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=-5.7 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 535B9C56201 for ; Thu, 19 Nov 2020 18:29:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CA07B241A6 for ; Thu, 19 Nov 2020 18:29:32 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="QHiwHIIe" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729144AbgKSS3L (ORCPT ); Thu, 19 Nov 2020 13:29:11 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44572 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727869AbgKSS3L (ORCPT ); Thu, 19 Nov 2020 13:29:11 -0500 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 06BB6C0613CF for ; Thu, 19 Nov 2020 10:29:11 -0800 (PST) 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=4LIkeskhd935eUwqijtsoAM0pfoT4xJqUA6L3kxJuH8=; b=QHiwHIIeWOa4GXy2EfG3PvLNfv El4CyX0m4VRDLmmHgs/MqsjjtO1m76j94PU7c8Roomj2tZJPPIq+kD5nM78g6OBFOE7iYRAjt9jQ2 mWUEXSOPUKVSwO0bWj7RnXzIzrtR80Edx0Wx1oMc8OdT4hPo6Eqwh1nfCP7QwDF34ZyF0hp24olZW kvSUIBxwEFDXRusQdYPgqZbsPzEjJqVmmh07Ybjrtp7UuqVAcu5CWzTZv9WKH+3pYu+RoopPDWn70 lu95GTGwxvNIVK31Qi/DPM8usTa0FvB2TTjPBTMj2ABok+EBoCk3b5EcRwpqBd2I/slfTs7/yShUj 5zknQSMQ==; 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 1kfofo-0008Bj-6o; Thu, 19 Nov 2020 18:28:48 +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 B03FC3060AE; Thu, 19 Nov 2020 19:28:43 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 8E27E20234C36; Thu, 19 Nov 2020 19:28:43 +0100 (CET) Date: Thu, 19 Nov 2020 19:28:43 +0100 From: Peter Zijlstra To: Linus Torvalds Cc: Mel Gorman , Thomas Gleixner , LKML , the arch/x86 maintainers , Christoph Hellwig , Matthew Wilcox , Daniel Vetter , Andrew Morton , Linux-MM , Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Daniel Bristot de Oliveira Subject: Re: [patch V4 4/8] sched: Make migrate_disable/enable() independent of RT Message-ID: <20201119182843.GA2414@hirez.programming.kicks-ass.net> References: <20201118194838.753436396@linutronix.de> <20201118204007.269943012@linutronix.de> <20201119093834.GH3306@suse.de> <20201119111411.GL3121378@hirez.programming.kicks-ass.net> 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-kernel@vger.kernel.org On Thu, Nov 19, 2020 at 09:23:47AM -0800, Linus Torvalds wrote: > On Thu, Nov 19, 2020 at 3:14 AM Peter Zijlstra wrote: > > > > I still hate all of this, and I really fear that with migrate_disable() > > available, people will be lazy and usage will increase :/ > > > > Case at hand is this series, the only reason we need it here is because > > per-cpu page-tables are expensive... > > No, I think you as a scheduler person just need to accept it. Well, I did do write the patches. > Because this is certainly not the only time migration limiting has > come up, and no, it has absolutely nothing to do with per-cpu page > tables being completely unacceptable. It is for this instance; but sure, it's come up before in other contexts. > The scheduler people need to get used to this. Really. Because ASMP is > just going to be a fact. ASMP is different in that it is a hardware constraint, you're just not going to be able to run more of X than there's X capable hardware units on (be if FPUs, Vector units, 32bit or whatever) > There are few things more futile than railing against reality, Peter. But, but, my windmills! :-) > Honestly, the only argument I've ever heard against limiting migration > is the whole "our scheduling theory doesn't cover it". > > So either throw the broken theory away, or live with it. Theory that > doesn't match reality isn't theory, it's religion. The first stage of throwing it out is understanding the problem, which is just about where we're at. Next is creating a new formalism (if possible) that covers this new issue. That might take a while. Thing is though; without a formalism to reason about timeliness guarantees, there is no Real-Time. So sure, I've written the patches, doesn't mean I have to like the place we're in due to it.