Messages in this thread | | | Subject | Re: [patch 4/9] Remove the TSC synchronization on SMP machines | From | john stultz <> | Date | Tue, 13 Feb 2007 16:25:31 -0800 |
| |
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/
| |