From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753302AbdE0HgN (ORCPT ); Sat, 27 May 2017 03:36:13 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:33890 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753083AbdE0HgM (ORCPT ); Sat, 27 May 2017 03:36:12 -0400 Date: Sat, 27 May 2017 09:36:08 +0200 From: Ingo Molnar To: John Stultz Cc: lkml , Thomas Gleixner , Miroslav Lichvar , Richard Cochran , Prarit Bhargava , Stephen Boyd , Kevin Brodsky , Will Deacon , Daniel Mentz Subject: Re: [RFC][PATCH 2/4] time: Fix CLOCK_MONOTONIC_RAW sub-nanosecond accounting Message-ID: <20170527073608.zb53ytgpfrv6cvut@gmail.com> References: <1495856035-6622-1-git-send-email-john.stultz@linaro.org> <1495856035-6622-3-git-send-email-john.stultz@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1495856035-6622-3-git-send-email-john.stultz@linaro.org> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * John Stultz wrote: > Due to how the MONOTONIC_RAW accumulation logic was handled, > there is the potential for a 1ns discontinuity when we do > accumulations. This small discontinuity has for the most part > gone un-noticed, but since ARM64 enabled CLOCK_MONOTONIC_RAW > in their vDSO clock_gettime implementation, we've seen failures > with the inconsistency-check test in kselftest. > > This patch addresses the issue by using the same sub-ns > accumulation handling that CLOCK_MONOTONIC uses, which avoids > the issue for in-kernel users. > > Since the ARM64 vDSO implementation has its own clock_gettime > calculation logic, this patch reduces the frequency of errors, > but failures are still seen. The ARM64 vDSO will need to be > updated to include the sub-nanosecond xtime_nsec values in its > calculation for this issue to be completely fixed. > > Cc: Thomas Gleixner > Cc: Ingo Molnar > Cc: Miroslav Lichvar > Cc: Richard Cochran > Cc: Prarit Bhargava > Cc: Stephen Boyd > Cc: Kevin Brodsky > Cc: Will Deacon > Cc: Daniel Mentz > Signed-off-by: John Stultz > --- > include/linux/timekeeper_internal.h | 4 ++-- > kernel/time/timekeeping.c | 21 ++++++++++++--------- > 2 files changed, 14 insertions(+), 11 deletions(-) > > diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h > index 110f453..528cc86 100644 > --- a/include/linux/timekeeper_internal.h > +++ b/include/linux/timekeeper_internal.h > @@ -58,7 +58,7 @@ struct tk_read_base { > * interval. > * @xtime_remainder: Shifted nano seconds left over when rounding > * @cycle_interval > - * @raw_interval: Raw nano seconds accumulated per NTP interval. > + * @raw_interval: Shifted raw nano seconds accumulated per NTP interval. > * @ntp_error: Difference between accumulated time and NTP time in ntp > * shifted nano seconds. > * @ntp_error_shift: Shift conversion between clock shifted nano seconds and > @@ -100,7 +100,7 @@ struct timekeeper { > u64 cycle_interval; > u64 xtime_interval; > s64 xtime_remainder; > - u32 raw_interval; > + u64 raw_interval; > /* The ntp_tick_length() value currently being used. > * This cached copy ensures we consistently apply the tick > * length for an entire tick, as ntp_tick_length may change > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c > index abc1968..35d3ba3 100644 > --- a/kernel/time/timekeeping.c > +++ b/kernel/time/timekeeping.c > @@ -282,7 +282,7 @@ static void tk_setup_internals(struct timekeeper *tk, struct clocksource *clock) > /* Go back from cycles -> shifted ns */ > tk->xtime_interval = interval * clock->mult; > tk->xtime_remainder = ntpinterval - tk->xtime_interval; > - tk->raw_interval = (interval * clock->mult) >> clock->shift; > + tk->raw_interval = interval * clock->mult; > > /* if changing clocks, convert xtime_nsec shift units */ > if (old_clock) { > @@ -1994,7 +1994,7 @@ static u64 logarithmic_accumulation(struct timekeeper *tk, u64 offset, > u32 shift, unsigned int *clock_set) > { > u64 interval = tk->cycle_interval << shift; > - u64 raw_nsecs; > + u64 nsecps; What does the 'ps' postfix stand for? It's not obvious (to me). > + tk->tkr_raw.xtime_nsec += (u64)tk->raw_time.tv_nsec > + << tk->tkr_raw.shift; > + tk->tkr_raw.xtime_nsec -= (u64)tk->raw_time.tv_nsec > + << tk->tkr_raw.shift; Please don't break the line in such an ugly, random way, just write: tk->tkr_raw.xtime_nsec += (u64)tk->raw_time.tv_nsec << tk->tkr_raw.shift; tk->tkr_raw.xtime_nsec -= (u64)tk->raw_time.tv_nsec << tk->tkr_raw.shift; Thanks, Ingo