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=-7.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_PASS, T_DKIMWL_WL_HIGH,USER_AGENT_MUTT autolearn=ham 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 87F5FC04AAB for ; Mon, 6 May 2019 15:16:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5E4D42087F for ; Mon, 6 May 2019 15:16:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1557155783; bh=PAUvR3Wb9ZgDXvvdn3Knt6Ky3jXq/dkf1xIHL5qQzN0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=FFf/7sDGnqFOUIqKuxYtju5rusemWvsrf/NoDsj2A0nCSiTx8IZBhuSJUK8/6Cl/Q 2WfXu3WUwvgBRfrXmiktSEEDrudVT3FOyyCIJxm5MZTAg6BbQC/YPtu7Ea+qChk5S0 Yd9E9n5g5w0ko2aqUsqMUIh9QOdxcunm9230LHAM= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726708AbfEFPQW (ORCPT ); Mon, 6 May 2019 11:16:22 -0400 Received: from mail.kernel.org ([198.145.29.99]:43672 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726653AbfEFPQU (ORCPT ); Mon, 6 May 2019 11:16:20 -0400 Received: from localhost (lfbn-1-18355-218.w90-101.abo.wanadoo.fr [90.101.143.218]) (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 AFC7D2053B; Mon, 6 May 2019 15:16:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1557155780; bh=PAUvR3Wb9ZgDXvvdn3Knt6Ky3jXq/dkf1xIHL5qQzN0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=07Mu5sKBKQ89GPgcRr/fFMGvEhchgNlu0LlC1/YEkesH/uGsoYB4XEwMkBjGeM5CA ymfGWjx6/Ng9dkUCt6ZCoeDuh7copZPALyal5AwpZSj88CS5qLXkDvK3gg5aIBONOv E64CAT5DxaeaN8DoefCPiCkiz7u7VD9HJuJUph6E= Date: Mon, 6 May 2019 17:16:17 +0200 From: Frederic Weisbecker To: Nicholas Piggin Cc: fweisbec@gmail.com, hpa@zytor.com, linux-kernel@vger.kernel.org, mingo@kernel.org, peterz@infradead.org, rafael.j.wysocki@intel.com, tglx@linutronix.de, torvalds@linux-foundation.org, linux-tip-commits@vger.kernel.org Subject: Re: [tip:sched/core] sched/isolation: Require a present CPU in housekeeping mask Message-ID: <20190506151615.GA14529@lenoir> References: <20190411033448.20842-5-npiggin@gmail.com> <20190504002733.GB19076@lenoir> <1556952021.2xpa7joi2y.astroid@bobo.none> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1556952021.2xpa7joi2y.astroid@bobo.none> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, May 04, 2019 at 04:59:12PM +1000, Nicholas Piggin wrote: > Frederic Weisbecker's on May 4, 2019 10:27 am: > > On Fri, May 03, 2019 at 10:47:37AM -0700, tip-bot for Nicholas Piggin wrote: > >> Commit-ID: 9219565aa89033a9cfdae788c1940473a1253d6c > >> Gitweb: https://git.kernel.org/tip/9219565aa89033a9cfdae788c1940473a1253d6c > >> Author: Nicholas Piggin > >> AuthorDate: Thu, 11 Apr 2019 13:34:47 +1000 > >> Committer: Ingo Molnar > >> CommitDate: Fri, 3 May 2019 19:42:58 +0200 > >> > >> sched/isolation: Require a present CPU in housekeeping mask > >> > >> During housekeeping mask setup, currently a possible CPU is required. > >> That does not guarantee the CPU would be available at boot time, so > >> check to ensure that at least one present CPU is in the mask. > > > > I have a doubt about the requirements and semantics of cpu_present_mask. > > IIUC a present CPU means that it is physically plugged in (from ACPI > > perspective) but might not be logically plugged in (set on cpu_online_mask). > > Right, a superset of cpu_possible_mask, subset of cpu_online_mask. It > means that CPU can be brought online at any time. > > > But do we have the guarantee that a present CPU _will_ be online at least once > > right after the boot? After all, kernel parameters such as "maxcpus=" can prevent > > from turning some CPUs on. I guess there are even more creative ways to achieve > > that. > > > > In any case we really require the housekeeper to be forced online. Perhaps > > I missed that enforcement somewhere in the patchset? > > No I think you're right, that may be able to boot without anything in > the housekeeping mask. Maybe we can just cpu_up() a CPU in the > housekeeping mask with a warning that it has overidden their SMP > command line option. I'll take a look at it. But then what if cpu_up() fails? In this case I can think of only two answers: * Force the boot CPU as the housekeeper. * Rollback the whole thing: nohz and all isolation. The second solution looks sane to me. After all if the user doesn't include CPU 0 in the housekeeping set, forcing it isn't going to help much. But that means we must enhance the isolation code (nohz included) to be able to dynamically add/del CPUs to the houseeeping/isolation set. That's not going to be easy but it's a necessary evolution of that subsystem since we want to drive it through cpusets. I should start working on that.