Re: [clocksource] 8901ecc231: stress-ng.lockbus.ops_per_sec -9.5% regression

From: Chao Gao
Date: Thu Aug 05 2021 - 01:32:33 EST

>> 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?

Tested-by: Chao Gao <chao.gao@xxxxxxxxx>

>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.


>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.

It makes sense to me.