[snip]

quoted

This patch works well; no false-positive (marking TSC unstable) in a
10hr stress test.

Very good, thank you!  May I add your Tested-by?

sure.
Tested-by: Chao Gao <redacted>

I expect that I will need to modify the patch a bit more to check for
a system where it is -never- able to get a good fine-grained read from
the clock.

Agreed.

And it might be that your test run ended up in that state.

Not that case judging from kernel logs. Coarse-grained check happened 6475
times in 43k seconds (by grep "coarse-grained skew check" in kernel logs).
So, still many checks were fine-grained.

My current thought is that if more than (say) 100 consecutive attempts
to read the clocksource get hit with excessive delays, it is time to at
least do a WARN_ON(), and maybe also time to disable the clocksource
due to skew.  The reason is that if reading the clocksource -always-
sees excessive delays, perhaps the clock driver or hardware is to blame.

Thoughts?

It makes sense to me.

Thanks
Chao