From RHEL-RT

Enabling TSC timer synchronization on AMD Opteron systems

The current generation of Opteron processors do not provide a frequency independent TSC. This causes wild gettimeofday skew on systems that enable cpufreq while using TSC as a gtod source.

MRG Realtime contains a kernel patch that provides a workaround for Opteron systems using TSC as the gettimeofday() time source. The work around is that all processors change to the same frequency at the same time, so that the TSC on each processor never increments at a different rate than the TSC on another processor.

The following boot options are necessary to force the use of TSC and enable simultaneous core processor frequency transitions i.e. "nopmtimer powernow-k8.tscsync=1"