lkml.org 
[lkml]   [2007]   [Feb]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [patch 4/9] Remove the TSC synchronization on SMP machines
From
Date
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 majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-02-14 01:29    [W:0.126 / U:0.172 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site