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 1CF0DC4167B for ; Tue, 28 Nov 2023 18:02:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345173AbjK1SCn (ORCPT ); Tue, 28 Nov 2023 13:02:43 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35584 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229921AbjK1SCn (ORCPT ); Tue, 28 Nov 2023 13:02:43 -0500 Received: from gentwo.org (gentwo.org [IPv6:2a02:4780:10:3cd9::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F39F191 for ; Tue, 28 Nov 2023 10:02:45 -0800 (PST) Received: by gentwo.org (Postfix, from userid 1003) id 5DF5F46624; Tue, 28 Nov 2023 10:02:45 -0800 (PST) Received: from localhost (localhost [127.0.0.1]) by gentwo.org (Postfix) with ESMTP id 5CC1341AEC; Tue, 28 Nov 2023 10:02:45 -0800 (PST) Date: Tue, 28 Nov 2023 10:02:45 -0800 (PST) From: "Christoph Lameter (Ampere)" To: Anshuman Khandual cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Valentin.Schneider@arm.com, Vanshidhar Konda , Jonathan Cameron , Catalin Marinas , Robin Murphy , Dave Kleikamp , Matteo Carlini Subject: Re: [PATCH ARM64]: Introduce CONFIG_MAXSMP to allow up to 512 cpus In-Reply-To: <52b8a7d8-aa4c-46db-a7f5-f36c92ad5380@arm.com> Message-ID: <01d7531b-3d5c-989d-58ac-60861249fd3e@linux.com> References: <6a854175-5f89-c754-17b8-deda18447f1f@gentwo.org> <52b8a7d8-aa4c-46db-a7f5-f36c92ad5380@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 28 Nov 2023, Anshuman Khandual wrote: > > > On 11/21/23 06:34, Christoph Lameter (Ampere) wrote: >> Ampere Computing develops high end ARM processors that support an ever >> increasing number of processors. The current default of 256 processors is >> not enough for our newer products. The default is used by Linux >> distros and therefore our customers cannot use distro kernels because >> the number of processors is not supported. > > In the previous thread mentioned below, Catalin had mentioned that the > distros do tweak the config for their needs. The default is applicable > for an wide range systems, hence just wondering why default NR_CPUS be > changed for all. We would like the standard kernel to be able to boot on our systems and those have more than the current NR_CPU processors. The distros only tweaks things on request and with this change the tweaking is no longer necessary. > Also just curious, what might be the concern for distros to have large > platform specific configs overriding the default. There are numerous distributions as well as individuals who built kernels. It is surprising if someone builds an upstream kernel with the defaults that should fit all supported platforms only to find that only a portion of their cpus come up. The work of discovery why this is and how to fix it has to be done by numerous individuals and organizations in order to enable all cpus. That work is not necessary if the default is such that a sufficient number of processors are supported by the default configuration accommodating all ARM hardware. The CONFIG_MAXSMP configuration on X86 was developed exactly for these situations and we have a special KCONFIG option to have potentially large bitmaps for cpus allocated as needed in the kernel core. The patch enables the use of that facility.