linux-kernel - Re: [clocksource] 8901ecc231: stress-ng.lockbus.ops_per

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Date:   Thu, 5 Aug 2021 08:37:27 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Chao Gao <chao.gao@...el.com>
Cc:     Feng Tang <feng.tang@...el.com>,
        kernel test robot <oliver.sang@...el.com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Mark Rutland <Mark.Rutland@....com>,
        Marc Zyngier <maz@...nel.org>, Andi Kleen <ak@...ux.intel.com>,
        Xing Zhengjun <zhengjun.xing@...ux.intel.com>,
        Chris Mason <clm@...com>, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        lkp@...ts.01.org, lkp@...el.com, ying.huang@...el.com,
        zhengjun.xing@...el.com
Subject: Re: [clocksource]  8901ecc231:  stress-ng.lockbus.ops_per_sec -9.5%
 regression

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@...el.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