tools/power turbostat: detect and work around syscall jitter

tools/power turbostat: detect and work around syscall jitter

The accuracy of Bzy_Mhz and Busy% depend on reading
the TSC, APERF, and MPERF close together in time.

When there is a very short measurement interval,
or a large system is profoundly idle, the changes
in APERF and MPERF may be very small.
They can be small enough that an expensive interrupt
between reading APERF and MPERF can cause the APERF/MPERF
ratio to become inaccurate, resulting in invalid
calculation and display of Bzy_MHz.

A dummy APERF read of APERF makes this problem
much more rare.  Apparently this 1st systemn call
after exiting a long stretch of idle is when we
typically see expensive timer interrupts that cause
large jitter.

For the cases that dummy APERF read fails to prevent,
we compare the latency of the APERF and MPERF reads.
If they differ by more than 2x, we re-issue them.

Signed-off-by: Len Brown <len.brown@intel.com>