recent CPUs have constant-freq TSCs so it's mostly a legacy issue, but
the affected installed base is still significant to warrant a fix.
we dont really have to worry about complications like grace periods -
higher layers in the scheduler protect against temporary sched_clock()
outliers. So i think this can all be done much simpler. Just get rid of
the global cpu_khz notion, sched_clock() should simply follow the ->freq
value - and that's it.
Ingo
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