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

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Date:   Fri, 6 Aug 2021 10:10:00 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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 08:37:27AM -0700, Paul E. McKenney wrote:
>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?

Yes. 5 consecutive course-grained clocksource checks. Note that
considering the reinitialization after course-grained check, in my
calculation, two course-grained checks are considered consecutive if
they happens in 1s(+/- 0.3s).

Thanks
Chao