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Date:   Thu, 5 Aug 2021 13:39:40 +0800
From:   Chao Gao <chao.gao@intel.com>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Feng Tang <feng.tang@intel.com>,
        kernel test robot <oliver.sang@intel.com>,
        John Stultz <john.stultz@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Mark Rutland <Mark.Rutland@arm.com>,
        Marc Zyngier <maz@kernel.org>, Andi Kleen <ak@linux.intel.com>,
        Xing Zhengjun <zhengjun.xing@linux.intel.com>,
        Chris Mason <clm@fb.com>, LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        lkp@lists.01.org, lkp@intel.com, ying.huang@intel.com,
        zhengjun.xing@intel.com
Subject: Re: [clocksource]  8901ecc231:  stress-ng.lockbus.ops_per_sec -9.5%
 regression
Message-ID: <20210805053938.GA12593@gao-cwp>
References: <20210522160827.GA2625834@paulmck-ThinkPad-P17-Gen-1>
 <20210526064922.GD5262@shbuild999.sh.intel.com>
 <20210526134911.GB4441@paulmck-ThinkPad-P17-Gen-1>
 <20210527182959.GA437082@paulmck-ThinkPad-P17-Gen-1>
 <20210802062008.GA24720@gao-cwp>
 <20210802170257.GL4397@paulmck-ThinkPad-P17-Gen-1>
 <20210803085759.GA31621@gao-cwp>
 <20210803134816.GO4397@paulmck-ThinkPad-P17-Gen-1>
 <20210805021646.GA11629@gao-cwp>
 <20210805040349.GD4397@paulmck-ThinkPad-P17-Gen-1>
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[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>

>
>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