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 61833C433F5 for ; Thu, 14 Apr 2022 21:09:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1346794AbiDNVMF (ORCPT ); Thu, 14 Apr 2022 17:12:05 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35404 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234903AbiDNVMD (ORCPT ); Thu, 14 Apr 2022 17:12:03 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3967528E2C; Thu, 14 Apr 2022 14:09:36 -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 ams.source.kernel.org (Postfix) with ESMTPS id DAA3BB818EB; Thu, 14 Apr 2022 21:09:34 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 97246C385A5; Thu, 14 Apr 2022 21:09:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1649970573; bh=K4e1NxHpGoyDCMiTXGChbZesTBHyq3kIDMII4WvrDyQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=TsnrUSm/DSRoSCVhBXkyNWwkbSDQ2YoEF3Ad/ZL48tqEOTNT66o0V5Rgl9Tyfr/pO kIRNb4WaE3CrnYsSuISKKMpm5hKyEKAEHGOGQQk5hgyPeuQyra14W4wyOUA7M7Y/8T S53v6sPHjjMwCXJrP5bIhSYACO0nlkfnKwjGD9pVDCp94I+ZF9Kbi48ovle2TMwBtX lTAErt+nam9fhpGz8unU5lUoqVPh+qrEFFvfV/oB8FS+hMw0AFlp78dgHrubFxmKur YbtdqSkAqxUvYTfwVMpnzbW5ii7qNzV4aTVU11f7hpdr0HeBdHcpgJNTNfN5PPTYL9 Xmu/p4uuigviw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 31C615C03A5; Thu, 14 Apr 2022 14:09:33 -0700 (PDT) Date: Thu, 14 Apr 2022 14:09:33 -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: <20220414210933.GW4285@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220408174908.GK4285@paulmck-ThinkPad-P17-Gen-1> <20220408205440.GL4285@paulmck-ThinkPad-P17-Gen-1> <20220411154109.GX4285@paulmck-ThinkPad-P17-Gen-1> <20220414194204.GU4285@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 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. Thanx, Paul > Thanks, > > - Joel > > > > > > > > > I believe that Steve Rostedt's review would carry weight for ChromeOS, > > > > > > however, I am suffering a senior moment on the right person for Android. > > > > > > > > > > I think for Android, Kalesh Singh is in the kernel team and Tim Murray > > > > > is the performance lead. They could appropriately represent their RCU > > > > > needs. > > > > > > > > Sounds good! Please collect a Reviewed-by from one or both of them. > > > > > > Ok. > > > > Thanx, Paul