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=-5.4 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 0802EC48BE5 for ; Wed, 16 Jun 2021 13:20:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DE6DC6102A for ; Wed, 16 Jun 2021 13:20:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233068AbhFPNWl (ORCPT ); Wed, 16 Jun 2021 09:22:41 -0400 Received: from foss.arm.com ([217.140.110.172]:37186 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232949AbhFPNWk (ORCPT ); Wed, 16 Jun 2021 09:22:40 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 832C931B; Wed, 16 Jun 2021 06:20:33 -0700 (PDT) Received: from [192.168.178.6] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 174803F719; Wed, 16 Jun 2021 06:20:31 -0700 (PDT) Subject: Re: sched: Question about big and little cores system with SMP and EAS To: Peter Zijlstra , Ley Foon Tan Cc: Vincent Guittot , Ingo Molnar , Greg KH , linux-kernel@vger.kernel.org, Quentin Perret References: From: Dietmar Eggemann Message-ID: Date: Wed, 16 Jun 2021 15:20:30 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org - Quentin Perret + Quentin Perret On 16/06/2021 13:39, Peter Zijlstra wrote: > On Wed, Jun 16, 2021 at 07:29:26PM +0800, Ley Foon Tan wrote: >> Hi all >> >> Would like to ask the experts here regarding the Symmetric >> Multi-Processing mode (SMP) with Energy aware scheduler (EAS) support >> on the big + little cores system. > > And the you ask a question unrelated to either Symmetric MP or EAS :-) > >> Hardware system: >> Big and little cores have almost the same ISA, but the big core has >> some extension instructions that little core doesn't have. > > That problem is unrelated to big.Little / EAS, also by definition that > is not SMP seeing how the 'S' is a blatant lie. > > The simplest solution is to simply disallow usage of the extended ISA > and force mandate the common subset. The complicated answer is something > along the lines of: > > https://lkml.kernel.org/r/20210608180313.11502-1-will@kernel.org We don't encourage asymmetric ISA extensions for EAS*/CAS** on big.Little systems. It would be simply a nightmare to schedule tasks on such systems. The exception to this is the 'asymmetric 32-bit Soc' to support 32bit legacy Apps. The nightmare for scheduling is reduced in this case to CPU affinity, something the task scheduler has to live with already today. (+ DL admission control for 32bit tasks). * Documentation/scheduler/sched-energy.rst ** Documentation/scheduler/sched-capacity.rst