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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,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 E0665C3A5A8 for ; Wed, 4 Sep 2019 14:11:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BB1972339E for ; Wed, 4 Sep 2019 14:11:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1567606278; bh=rFvqx1t57nbc+Hg21OUbKEcYTo2/giLTrk1qEwOXB0A=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=InKLdT6trhj45WIKy7D+aaq2GF9jsQpFojoULrKk4TK7223UVFLcMbn3pYKrvTece 7BNl9NEX+S41UhKyueAAg55+1QwxqqYKGoI6sqArnQVFul2nE4VFsk5VhNbWfG8X7D oPVlGQNYNm+vYyeYHLtSkrV025dVqy7BNWzACA9c= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730538AbfIDOLS (ORCPT ); Wed, 4 Sep 2019 10:11:18 -0400 Received: from mail.kernel.org ([198.145.29.99]:47792 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730576AbfIDOLR (ORCPT ); Wed, 4 Sep 2019 10:11:17 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 95CF322CEA; Wed, 4 Sep 2019 14:11:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1567606276; bh=rFvqx1t57nbc+Hg21OUbKEcYTo2/giLTrk1qEwOXB0A=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=URIUSDFJKgoNmYvjUlR5n4yVeTKuFkwBbb7wGzVnt/h20wF1bZJlain5U2KU1ae9j 4zFKKrp0oJHIJZ48DD6amu1aNZ+R6iAGiqHG5dsg77ltmug3+o0h0+jiDDqTvBNo7u Qnt6hA7EV7pClHf1mKD0KFukCvIRxTZAEY3EwaUg= Date: Wed, 4 Sep 2019 15:11:10 +0100 From: Will Deacon To: Joel Fernandes Cc: Peter Zijlstra , Steven Rostedt , Alessio Balsini , mingo@kernel.org, juri.lelli@redhat.com, linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com, luca.abeni@santannapisa.it, bristot@redhat.com, dvyukov@google.com, tglx@linutronix.de, vpillai@digitalocean.com, kernel-team@android.com, will@kernel.org Subject: Re: [RFC][PATCH 01/13] sched/deadline: Impose global limits on sched_attr::sched_period Message-ID: <20190904141109.4v6o2cxklpdlmldb@willie-the-truck> References: <20190726161357.397880775@infradead.org> <20190802172104.GA134279@google.com> <20190805115309.GJ2349@hirez.programming.kicks-ass.net> <20190822122949.GA245353@google.com> <20190822165125.GW2369@hirez.programming.kicks-ass.net> <20190831144117.GA133727@google.com> <20190902091623.GQ2349@hirez.programming.kicks-ass.net> <20190904061616.25ce79e1@oasis.local.home> <20190904113038.GE2349@hirez.programming.kicks-ass.net> <20190904132418.GA237277@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190904132418.GA237277@google.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Joel, On Wed, Sep 04, 2019 at 09:24:18AM -0400, Joel Fernandes wrote: > On Wed, Sep 04, 2019 at 01:30:38PM +0200, Peter Zijlstra wrote: > > On Wed, Sep 04, 2019 at 06:16:16AM -0400, Steven Rostedt wrote: > > > On Mon, 2 Sep 2019 11:16:23 +0200 > > > Peter Zijlstra wrote: > > > > > > > in sched_dl_period_handler(). And do: > > > > > > > > + preempt_disable(); > > > > max = (u64)READ_ONCE(sysctl_sched_dl_period_max) * NSEC_PER_USEC; > > > > min = (u64)READ_ONCE(sysctl_sched_dl_period_min) * NSEC_PER_USEC; > > > > + preempt_enable(); > > > > > > Hmm, I'm curious. Doesn't the preempt_disable/enable() also add > > > compiler barriers which would remove the need for the READ_ONCE()s here? > > > > They do add compiler barriers; but they do not avoid the compiler > > tearing stuff up. > > Neither does WRITE_ONCE() on some possibly buggy but currently circulating > compilers :( Hmm. The example above is using READ_ONCE, which is a different kettle of frogs. > As Will said in: > https://lore.kernel.org/lkml/20190821103200.kpufwtviqhpbuv2n@willie-the-truck/ > > void bar(u64 *x) > { > *(volatile u64 *)x = 0xabcdef10abcdef10; > } > > gives: > > bar: > mov w1, 61200 > movk w1, 0xabcd, lsl 16 > str w1, [x0] > str w1, [x0, 4] > ret > > Speaking of which, Will, is there a plan to have compiler folks address this > tearing issue and are bugs filed somewhere? I believe aarch64 gcc is buggy, > and clang is better but is still buggy? Well, it depends on your point of view. Personally, I think tearing a volatile access (e.g. WRITE_ONCE) is buggy and it seems as though the GCC developers agree: https://gcc.gnu.org/ml/gcc-patches/2019-08/msg01500.html so it's likely this will be fixed for AArch64 GCC. I couldn't persuade clang to break the volatile case, so think we're good there too. For the non-volatile case, I don't actually consider it to be a bug, although I sympathise with the desire to avoid a retrospective tree-wide sweep adding random WRITE_ONCE invocations to stores that look like they might be concurrent. In other words, I think I'd suggest: * The use of WRITE_ONCE in new code (probably with a comment justifying it) * The introduction of WRITE_ONCE to existing code where it can be shown to be fixing a real bug (e.g. by demonstrating that a compiler actually gets it wrong) For the /vast/ majority of cases, the compiler will do the right thing even without WRITE_ONCE, simply because that's going to be the most performant choice as well. Will