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=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 1EC14C388F9 for ; Mon, 26 Oct 2020 17:12:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BA4D82070E for ; Mon, 26 Oct 2020 17:12:29 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="KMYHIio0" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1787377AbgJZRM3 (ORCPT ); Mon, 26 Oct 2020 13:12:29 -0400 Received: from merlin.infradead.org ([205.233.59.134]:34504 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1787215AbgJZRM3 (ORCPT ); Mon, 26 Oct 2020 13:12:29 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=6BL74GG/GU7T+OZHWA4ZwCz98Vw3V+JJr/MYkeOc1pw=; b=KMYHIio0bD27ubImFg/uC09LdZ ThlxstJV1PpLhIweeWjkIbu6I85HAlUOTHo0lBd8V8wR5woEWEB9hWGnGv+bnSh/UmMB23PNAQUwp PvgxPDYtlI8CU5il6r3tJrozpU5+/odZQK5/APHnUzyrxL/m9H5RRrqSsbnpfUtcvO9L040izpkT7 5XoPdYkYQ5rVmodTu2F94mjEYaJFWLmHLpTD+ZSr0tZA6nHIFNsf1MVrm+QnvRu6RlK6XA6hSi1C5 QIfWyndmIdnnS68XgLd6PvV1T6bW3hyiS5CKJD4eB/tXJetGqEadYX2o36sRVAZVtID702biRTGhR 2wTYyCDg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kX62j-0000OZ-Hs; Mon, 26 Oct 2020 17:12:25 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 39F873006D0; Mon, 26 Oct 2020 18:12:24 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 2C5E02BAA7DD6; Mon, 26 Oct 2020 18:12:24 +0100 (CET) Date: Mon, 26 Oct 2020 18:12:24 +0100 From: Peter Zijlstra To: psodagud@codeaurora.org Cc: Thomas Gleixner , Elliot Berman , "Paul E. McKenney" , Jonathan Corbet , Trilok Soni , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org Subject: Re: [PATCH] smp: Add bootcpus parameter to boot subset of CPUs Message-ID: <20201026171224.GV2611@hirez.programming.kicks-ass.net> References: <1603404243-5536-1-git-send-email-eberman@codeaurora.org> <87v9f04n8r.fsf@nanos.tec.linutronix.de> 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-doc@vger.kernel.org On Mon, Oct 26, 2020 at 10:08:47AM -0700, psodagud@codeaurora.org wrote: > On 2020-10-23 14:59, Thomas Gleixner wrote: > > On Thu, Oct 22 2020 at 15:04, Elliot Berman wrote: > > > In a heterogeneous multiprocessor system, specifying the 'maxcpus' > > > parameter on kernel command line does not provide sufficient control > > > over which CPUs are brought online at kernel boot time, since CPUs may > > > have nonuniform performance characteristics. Thus, add bootcpus kernel > > > parameter to control which CPUs should be brought online during kernel > > > boot. When both maxcpus and bootcpus is set, the more restrictive of > > > the > > > two are booted. > > > > What for? 'maxcpus' is a debug hack at best and outright dangerous on > > certain architectures. Why do we need more of that? Just let the machine > > boot and offline the CPUs from user space. > > Hi Thomas and Peter, > > Based on my understanding with maxcpus option provides, maximum no of CPUs > are brough up during the device boot up. There is a different case, in which > we want to restrict which CPUs to be brough up. > On a system with 8 cpus, if we set maxcpus as 3, cpu0, cpu1, and cpu2 are > brough up during the bootup. For example, if we want to bring core0, core3 > and core4 current maxcpu(as 3) setting would not help us. > On some platform we want the flexibility on which CPUs to bring up during > the device bootup. bootcpus command line is helping to bring specific CPUs > and these patches are working downstream. That's a lot of words, but exactly 0 on _WHY_ you would want to do that.