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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7A3B0C433F5 for ; Thu, 11 Nov 2021 00:04:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5DDC661246 for ; Thu, 11 Nov 2021 00:04:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234350AbhKKAHF (ORCPT ); Wed, 10 Nov 2021 19:07:05 -0500 Received: from mail.kernel.org ([198.145.29.99]:36948 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234339AbhKKAHD (ORCPT ); Wed, 10 Nov 2021 19:07:03 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 93A2D61105; Thu, 11 Nov 2021 00:04:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1636589054; bh=VbwJff2ix6rHkK73bDpQue47690xvnlowivXigHSQBA=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=Bhzk8Hg02W5B9ULNm5HJ6mYlw8DjPWN6oyCTYq9wbO9d8WwpCpUwgDvLGpZuJmQ/m F6aR6IKpNFU4i3+bfJTKHP6m5uLBe8uGgk16jzVZb+BJq9GozPTffwERUNpCuw0GOM Oxy6MCxQ38Zuk9xj/KDtxz9XDW29zo5fYhrtJ6nzuewd2l4JdJCyjV3R834Hg9oZsy FfOeeSPJ6G1stCwH4vkY18MgMy15Vv6VpkkGl4Fn7sKFfcLplNuksybm5LQjpwKVzr IN6CYE4CoMTc43WEQFWU71pTUDEBV+Kx/rNqJmwomOHhaXtk8MsCuZfNg/mrvjBWap VW7NTglKQYHUA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 61A605C0A70; Wed, 10 Nov 2021 16:04:14 -0800 (PST) Date: Wed, 10 Nov 2021 16:04:14 -0800 From: "Paul E. McKenney" To: Waiman Long Cc: John Stultz , Thomas Gleixner , Stephen Boyd , linux-kernel@vger.kernel.org, Peter Zijlstra , Cassio Neri , Linus Walleij , Frederic Weisbecker , Feng Tang Subject: Re: [PATCH 0/2] clocksource: Avoid incorrect hpet fallback Message-ID: <20211111000414.GH641268@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20211110221732.272986-1-longman@redhat.com> <20211110223250.GG641268@paulmck-ThinkPad-P17-Gen-1> <1f43bfad-434f-88d5-b794-4cf1116e9924@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1f43bfad-434f-88d5-b794-4cf1116e9924@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 10, 2021 at 06:25:14PM -0500, Waiman Long wrote: > > On 11/10/21 17:32, Paul E. McKenney wrote: > > On Wed, Nov 10, 2021 at 05:17:30PM -0500, Waiman Long wrote: > > > It was found that when an x86 system was being stressed by running > > > various different benchmark suites, the clocksource watchdog might > > > occasionally mark TSC as unstable and fall back to hpet which will > > > have a signficant impact on system performance. > > > > > > The current watchdog clocksource skew threshold of 50us is found to be > > > insufficient. So it is changed back to 100us before commit 2e27e793e280 > > > ("clocksource: Reduce clocksource-skew threshold") in patch 1. Patch 2 > > > adds a Kconfig option to allow kernel builder to control the actual > > > threshold to be used. > > > > > > Waiman Long (2): > > > clocksource: Avoid accidental unstable marking of clocksources > > > clocksource: Add a Kconfig option for WATCHDOG_MAX_SKEW > > The ability to control the fine-grained threshold seems useful, but is > > the TSC still marked unstable when this commit from -rcu is applied? > > It has passed significant testing on other workloads. > > > > 2a43fb0479aa ("clocksource: Forgive repeated long-latency watchdog clocksource reads") > > > > If the patch below takes care of your situation, my thought is to > > also take your second patch, which would allow people to set the > > cutoff more loosely or more tightly, as their situation dictates. > > > > Thoughts? > > That is commit 14dbb29eda51 ("clocksource: Forgive repeated long-latency > watchdog clocksource reads") in your linux-rcu git tree. From reading the > patch, I believe it should be able to address the hpet fallback problem that > Red Hat had encountered. Your patch said it was an out-of-tree patch. Are > you planning to mainline it? Yes, I expect to submit it into the next merge window (not the current v5.16 merge window, but v5.17). However, if your situation is urgent, and if it works for you, I could submit it as a fix for an earlier regression. > Patch 1 of this series contains some testing data that caused hpet fallback > in our testing runs. In summary, a clock skew of 100us is found to be enough > to avoid the problem with benchmark runs. However, we have some cases where > TSC was marked unstable at bootup time with a skew of 200us or more which, I > believe, was caused by the thermal stress that the system was experiencing > after running stressful benchmarks for hours. This sort of thing does show some value for allowing the threshold to be adjusted. I hope that it does not prove necessary to dynamically adjust the threshold based on CPU clock frequency, but you never know. > At the end, we have to revert your clocksource patches before shipping RHEL9 > beta last week. Which has the disadvantage of leaving the initial clock-skew issues, but I do understand that introducing one problem even while fixing another one still counts as a regression. Thanx, Paul