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McKenney" To: Chao Gao Cc: Feng Tang , kernel test robot , John Stultz , Thomas Gleixner , Stephen Boyd , Jonathan Corbet , Mark Rutland , Marc Zyngier , Andi Kleen , Xing Zhengjun , Chris Mason , LKML , Linux Memory Management List , 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> X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 748A04001E8D Authentication-Results: imf18.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=gsNqgUPG; dmarc=pass (policy=none) header.from=kernel.org; spf=pass (imf18.hostedemail.com: domain of "SRS0=aZYB=M4=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org" designates 198.145.29.99 as permitted sender) smtp.mailfrom="SRS0=aZYB=M4=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org" X-Stat-Signature: rjpdtjfz34teqtnnkrknyix3n5x5j1sx X-HE-Tag: 1628177848-192304 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 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