20081006225949.357365087@mini.kroah.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline; filename="timer-fix-10.patch" In-Reply-To: <20081006231639.GA20567@suse.de> User-Agent: Mutt/1.5.16 (2007-06-09) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2622 Lines: 63 2.6.25-stable review patch. If anyone has any objections, please let us know. ------------------ From: Maciej W. Rozycki commit 4ff4b9e19a80b73959ebeb28d1df40176686f0a8 upstream We have a bug in the calculation of the next jiffie to trigger the RTC synchronisation. The aim here is to run sync_cmos_clock() as close as possible to the middle of a second. Which means we want this function to be called less than or equal to half a jiffie away from when now.tv_nsec equals 5e8 (500000000). If this is not the case for a given call to the function, for this purpose instead of updating the RTC we calculate the offset in nanoseconds to the next point in time where now.tv_nsec will be equal 5e8. The calculated offset is then converted to jiffies as these are the unit used by the timer. Hovewer timespec_to_jiffies() used here uses a ceil()-type rounding mode, where the resulting value is rounded up. As a result the range of now.tv_nsec when the timer will trigger is from 5e8 to 5e8 + TICK_NSEC rather than the desired 5e8 - TICK_NSEC / 2 to 5e8 + TICK_NSEC / 2. As a result if for example sync_cmos_clock() happens to be called at the time when now.tv_nsec is between 5e8 + TICK_NSEC / 2 and 5e8 to 5e8 + TICK_NSEC, it will simply be rescheduled HZ jiffies later, falling in the same range of now.tv_nsec again. Similarly for cases offsetted by an integer multiple of TICK_NSEC. This change addresses the problem by subtracting TICK_NSEC / 2 from the nanosecond offset to the next point in time where now.tv_nsec will be equal 5e8, effectively shifting the following rounding in timespec_to_jiffies() so that it produces a rounded-to-nearest result. Signed-off-by: Maciej W. Rozycki Signed-off-by: Andrew Morton Signed-off-by: Ingo Molnar Signed-off-by: Greg Kroah-Hartman --- kernel/time/ntp.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/kernel/time/ntp.c +++ b/kernel/time/ntp.c @@ -205,7 +205,7 @@ static void sync_cmos_clock(unsigned lon if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec / 2) fail = update_persistent_clock(now); - next.tv_nsec = (NSEC_PER_SEC / 2) - now.tv_nsec; + next.tv_nsec = (NSEC_PER_SEC / 2) - now.tv_nsec - (TICK_NSEC / 2); if (next.tv_nsec <= 0) next.tv_nsec += NSEC_PER_SEC; -- -- 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/