Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 5 Aug 2021 08:37:27 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Chao Gao <chao.gao@intel.com>
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: <20210805153727.GG4397@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <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>
 <20210805053938.GA12593@gao-cwp>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20210805053938.GA12593@gao-cwp>
Precedence: bulk

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