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

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Aug 05, 2021 at 01:39:40PM +0800, Chao Gao wrote:
> [snip]
> >> 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 <chao.gao@intel.com>

Very good, thank you!  I will apply this on the next rebase.

> >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.

Whew!  ;-)

So about once per 13 clocksource watchdog checks.

To Andi's point, do you have enough information in your console log to
work out the longest run of course-grained clocksource checks?

> >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.

Sounds good!

							Thanx, Paul