Messages in this thread |  | | Date | Tue, 2 Jun 2015 11:01:54 +0200 | From | Ingo Molnar <> | Subject | Re: [RFC][PATCH 4/4] time: Do leapsecond adjustment in gettime fastpaths |
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* John Stultz <john.stultz@linaro.org> wrote:
> Currently, leapsecond adjustments are done at tick time. > > As a result, the leapsecond was applied at the first timer > tick *after* the leapsecond (~1-10ms late depending on HZ), > rather then exactly on the second edge. > > This was in part historical from back when we were always > tick based, but correcting this since has been avoided since > it adds extra conditional checks in the gettime fastpath, > which has performance overhead. > > However, it was recently pointed out that ABS_TIME > CLOCK_REALTIME timers set for right after the leapsecond > could fire a second early, since some timers may be expired > before we trigger the timekeeping timer, which then applies > the leapsecond. > > This isn't quite as bad as it sounds, since behaviorally > it is similar to what is possible w/ ntpd made leapsecond > adjustments done w/o using the kernel discipline. Where > due to latencies, timers may fire just prior to the > settimeofday call. (Also, one should note that all > applications using CLOCK_REALTIME timers should always be > careful, since they are prone to quirks from settimeofday() > disturbances.) > > However, the purpose of having the kernel do the leap adjustment > is to avoid such latencies, so I think this is worth fixing. > > So in order to properly keep those timers from firing a second > early, this patch modifies the gettime accessors to do the > extra checks to apply the leapsecond adjustment on the second > edge. This prevents the timer core from expiring timers too > early. > > This patch does not handle VDSO time implementations, so > userspace using vdso gettime will still see the leapsecond > applied at the first timer tick after the leapsecond. > This is a bit of a tradeoff, since the performance impact > would be greatest to VDSO implementations, and since vdso > interfaces don't provide the TIME_OOP flag, one can't > distinquish the leapsecond from a time discontinuity (such > as settimeofday), so correcting the VDSO may not be as > important there. > > Apologies to Richard Cochran, who pushed for such a change > years ago, which I resisted due to the concerns about the > performance overhead. > > While I suspect this isn't extremely critical, folks who > care about strict leap-second correctness will likely > want to watch this, and it will likely be a -stable candidate. > > Cc: Prarit Bhargava <prarit@redhat.com> > Cc: Daniel Bristot de Oliveira <bristot@redhat.com> > Cc: Richard Cochran <richardcochran@gmail.com> > Cc: Jan Kara <jack@suse.cz> > Cc: Jiri Bohac <jbohac@suse.cz> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: Shuah Khan <shuahkh@osg.samsung.com> > Originally-suggested-by: Richard Cochran <richardcochran@gmail.com> > Reported-by: Daniel Bristot de Oliveira <bristot@redhat.com> > Reported-by: Prarit Bhargava <prarit@redhat.com> > Signed-off-by: John Stultz <john.stultz@linaro.org> > --- > include/linux/time64.h | 1 + > include/linux/timekeeper_internal.h | 7 +++ > kernel/time/ntp.c | 73 +++++++++++++++++++++++++--- > kernel/time/ntp_internal.h | 1 + > kernel/time/timekeeping.c | 97 ++++++++++++++++++++++++++++++++----- > 5 files changed, 159 insertions(+), 20 deletions(-)
So I don't like the complexity of this at all: why do we add over 100 lines of code for something that occurs (literally) once in a blue moon?
... and for that reason I'm not surprised at all that it broke in non-obvious ways.
Instead of having these super rare special events, how about implementing leap second smearing instead? That's far less radical and a lot easier to test as well, as it's a continuous mechanism. It will also confuse user-space a lot less, because there are no sudden time jumps.
Secondly, why is there a directional flag? I thought leap seconds can only be inserted.
So all in one, the leap second code is fragile and complex - lets re-think the whole topic instead of complicating it even more ...
Thanks,
Ingo
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