From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9872CC4708A for ; Thu, 27 May 2021 21:05:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6A3CA613EC for ; Thu, 27 May 2021 21:05:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234266AbhE0VG7 (ORCPT ); Thu, 27 May 2021 17:06:59 -0400 Received: from mail.kernel.org ([198.145.29.99]:51010 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233788AbhE0VG6 (ORCPT ); Thu, 27 May 2021 17:06:58 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 6E8A4613E3; Thu, 27 May 2021 21:05:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1622149524; bh=usTkYUDMMgRLdDdQrjUeWJsC5bIfE442l+uBD1FDr1g=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=VLIMbVJsyqy5YGjl8qDmNMxdr1ATAL1r6WsSQmjjcVj+dBMkew+wV94lTdL2HxB4v QSvry6tjjJjeDJ3vw3kujsAC83M60yt9G45NGzyPk8RQZeJi00ZOHRSEmsoi/sM4LI Zs59P7WujpyFz379uIrEnPbnnPlTbjDQOIz5G8q30R9yhpyRQcrJtkt+ioCu9VpHU1 ED2genEOurwDsXL9gglF9yK4GJkF3meMx/lp6LXhGwNLlWO1B2FpWQrM167zVOBYEw 8qETaypfuU1OtOd1ZntWEXSXJcC3pqL/vg8xU03EPXz/LCOGibJjhNcf3l+fRae5+7 Y0gevS/QT/fbA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 3C9C45C032C; Thu, 27 May 2021 14:05:24 -0700 (PDT) Date: Thu, 27 May 2021 14:05:24 -0700 From: "Paul E. McKenney" To: Matthew Wilcox Cc: Andi Kleen , Feng Tang , kernel test robot , John Stultz , Thomas Gleixner , Stephen Boyd , Jonathan Corbet , Mark Rutland , Marc Zyngier , 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: <20210527210524.GE4397@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210521083322.GG25531@xsang-OptiPlex-9020> <20210521135617.GT4441@paulmck-ThinkPad-P17-Gen-1> <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> <138f81df-08e1-f96e-1915-c58b44f96a41@linux.intel.com> <20210527191923.GD4397@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 27, 2021 at 08:29:32PM +0100, Matthew Wilcox wrote: > On Thu, May 27, 2021 at 12:19:23PM -0700, Paul E. McKenney wrote: > > On Thu, May 27, 2021 at 12:01:23PM -0700, Andi Kleen wrote: > > > > > > > Nevertheless, it is quite possible that real-world use will result in > > > > some situation requiring that high-stress workloads run on hardware > > > > not designed to accommodate them, and also requiring that the kernel > > > > refrain from marking clocksources unstable. > > > > Therefore, provide an out-of-tree patch that reacts to this situation > > > > > > out-of-tree means it will not be submitted? > > > > > > I think it would make sense upstream, but perhaps guarded with some option. > > > > The reason I do not intend to immediately upstream this patch is that > > it increases the probability that a real clocksource read-latency issue > > will be ignored, for example, during hardware bringup. Furthermore, > > the only known need from it comes from hardware that is, in the words > > of the stress-ng man page, "poorly designed". And the timing of this > > email thread leads me to believe that such hardware is not easy to obtain. > > I think you're placing a little too much weight on the documentation > here. It seems that a continuous stream of locked operations executed > in userspace on a single CPU can cause this problem to occur. If that's > true all the way out to one guest in a hypervisor can cause problems > for the hypervisor itself, I think cloud providers everywhere are > going to want this patch? Only those cloud provides making heavy use of the aforementioned "poorly designed" hardware, correct? Thanx, Paul > > My thought is therefore to keep this patch out of tree for now. > > If it becomes clear that long-latency clocksource reads really are > > a significant issue in their own right (as opposed to merely being a > > symptom of a hardware or firmware bug), then this patch is available to > > immediately respond to that issue. > > > > And there would then be strong evidence in favor of me biting the bullet, > > adding the complexity and the additional option (with your Suggested-by), > > and getting that upstream and into -stable. > > > > Seem reasonable? > > > > Thanx, Paul > >