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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,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 9EDF0C47089 for ; Thu, 27 May 2021 19:30:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 70404613DD for ; Thu, 27 May 2021 19:30:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235135AbhE0Tbh (ORCPT ); Thu, 27 May 2021 15:31:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60508 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229764AbhE0Tbf (ORCPT ); Thu, 27 May 2021 15:31:35 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1B1C8C061574 for ; Thu, 27 May 2021 12:30:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=OmpHNr4TAP41rJ2r2wggyNBZl+LhZ+GtvDxjkk/dx1o=; b=vzGfouo5ojxroNYcpHA0i7jS7G ewKzzmf2ehPIB8v3KD52FK+eFXvJVkit+agK8EcnuYjjOO9cZptqgt7/O4dlaXv0umNbFS9peuINi wu7mqCXzhwqCzvfBtMNc8Anwb9BesjlGb8Q7VS47lvFKrgH+ZM4GKJW5ZdfpozNTeMN1Hym6m9LL8 XXsm60i4YupzGbEi9I2SV3UZ3J0eai2uvrpQO+qG9u0XIIBdudx2S7k4KKpZo8go+ZdN1ip9b3gM2 FCFKCzX7iIJs6cyj9kFMa6rRl1kP0p+5Y0MqS5yhclRgczB+FOn1OtfpDQdyf0e9Lc4G8DcaQtBhe RYdZKmyA==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lmLhE-005r04-Bi; Thu, 27 May 2021 19:29:34 +0000 Date: Thu, 27 May 2021 20:29:32 +0100 From: Matthew Wilcox To: "Paul E. McKenney" 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: 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: <20210527191923.GD4397@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? > 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 >