> Notice that in the Skylake Client microarchitecture the RDTSC instruction counts at the machine’s guaranteed P1 frequency independently of the current processor clock (see the INVARIANT TSC property), and therefore, when running in Intel® Turbo-Boost-enabled mode, the delay will remain constant, but the number of instructions that could have been executed will change.

rdtsc may execute out of order, so sometimes an lfence (previously cpuid) can be used and there is also rdtscp

See https://github.com/torvalds/linux/blob/master/arch/x86/inclu...

And just because rdtsc is constant doesn't mean the processor clock will be constant that could be fluctuating.