From mboxrd@z Thu Jan 1 00:00:00 1970 From: akpm@linux-foundation.org Subject: + ntp-fix-calculation-of-the-next-jiffie-to-trigger-rtc-sync.patch added to -mm tree Date: Mon, 12 May 2008 17:22:36 -0700 Message-ID: <200805130022.m4D0Map2016452@imap1.linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Return-path: Received: from smtp1.linux-foundation.org ([140.211.169.13]:55998 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753112AbYEMAXd (ORCPT ); Mon, 12 May 2008 20:23:33 -0400 Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: mm-commits@vger.kernel.org Cc: macro@linux-mips.org, tglx@linutronix.de, zippel@linux-m68k.org The patch titled ntp: fix calculation of the next jiffie to trigger RTC sync has been added to the -mm tree. Its filename is ntp-fix-calculation-of-the-next-jiffie-to-trigger-rtc-sync.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find out what to do about this The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: ntp: fix calculation of the next jiffie to trigger RTC sync From: "Maciej W. Rozycki" 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 Cc: Thomas Gleixner Cc: Roman Zippel Signed-off-by: Andrew Morton --- kernel/time/ntp.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff -puN kernel/time/ntp.c~ntp-fix-calculation-of-the-next-jiffie-to-trigger-rtc-sync kernel/time/ntp.c --- a/kernel/time/ntp.c~ntp-fix-calculation-of-the-next-jiffie-to-trigger-rtc-sync +++ a/kernel/time/ntp.c @@ -245,7 +245,7 @@ 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; _ Patches currently in -mm which might be from macro@linux-mips.org are origin.patch rtc-rtc_time_to_tm-use-unsigned-arithmetic.patch rtc-watchdog-m41t80-fix-a-potential-use-of-unitialized-data.patch rtc-m41t80-include-linux-kernelh-for-printk.patch linux-next.patch ntp-make-the-rtc-sync-mode-11-minute-again.patch ntp-fix-calculation-of-the-next-jiffie-to-trigger-rtc-sync.patch rtc-m41t80-sort-header-inclusions-for-readability.patch rtc-m41t80-use-pr_info-as-appropriate.patch