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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 1CFD1C433FF for ; Fri, 2 Aug 2019 09:50:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EB8F620679 for ; Fri, 2 Aug 2019 09:50:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2405875AbfHBJuo (ORCPT ); Fri, 2 Aug 2019 05:50:44 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:39175 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2404933AbfHBJuf (ORCPT ); Fri, 2 Aug 2019 05:50:35 -0400 Received: from [5.158.153.52] (helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1htUCg-0001sw-HD; Fri, 02 Aug 2019 11:50:26 +0200 Date: Fri, 2 Aug 2019 11:50:20 +0200 (CEST) From: Thomas Gleixner To: Peter Zijlstra cc: Qais Yousef , mingo@kernel.org, linux-kernel@vger.kernel.org, Geert Uytterhoeven Subject: Re: [PATCH 0/5] Fix FIFO-99 abuse In-Reply-To: <20190802093244.GF2332@hirez.programming.kicks-ass.net> Message-ID: References: <20190801111348.530242235@infradead.org> <20190801131707.5rpyydznnhz474la@e107158-lin.cambridge.arm.com> <20190802093244.GF2332@hirez.programming.kicks-ass.net> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2 Aug 2019, Peter Zijlstra wrote: > On Thu, Aug 01, 2019 at 02:17:07PM +0100, Qais Yousef wrote: > > On 08/01/19 13:13, Peter Zijlstra wrote: > > > I noticed a bunch of kthreads defaulted to FIFO-99, fix them. > > > > > > The generic default is FIFO-50, the admin will have to configure the system > > > anyway. > > > > > > For some the purpose is to be above OTHER and then FIFO-1 really is sufficient. > > > > I was looking in this area too and was thinking of a way to consolidate the > > creation of RT/DL tasks in the kernel and the way we set the priority. > > > > Does it make sense to create a new header for RT priorities for kthreads > > created in the kernel so that we can easily track and rationale about the > > relative priorities of in-kernel RT tasks? > > > > When working in the FW world such a header helped a lot in understanding what > > runs at each priority level and how to reason about what priority level makes > > sense for a new item. It could be a nice single point of reference; even for > > admins. > > Well, SCHED_FIFO is a broken scheduler model; that is, it is > fundamentally incapable of resource management, which is the one thing > an OS really should be doing. > > This is of course the reason it is limited to privileged users only. > > Worse still; it is fundamentally impossible to compose static priority > workloads. You cannot take two correctly working static prio workloads > and smash them together and still expect them to work. > > For this reason 'all' FIFO tasks the kernel creates are basically at: > > MAX_RT_PRIO / 2 > > The administrator _MUST_ configure the system, the kernel simply doesn't > know enough information to make a sensible choice. > > Now, Geert suggested so make make a define for that, but how about we do > something like: > > /* > * ${the above explanation} > */ > int kernel_setscheduler_fifo(struct task_struct *p) > { > struct sched_param sp = { .sched_priority = MAX_RT_PRIO / 2 }; > return sched_setscheduler_nocheck(p, SCHED_FIFO, &sp); > } > > And then take away sched_setscheduler*(). Yes, please. tglx