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=-7.9 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 C3B4CC433B4 for ; Thu, 20 May 2021 18:01:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A7F716100A for ; Thu, 20 May 2021 18:01:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236564AbhETSDI (ORCPT ); Thu, 20 May 2021 14:03:08 -0400 Received: from mail.kernel.org ([198.145.29.99]:54602 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235935AbhETSDH (ORCPT ); Thu, 20 May 2021 14:03:07 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 000336101D; Thu, 20 May 2021 18:01:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1621533705; bh=VuH0uDCYxYFobOO1melI2DB00iQVhYlPjm02AAHgSXw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=V5pc1Px8rzn8IhwbdUoSYWWWejC1TgCV17xBQCMOLjXEBZEjlMswG0AI53R8TGe0X ziQotYU6LyAPJ1/oBDH7ifo7/RLwT888UuYvlm+466ewL4txm3yT2R2mlwJNNaNP+s pIr5i6APPSQqxaDzIdJ7kuJuwmlENdcwzHN95MSbJahJPoHxpL1tWt57gzvpnBCoTx 23TZChJjXt/Vd3Mg38fYBq5UFA4yBGqJ71GeAX0DV56PVHsgibamPPt7EeiCVRtrwy RlhQXPMc/n1D+1dtPwV3j0/8ST8rCHltHJaZAPVYhx288Wy54PuB8H8j/lNYuutzm1 gWiPFeRQbu0BQ== Date: Thu, 20 May 2021 19:01:39 +0100 From: Will Deacon To: Daniel Bristot de Oliveira Cc: Quentin Perret , Juri Lelli , linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Catalin Marinas , Marc Zyngier , Greg Kroah-Hartman , Peter Zijlstra , Morten Rasmussen , Qais Yousef , Suren Baghdasaryan , Tejun Heo , Johannes Weiner , Ingo Molnar , Vincent Guittot , "Rafael J. Wysocki" , kernel-team@android.com Subject: Re: [PATCH v6 13/21] sched: Admit forcefully-affined tasks into SCHED_DEADLINE Message-ID: <20210520180138.GA10523@willie-the-truck> References: <20210518094725.7701-14-will@kernel.org> <20210518102833.GA7770@willie-the-truck> <20210518105951.GC7770@willie-the-truck> <20210520101640.GA10065@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 20, 2021 at 02:38:55PM +0200, Daniel Bristot de Oliveira wrote: > On 5/20/21 12:33 PM, Quentin Perret wrote: > > On Thursday 20 May 2021 at 11:16:41 (+0100), Will Deacon wrote: > >> Ok, thanks for the insight. In which case, I'll go with what we discussed: > >> require admission control to be disabled for sched_setattr() but allow > >> execve() to a 32-bit task from a 64-bit deadline task with a warning (this > >> is probably similar to CPU hotplug?). > > > > Still not sure that we can let execve go through ... It will break AC > > all the same, so it should probably fail as well if AC is on IMO > > > > If the cpumask of the 32-bit task is != of the 64-bit task that is executing it, > the admission control needs to be re-executed, and it could fail. So I see this > operation equivalent to sched_setaffinity(). This will likely be true for future > schedulers that will allow arbitrary affinities (AC should run on affinity > change, and could fail). > > I would vote with Juri: "I'd go with fail hard if AC is on, let it > pass if AC is off (supposedly the user knows what to do)," (also hope nobody > complains until we add better support for affinity, and use this as a motivation > to get back on this front). I can have a go at implementing it, but I don't think it's a great solution and here's why: Failing an execve() is _very_ likely to be fatal to the application. It's also very likely that the task calling execve() doesn't know whether the program it's trying to execute is 32-bit or not. Consequently, if we go with failing execve() then all that will happen is that people will disable admission control altogether. That has a negative impact on "pure" 64-bit applications and so I think we end up with the tail wagging the dog because admission control will be disabled for everybody just because there is a handful of 32-bit programs which may get executed. I understand that it also means that RT throttling would be disabled. Allowing the execve() to continue with a warning is very similar to the case in which all the 64-bit CPUs are hot-unplugged at the point of execve(), and this is much closer to the illusion that this patch series intends to provide. So, personally speaking, I would prefer the behaviour where we refuse to admit 32-bit tasks vioa sched_set_attr() if the root domain contains 64-bit CPUs, but we _don't_ fail execve() of a 32-bit program from a 64-bit deadline task. However, you're the deadline experts so ultimately I'll implement what you prefer. I just wanted to explain why I think it's a poor interface. Have I changed anybody's mind? Will