On Wed, 2007-02-14 at 11:18 +1100, Paul Mackerras wrote:
> Andi Kleen writes:
>
> > Just to avoid spreading misinformation: modulo some new broken hardware
> > (which we always try to work around when found) i386/x86-64 gettimeofday
> > is monotonic. AFAIK on the currently known hardware it should be generally
> > ok.
> >
> > However ntpd can always screw you up, but that's inherent in the design.
>
> On powerpc we manage to keep gettimeofday monotonic even when ntpd is
> adjusting the clock. We have 3 parameters used to convert a value
> from the timebase register to the time of day, and these parameters
> are adjusted if necessary at the beginning of each tick, based on the
> value returned by current_tick_length(). The point is that
> current_tick_length() tells you at the *beginning* of each tick how
> much time will be added on to xtime at the *end* of that tick, and
> that makes it possible to aim the interpolation to hit the same value
> as xtime at the end of each tick.
>
> Clearly if you make a discrete jump backwards with settimeofday or
> adjtime, it's impossible to keep gettimeofday monotonic, but apart
> from that it's monotonic on powerpc.
>
> At least, that's the way it's supposed to work. I hope the recent
> timekeeping changes haven't broken it. :)
No. Just to even further clarify (and since everyone is speaking up),
the generic timekeeping does a similar scaling adjustment of the
clocksource frequency for NTP adjustments made via sys_adjtimex().
I believe Andi was just referring to ntpd calling settimeofday(), which
will cause clock_gettime(CLOCK_REALTIME,...)/gettimeofday() to possibly
jump backwards. This behavior of NTP is of course configurable (see the
-x option, or the "tinker step 0" option combined w/ "disable kernel" in
ntp.conf)
-john
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
