CPU excessively long times between frequency scaling driver calls - bisected

["Doug Smythies" <dsmythies@telus.net>]

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Hi All,

Note before: I do not know If I have the e-mail address list correct,
nor am I actually a member of the x86 distribution list. I am on
the linux-pm email list.

When using the intel_pstate CPU frequency scaling driver with HWP disabled,
active mode, powersave scaling governor, the times between calls to the driver
have never exceeded 10 seconds.

Since kernel 5.16-rc4 and commit: b50db7095fe002fa3e16605546cba66bf1b68a3e
" x86/tsc: Disable clocksource watchdog for TSC on qualified platorms"

There are now occasions where times between calls to the driver can be
over 100's of seconds and can result in the CPU frequency being left
unnecessarily high for extended periods.

From the number of clock cycles executed between these long
durations one can tell that the CPU has been running code, but
the driver never got called.

Attached are some graphs from some trace data acquired using
intel_pstate_tracer.py where one can observe an idle system between
about 42 and well over 200 seconds elapsed time, yet CPU10 never gets
called, which would have resulted in reducing it's pstate request, until
an elapsed time of 167.616 seconds, 126 seconds since the last call. The
CPU frequency never does go to minimum.

For reference, a similar CPU frequency graph is also attached, with
the commit reverted. The CPU frequency drops to minimum,
over about 10 or 15 seconds.

Processor: Intel(R) Core(TM) i5-10600K CPU @ 4.10GHz

Why this particular configuration, i.e. no-hwp, active, powersave?
Because it is, by far, the easiest to observe what is going on.

... Doug


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