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=-6.0 required=3.0 tests=BAYES_00,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 6D56EC433F5 for ; Thu, 16 Sep 2021 17:42:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5666C60C40 for ; Thu, 16 Sep 2021 17:42:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349705AbhIPRn2 (ORCPT ); Thu, 16 Sep 2021 13:43:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:57062 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1355630AbhIPRlx (ORCPT ); Thu, 16 Sep 2021 13:41:53 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id CF70361A89; Thu, 16 Sep 2021 16:53:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1631811204; bh=44P1ANeIAD9swC18fBnaZOjVI5kO3dW9XWUbhixKd10=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=OFlD1ELi/aKDiS+3aLndNJJb7vlkstd0PA+w+ndcU2iUgoyqvR31Ii/T8ooimo/Iy 5BMjedmvQiepbsxnpVlPavnluEyt6yABTvWdWYQROLOijC/xsKGt6A5NEfwc6PVyeo 2k2jcO/aSExftl32ZMZJUv8BRAAbaLBcTD8PALpJq+NjlvMu+Sym+8Y7Zbe572AZBI Mbb/lUa7x+/gtvDDjchIrWVv9Jgno4mXMRskQQOidloE+XiaZvHCneY3vL6PdShZsw 1EnXPOWBGxSkGsOwHU92oIiEDRhenIeWKjMpUsgIJepDHg32QGotIweM49IcW5CGRG Iwmn5wUH26IIw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id A194C5C02AB; Thu, 16 Sep 2021 09:53:24 -0700 (PDT) Date: Thu, 16 Sep 2021 09:53:24 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, Juri Lelli Subject: Re: [PATCH rcu 11/14] rcu: Make rcu_normal_after_boot writable again Message-ID: <20210916165324.GG4156@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210915233305.GA3906641@paulmck-ThinkPad-P17-Gen-1> <20210915233343.3906738-11-paulmck@kernel.org> <20210916135712.GB4156@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: rcu@vger.kernel.org On Thu, Sep 16, 2021 at 04:27:15PM +0200, Peter Zijlstra wrote: > On Thu, Sep 16, 2021 at 06:57:12AM -0700, Paul E. McKenney wrote: > > RT systems they avoid expedited grace periods by booting with either > > the rcupdate.rcu_normal or the rcupdate.rcu_normal_after_boot kernel > > boot parameters. And here is the definition for the latter: > > > > static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT); > > > > In other words, RT systems shut off expedited grace periods by default, > > and are thus free to use nohz_full CPU or not, as they choose. When using > > nohz_full, they can also enable expedited grace periods by booting with > > rcupdate.rcu_normal_after_boot=0. Or not, sysadm's choice. > > > > So I am not seeing a problem here. What am I missing? > > That wasn't at all clear to me from the Changelog. I thought it was > enabling expedited crud for RT. This paragraph of the current commit log covers that point: Make rcupdate.rcu_normal_after_boot= again writeable on RT (if NO_HZ_ FULL is defined), but keep its default value to 1 (enabled) to avoid regressions. Users who need synchronize_rcu_expedited() will boot with rcupdate.rcu_normal_after_ boot=0 in the kernel cmdline. Does that cover it from your viewpoint? If not, any suggested changes for clarification? Thanx, Paul