From mboxrd@z Thu Jan 1 00:00:00 1970 From: akpm@linux-foundation.org Subject: + time-compensate-for-rounding-on-odd-frequency-clocksources.patch added to -mm tree Date: Fri, 03 Sep 2010 16:48:16 -0700 Message-ID: <201009032348.o83NmGuR019709@imap1.linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Return-path: Received: from smtp1.linux-foundation.org ([140.211.169.13]:42292 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752517Ab0ICXso (ORCPT ); Fri, 3 Sep 2010 19:48:44 -0400 Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: mm-commits@vger.kernel.org Cc: kkp2010@kasperkp.dk, jkacur@redhat.com, johnstul@us.ibm.com, schwidefsky@de.ibm.com, tglx@linutronix.de, williams@redhat.com The patch titled time: compensate for rounding on odd-frequency clocksources has been added to the -mm tree. Its filename is time-compensate-for-rounding-on-odd-frequency-clocksources.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://userweb.kernel.org/~akpm/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: time: compensate for rounding on odd-frequency clocksources From: Kasper Pedersen When the clocksource is not a multiple of HZ, the clock will be off. For acpi_pm, HZ=1000 the error is 127.111 ppm: The rounding of cycle_interval ends up generating a false error term in ntp_error accumulation since xtime_interval is not exactly 1/HZ. So, we subtract out the error caused by the rounding. This has been visible since 2.6.32-rc2 commit a092ff0f90cae22b2ac8028ecd2c6f6c1a9e4601 time: Implement logarithmic time accumulation That commit raised NTP_INTERVAL_FREQ and exposed the rounding error. testing tool: http://n1.taur.dk/permanent/testpmt.c Also tested with ntpd and a frequency counter. Signed-off-by: Kasper Pedersen Cc: Thomas Gleixner Cc: john stultz Cc: John Kacur Cc: Clark Williams Cc: Martin Schwidefsky Signed-off-by: Andrew Morton --- kernel/time/timekeeping.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff -puN kernel/time/timekeeping.c~time-compensate-for-rounding-on-odd-frequency-clocksources kernel/time/timekeeping.c --- a/kernel/time/timekeeping.c~time-compensate-for-rounding-on-odd-frequency-clocksources +++ a/kernel/time/timekeeping.c @@ -32,6 +32,8 @@ struct timekeeper { cycle_t cycle_interval; /* Number of clock shifted nano seconds in one NTP interval. */ u64 xtime_interval; + /* shifted nano seconds left over when rounding cycle_interval */ + s64 xtime_remainder; /* Raw nano seconds accumulated per NTP interval. */ u32 raw_interval; @@ -62,7 +64,7 @@ struct timekeeper timekeeper; static void timekeeper_setup_internals(struct clocksource *clock) { cycle_t interval; - u64 tmp; + u64 tmp, ntpinterval; timekeeper.clock = clock; clock->cycle_last = clock->read(clock); @@ -70,6 +72,7 @@ static void timekeeper_setup_internals(s /* Do the ns -> cycle conversion first, using original mult */ tmp = NTP_INTERVAL_LENGTH; tmp <<= clock->shift; + ntpinterval = tmp; tmp += clock->mult/2; do_div(tmp, clock->mult); if (tmp == 0) @@ -80,6 +83,7 @@ static void timekeeper_setup_internals(s /* Go back from cycles -> shifted ns */ timekeeper.xtime_interval = (u64) interval * clock->mult; + timekeeper.xtime_remainder = ntpinterval - timekeeper.xtime_interval; timekeeper.raw_interval = ((u64) interval * clock->mult) >> clock->shift; @@ -746,7 +750,8 @@ static cycle_t logarithmic_accumulation( /* Accumulate error between NTP and clock interval */ timekeeper.ntp_error += tick_length << shift; - timekeeper.ntp_error -= timekeeper.xtime_interval << + timekeeper.ntp_error -= + (timekeeper.xtime_interval + timekeeper.xtime_remainder) << (timekeeper.ntp_error_shift + shift); return offset; _ Patches currently in -mm which might be from kkp2010@kasperkp.dk are time-compensate-for-rounding-on-odd-frequency-clocksources.patch