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=-6.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 BA2ECC48BDF for ; Thu, 10 Jun 2021 20:17:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 937A1613BC for ; Thu, 10 Jun 2021 20:17:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230453AbhFJUTL (ORCPT ); Thu, 10 Jun 2021 16:19:11 -0400 Received: from mail.kernel.org ([198.145.29.99]:52432 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230130AbhFJUTK (ORCPT ); Thu, 10 Jun 2021 16:19:10 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id D1BE96139A; Thu, 10 Jun 2021 20:17:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1623356233; bh=nhEn6DbWrwaK255UgkRjv7t7Hm+VJsFrnmQKWYlFQTc=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=VLZmoep3lo4evx0jYG0euA5dEpwIkS0KwTCiK83JkcoTvTApqQtoutG9uLVHJTjWY RHh1Lc+KpehDi7+aon62xUTzqTlEeGjkrStw40IKyvhsFbCZ90tddzkpsKV58o2ANc SJF9tdMSh0ylkK4x8/hHyT9srwmoVxgfer5buWvvnC/me441m8PHdwpj5Lj6IwdPyA LD2kBVmd3qA/7HXX2BMxeEkHII+pvE3ss6av3IYCjbtGmv9gs8HASUXxzTflB/SBBy pt4s3habv5FVkcsl9CYd6AjaFlaLTfTMiJTdKNG0zY65EgvGttNkH1yQZT99Hw5nyI ize0X2cHb6H5Q== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 9B9345C04C6; Thu, 10 Jun 2021 13:17:13 -0700 (PDT) Date: Thu, 10 Jun 2021 13:17:13 -0700 From: "Paul E. McKenney" To: Valentin Schneider Cc: frederic@kernel.org, linux-kernel@vger.kernel.org Subject: Re: Question about a8ea6fc9b089 ("sched: Stop PF_NO_SETAFFINITY from being inherited by various init system threads") Message-ID: <20210610201713.GU4397@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210610170435.GA2187550@paulmck-ThinkPad-P17-Gen-1> <8735tpd15i.mognet@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8735tpd15i.mognet@arm.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 10, 2021 at 07:28:57PM +0100, Valentin Schneider wrote: > On 10/06/21 10:04, Paul E. McKenney wrote: > > Hi, > > Hello, Frederic, > > > > This commit works well, but has the unfortunate side-effect of making > > smp_processor_id() complain when used in a preemptible region even > > though the kthread has been pinned onto a single CPU by a call to > > set_cpus_allowed_ptr(). (Which did return success.) > > > > On which tree are you encountering this? I bisected to this commit in -next tag next-20210609, and this commit could of course be an innocent bystander caught in the crossfire. > Looking at check_preemption_disabled() and CPU affinity, v5.13-rc5 has: > > /* > * Kernel threads bound to a single CPU can safely use > * smp_processor_id(): > */ > if (current->nr_cpus_allowed == 1) > goto out; > > tip/sched/core additionally hinges that on PF_NO_SETAFFINITY: > > 570a752b7a9b ("lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed") > > The former shouldn't be affected by Frederic's patch, and the latter should > only cause warnings if the pinned task isn't a "proper" kthread (thus > doesn't have PF_NO_SETAFFINITY)... Exceptions that come to mind are things > like UMH which doesn't use kthread_create(). And reverting 570a752b7a9b ("lib/smp_processor_id: Use is_percpu_thread() instead of nr_cpus_allowed") causes the kernel to once again be OK with smp_processor_id(), so thank you! And apologies to Frederic for the false alarm. Added Yejune on CC. Thoughts? Thanx, Paul > > This isn't a big deal -- I can easily switch to raw_smp_processor_id(), > > which is arguably a better choice anyway because it prevents the > > complaints from flooding out any real warnings due to error returns > > from set_cpus_allowed_ptr() or something else unpinning the kthread. > > Which I am in the process of doing: > > > > 516e52e9f5ec ("scftorture: Avoid excess warnings") > > 475d6d49f21d ("refscale: Avoid excess warnings in ref_scale_reader()") > > > > But I figured that I should check to see if this change was in fact > > intentional. > > > > Thanx, Paul