[LITMUS^RT] Overhead measurement with switching frequencies

[Gabriel Lozano]

Hi all, I'm working on a DVFS scheduler prototype using LITMUS^RT as my
main platform.

I wanted to know if LITMUS^RT mechanism for measuring overheads is still
reliable even when the frequency of the processor is constantly changing
during the schedule. Digging into the feather trace code, I found it relies
on calls to __native_read_tsc(), which I suppose triggers the rdtsc
instruction.

My x86_64 system features the constant_tsc flag, which according to
documentation means that working of the tsc goes unaffected by frequency
changes or sleep states. This way, is it safe to assume that the feather
trace infrastructure will work correctly even when switching frequencies? I
suppose overheads would have to be accounted for in terms of cycles instead
of time.

Thanks in advance for your help. Regards!
GL
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