No, new Linux kernels will use rdtsc in all situations that older kernels
used to as well.
HOWEVER, there are a set of circumstances where an old kernel would
_never_ have used rdtsc, and a new kernel will. Most of those are so
performance-critical that some people (namely me and Ingo) felt that the
only way to support it was to just require it at compile time.
So think of the compile-time option as giving up an extra choice that
wasn't available before.
Linus
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