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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9B494C433FE for ; Thu, 14 Apr 2022 22:37:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347058AbiDNWja (ORCPT ); Thu, 14 Apr 2022 18:39:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36278 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232386AbiDNWj2 (ORCPT ); Thu, 14 Apr 2022 18:39:28 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A0BDEC6B54; Thu, 14 Apr 2022 15:37:02 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 3D66B620B1; Thu, 14 Apr 2022 22:37:02 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 87F1DC385A1; Thu, 14 Apr 2022 22:37:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1649975821; bh=nZQEIAbV5ImpOLfoFMOS/d41QAE77juoAmcn5Yjj7tA=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=B/Hf6TyRY7JsgqhNQ3nTtIGf3lGpUXrfXJW678cB0qzeQLDuY9H2g2EcImCya6n3R hGHP4XHgAb4/CLP9O1kJDkKunKQlBlsu2Rdh/aRs/zDUwD1ZDrnFkqHHEj9g2q7ls7 jFGvG6p106PbNw76zvltWV9HNgCgGg3leAcMof7J7tFGXSLijjmsNPPT/bIXmeWKbU aUoVz+eG27Zel9GOyAdtTi60sxVGkQ9pwgNO5zDPHPSqPmP3eWFQ8MGS8jRfgskDaS X/LJUcDIoJFtG/PcYd5IPFbBQH28crgO+fe0eZVdoBXROd+7ZoS8lC4dsXQYMLSEvT 3dHw8s92/hbxQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 158315C013A; Thu, 14 Apr 2022 15:37:01 -0700 (PDT) Date: Thu, 14 Apr 2022 15:37:01 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: LKML , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , rcu , Steven Rostedt , Frederic Weisbecker Subject: Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB_CPU=y Message-ID: <20220414223701.GD4285@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220408205440.GL4285@paulmck-ThinkPad-P17-Gen-1> <20220411154109.GX4285@paulmck-ThinkPad-P17-Gen-1> <20220414194204.GU4285@paulmck-ThinkPad-P17-Gen-1> <20220414210933.GW4285@paulmck-ThinkPad-P17-Gen-1> <20220414213156.GZ4285@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, Apr 14, 2022 at 05:38:08PM -0400, Joel Fernandes wrote: > On Thu, Apr 14, 2022 at 5:31 PM Paul E. McKenney wrote: > > > > On Thu, Apr 14, 2022 at 09:14:05PM +0000, Joel Fernandes wrote: > > > On Thu, Apr 14, 2022 at 02:09:33PM -0700, Paul E. McKenney wrote: > > > > On Thu, Apr 14, 2022 at 03:49:16PM -0400, Joel Fernandes wrote: > > > > > On Thu, Apr 14, 2022 at 3:42 PM Paul E. McKenney wrote: > > > > > > > > > > > > On Thu, Apr 14, 2022 at 07:19:48PM +0000, Joel Fernandes wrote: > > > > > > > On Mon, Apr 11, 2022 at 08:41:09AM -0700, Paul E. McKenney wrote: > > > > > > > [..] > > > > > > > > > > [4] All CPUs are offloaded at boot, and any CPU can be de-offloaded > > > > > > > > > > and offloaded at runtime. This is the same behavior that > > > > > > > > > > you would currently get with CONFIG_RCU_NOCB_CPU_ALL=n and > > > > > > > > > > rcu_nocbs=0-N. > > > > > > > > > > > > > > > > > > Yes, this is the behavior I intend. So then there would not be a need > > > > > > > > > to pass a mask (and I suspect for a large number of users, it > > > > > > > > > simplifies boot params). > > > > > > > > > > > > > > > > Very good, and from what I can see, this should work for everyone. > > > > > > > > > > > > > > Just to clarify, what I am going to do is, if this new option =y, then > > > > > > > rcu_nocbs effectively wont do anything. i.e. All CPUs are offloaded at boot. > > > > > > > Let me know if we are not on the same page about it though. I do feel that is > > > > > > > a sensible choice given =y. If we are on same page, please ignore my comment. > > > > > > > > > > > > I was assuming that the rcu_nocbs=??? for non-empty "???" would override > > > > > > the CONFIG_RCU_NOCB_CPU_ALL=y. If you choose not to do that, shouldn't > > > > > > you at least issue some sort of diagnostic? After all, the sysadmin > > > > > > gave a kernel-boot parameter asking the code to do something and the > > > > > > code is choosing not to do that something. > > > > > > > > > > > > Of course, such a sysadmin might want the CONFIG_RCU_NOCB_CPU_ALL=y > > > > > > Kconfig option to affect only the default, that is, when no rcu_nocbs > > > > > > kernel boot parameter is specified. This would change the second "[4]" > > > > > > in my original table to "[2]". > > > > > > > > > > > > Thoughts? > > > > > > > > > > I thought about that. I feel that since we are defaulting the new > > > > > config option to =n , it is a conscious choice by the distro to set it > > > > > to =y. In such a case, they should be Ok with offloading all CPUs. If > > > > > they decide to selectively offload some CPUs in the future, then they > > > > > could revisit the config option at that time. > > > > > > > > > > I feel the kernel config should override the boot parameter behavior. > > > > > It is the same effect as a sysadmin passing kernel parameter X > > > > > assuming the kernel does something but the CONFIG option might not > > > > > even build code corresponding to X. > > > > > > > > > > I feel to address your concern, we can document in kernel command line > > > > > documentation that rcu_nocbs= does not have an effect if > > > > > CONFIG_RCU_NOCB_CPU_ALL=y, would that work for you? > > > > > > > > Not me so much, because I would just set CONFIG_RCU_NOCB_CPU_ALL=n so > > > > as to not worry about it. > > > > > > > > But I am not at all looking forward to complaints about rcu_nocbs not > > > > working the way people expect. So let's take some time to think more > > > > carefully about this. > > > > > > That's a fair concern. But we are defaulting it to 'n' so I think if it is > > > unconsciously enabled without someone reading documentation, then that's a > > > slightly different issue. > > > > Suppose that one group decides to change to CONFIG_RCU_NOCB_CPU_ALL=y, > > and some other group on some other continent happens to be using the > > rcu_nocbs boot parameter (having read the documentation). And suppose > > that the level of communication between the two groups is typical, > > that is to say, nonexistent. > > > > Sure, we can argue that groups should communicate, but our making that > > argument won't necessarily prevent the group using rcu_nocbs from taking > > us to task in the course of their debugging effort. > > > > > On the other hand, I can also make it such that if rcu_nocbs= is passed, then > > > the CONFIG does not take effect. That's quite a bit weird/quirky IMHO. > > > > Not at all. We can simply say that CONFIG_RCU_NOCB_CPU_ALL controls > > only the default situation, that is, when rcu_nocbs is not specified. > > Then it should be called: CONFIG_RCU_NOCB_CPU_DEFAULT_ALL , or > something. Otherwise I can tell you that I will be the first one to be > confused by menuconfig unless I also read the code :-D I am OK with CONFIG_RCU_NOCB_CPU_DEFAULT_ALL. Thanx, Paul