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=-2.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,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 BE227C433E0 for ; Fri, 12 Feb 2021 00:24:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 111B164DFF for ; Fri, 12 Feb 2021 00:24:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230334AbhBLAYZ (ORCPT ); Thu, 11 Feb 2021 19:24:25 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54738 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229895AbhBLAYX (ORCPT ); Thu, 11 Feb 2021 19:24:23 -0500 Received: from mail-qv1-xf2f.google.com (mail-qv1-xf2f.google.com [IPv6:2607:f8b0:4864:20::f2f]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 02D15C061786 for ; Thu, 11 Feb 2021 16:23:43 -0800 (PST) Received: by mail-qv1-xf2f.google.com with SMTP id e9so2915867qvy.3 for ; Thu, 11 Feb 2021 16:23:42 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=KqTLTsbdrTGVIghcDkYt5epBaafbNevmlo91ukcKzxU=; b=Yy799SB63qjaBhZielaGtbPOh1rjmIDChZi+LYiZcpZXC0aWJ2lyTjZgdnzAL7J67p taBk6UjxPq6LLWQHYxum9OxangrtJmpXgyizwIGQwIznHoLWmFr68oHvBzDFgwiXB5Th ZjOlajrj0t23Ll4Z7WBLQkhLyx0JUd/L2efkN7tV5gxMAK8GwdqayK5xx5uMa/resQi+ CR23SI3jq6R8mg+n/vRxOaOY4a8Qvzme3WB7ZlZJ3GeZI7n3Jw+0PVX0zLOsPURV+TGV yUFyp0ldTVBKWjBVJW1CsZJ+3IBd7YvBZYaiIuJaFtczo439S1SfzapFimEMwhzP1Z83 i5XQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=KqTLTsbdrTGVIghcDkYt5epBaafbNevmlo91ukcKzxU=; b=lLlRWBvrrkIKUsqz6U5eqIOkNWdkw8NKuODJVg255NsALKYSg87DxQNOw0TDDHWXO1 oWptkGJwhHFPoMFgG48So4y0KFevdoNNGutNfvSRoPD8FjwGHVswvki+MM/dgf++NS/2 rfH2lWsAA9IdRUUmzTYApChR/0xWBtMJWhLm8n4BZJm7vAyUDCbJCMApOjGuXGqNEm+K N20tzFYtBMwauDAerR+ZFA3iNMAsmz01HAh9ICR7vcyIt0wcD/9IfwszwIH+pxGUI5du rRHD96k19q0JCIHfL35qWK361hnm6wOL7ziQ6NrcweHZF1y9MTzrU3t5ylLLWKx1IQ6z KLRw== X-Gm-Message-State: AOAM530daFTiBlre0R4Y74i5EA7gf01kPL/vp5xJg4JT5TnzxEZhyTMn N06ah7x71jKM2wzifPv0Xfw= X-Google-Smtp-Source: ABdhPJzcLgjAf93023wGBitvdrQdwwuDuSDRJmJa2+CYYFABkHRyorcPUsix9ADjCiP4/ZjetCXDvQ== X-Received: by 2002:a0c:fd64:: with SMTP id k4mr489415qvs.3.1613089421392; Thu, 11 Feb 2021 16:23:41 -0800 (PST) Received: from localhost ([24.214.13.233]) by smtp.gmail.com with ESMTPSA id i6sm4634753qti.30.2021.02.11.16.23.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 11 Feb 2021 16:23:40 -0800 (PST) Date: Thu, 11 Feb 2021 16:23:39 -0800 From: Yury Norov To: "Paul E. McKenney" 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: <20210212002339.GA167389@yury-ThinkPad> References: <20210209225907.78405-1-paul.gortmaker@windriver.com> <20210210175751.GH2743@paulmck-ThinkPad-P72> <20210211002309.GL2743@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210211002309.GL2743@paulmck-ThinkPad-P72> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Yury