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=-8.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 82E33C433DB for ; Fri, 12 Feb 2021 00:39:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 517ED64E2D for ; Fri, 12 Feb 2021 00:39:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229762AbhBLAjJ (ORCPT ); Thu, 11 Feb 2021 19:39:09 -0500 Received: from mail.kernel.org ([198.145.29.99]:56978 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229521AbhBLAjD (ORCPT ); Thu, 11 Feb 2021 19:39:03 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 695FA61493; Fri, 12 Feb 2021 00:38:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1613090302; bh=WcNPuJnmk5vyURXYl4skxvJr55pfhPAMsl6062FOmFo=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=tArgsQL/B1R0XDiuFVacP9amj2196ZK+/Zc4leJpmnIxSAlti1D2sawqLQEyaH+4+ zaZktbG0OvaoP4+nLqBUWw3m6N5ayZ7e7ECoCRI1GEYRRV8RsckN1x+x8G5nJTagLY T1Hwyf3OrvKKdm0IqrqBL0FvKOXebePa8zRaAzBYTSeZ5MfLfkWC/GJfbQNclkSuz0 fkQn+rsw0hHLUpvAeMPWWgLfzyLRMi4O9lC4j/GbI/Vj/FMRggE7OEwtMeV+e642xP h+STIpxuNZ4BWgiASh0CaiUjtXt1O24YVnD7eUjZJZzYHq6zIdSsi3UG15wSPETRVh gtu26YvVrZVOA== Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 3409D3522694; Thu, 11 Feb 2021 16:38:22 -0800 (PST) Date: Thu, 11 Feb 2021 16:38:22 -0800 From: "Paul E. McKenney" To: Yury Norov Cc: Andy Shevchenko , Paul Gortmaker , Linux Kernel Mailing List , Li Zefan , Ingo Molnar , Thomas Gleixner , Josh Triplett , Peter Zijlstra , Frederic Weisbecker , Rasmus Villemoes Subject: Re: [PATCH v4 0/8] support for bitmap (and hence CPU) list "N" abbreviation Message-ID: <20210212003822.GN2743@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20210209225907.78405-1-paul.gortmaker@windriver.com> <20210210175751.GH2743@paulmck-ThinkPad-P72> <20210211002309.GL2743@paulmck-ThinkPad-P72> <20210212002339.GA167389@yury-ThinkPad> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210212002339.GA167389@yury-ThinkPad> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 11, 2021 at 04:23:39PM -0800, Yury Norov wrote: > On Wed, Feb 10, 2021 at 04:23:09PM -0800, Paul E. McKenney wrote: > > On Wed, Feb 10, 2021 at 03:50:07PM -0800, Yury Norov wrote: > > > On Wed, Feb 10, 2021 at 9:57 AM Paul E. McKenney wrote: > > > > > > > > On Wed, Feb 10, 2021 at 06:26:54PM +0200, Andy Shevchenko wrote: > > > > > On Tue, Feb 09, 2021 at 05:58:59PM -0500, Paul Gortmaker wrote: > > > > > > The basic objective here was to add support for "nohz_full=8-N" and/or > > > > > > "rcu_nocbs="4-N" -- essentially introduce "N" as a portable reference > > > > > > to the last core, evaluated at boot for anything using a CPU list. > > > > > > > > > > I thought we kinda agreed that N is confusing and L is better. > > > > > N to me is equal to 32 on 32 core system as *number of cores / CPUs*. While L > > > > > sounds better as *last available CPU number*. > > > > > > > > The advantage of "N" is that people will automatically recognize it as > > > > "last thing" or number of things" because "N" has long been used in > > > > both senses. In contrast, someone seeing "0-L" for the first time is > > > > likely to go "What???". > > > > > > > > Besides, why would someone interpret "N" as "number of CPUs" when doing > > > > that almost always gets you an invalid CPU number? > > > > > > > > Thanx, Paul > > > > > > I have no strong opinion about a letter, but I like Andy's idea to make it > > > case-insensitive. > > > > > > There is another comment from the previous iteration not addressed so far. > > > > > > This idea of the N notation is to make the bitmap list interface more robust > > > when we share the configs between different machines. What we have now > > > is definitely a good thing, but not completely portable except for cases > > > 'N', '0-N' and 'N-N'. > > > > > > For example, if one user adds rcu_nocbs= '4-N', and it works perfectly fine for > > > him, another user with s NR_CPUS == 2 will fail to boot with such a config. > > > > > > This is not a problem of course in case of absolute values because nobody > > > guaranteed robustness. But this N feature would be barely useful in practice, > > > except for 'N', '0-N' and 'N-N' as I mentioned before, because there's always > > > a chance to end up with a broken config. > > > > > > We can improve on robustness a lot if we take care about this case.For me, > > > the more reliable interface would look like this: > > > 1. chunks without N work as before. > > > 2. if 'a-N' is passed where a>=N, we drop chunk and print warning message > > > 3. if 'a-N' is passed where a>=N together with a control key, we set last bit > > > and print warning. > > > > > > For example, on 2-core CPU: > > > "4-2" --> error > > > "4-4" --> error > > > "4-N" --> drop and warn > > > "X, 4-N" --> set last bit and warn > > > > > > Any comments? > > > > We really don't know the user's intent, and we cannot have complete > > portability without knowing the user's intent. For example, "4-N" means > > "all but the first four CPUs", in which case an error is appropriate > > because "4-N" makes no more sense on a 2-CPU system than does "4-1". > > I could see a potential desire for some notation for "the last two CPUs", > > but let's please have a real need for such a thing before overengineering > > this patch series any further. > > > > To get the level of portability you seem to be looking for, we need some > > higher-level automation that knows how many CPUs there are and what > > the intent is. That automation can then generate the cpumasks for a > > given system. But for more typical situations, what Paul has now will > > work fine. > > > > Paul Gortmaker's patch series is doing something useful. We should > > not let potential future desires prevent us from taking a very useful > > step forward. > > > > Thanx, Paul > > No problem, we can do it later if it will become a real concern. > > Can you please remove this series from linux-next unless we finish > the review? It prevents me from applying the series from the LKML. That will happen shortly, but in the meantime, just do the following on top of -next before applying Paul's latest series: git revert b3c314b ed78166 1e792c4 e831c73 Thanx, Paul