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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 51FA6C2D0E4 for ; Tue, 24 Nov 2020 08:33:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ED9932076E for ; Tue, 24 Nov 2020 08:33:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="TUOxQiaj" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730687AbgKXIdH (ORCPT ); Tue, 24 Nov 2020 03:33:07 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44036 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730476AbgKXIdH (ORCPT ); Tue, 24 Nov 2020 03:33:07 -0500 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CC047C0613CF for ; Tue, 24 Nov 2020 00:33:06 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=1z5cl4E+1Kt5ZTJVS7bSlXS5dAz5x84o0QFQGPW0MwI=; b=TUOxQiajN4MT/SrV3n5T0XJHNs ibPQSsgjFWHHfNC5igX2BOc3Z0l0Kei3PjbMXAmYx3v7QUjbfibKH6sDzGfuA6mGdwMS8JHCsa6Sy eo4Z/QwO9HQ/sTWR5yaiO48WyVmW4lUuNC1UrY1ktB8SD9Y2+Q51Dsq4+9RCmORBP1x0fpYacjlxa FRPbkkmzGadNf4GcflgnxN8nfwaubGPgr34iUUgY64tKaxZEsMhOsbJpkLs8YRZ1jt6eftqrsqqoF ZuSYfI65olro/Hxc5qrl/4EcXYrAd1ymfTsVjEWdWSylSSsCUMru6dUpuZ0Qgsb3HBrrQdpNFvqRx LdRQFF0Q==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1khTjz-0003zd-Pz; Tue, 24 Nov 2020 08:32:00 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id EBE563012DF; Tue, 24 Nov 2020 09:31:56 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id A103024C2AFD3; Tue, 24 Nov 2020 09:31:56 +0100 (CET) Date: Tue, 24 Nov 2020 09:31:56 +0100 From: Peter Zijlstra To: "Singh, Balbir" Cc: "Joel Fernandes (Google)" , Nishanth Aravamudan , Julien Desfossez , Tim Chen , Vineeth Pillai , Aaron Lu , Aubrey Li , tglx@linutronix.de, linux-kernel@vger.kernel.org, mingo@kernel.org, torvalds@linux-foundation.org, fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com, Phil Auld , Valentin Schneider , Mel Gorman , Pawan Gupta , Paolo Bonzini , vineeth@bitbyteword.org, Chen Yu , Christian Brauner , Agata Gruza , Antonio Gomez Iglesias , graf@amazon.com, konrad.wilk@oracle.com, dfaggioli@suse.com, pjt@google.com, rostedt@goodmis.org, derkling@google.com, benbjiang@tencent.com, Alexandre Chartre , James.Bottomley@hansenpartnership.com, OWeisse@umich.edu, Dhaval Giani , Junaid Shahid , jsbarnes@google.com, chris.hyser@oracle.com, Ben Segall , Josh Don , Hao Luo , Tom Lendacky , Aubrey Li , "Paul E. McKenney" , Tim Chen Subject: Re: [PATCH -tip 03/32] sched/fair: Fix pick_task_fair crashes due to empty rbtree Message-ID: <20201124083156.GU3021@hirez.programming.kicks-ass.net> References: <20201117232003.3580179-1-joel@joelfernandes.org> <20201117232003.3580179-4-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 20, 2020 at 09:15:28PM +1100, Singh, Balbir wrote: > On 18/11/20 10:19 am, Joel Fernandes (Google) wrote: > > From: Peter Zijlstra > > > > pick_next_entity() is passed curr == NULL during core-scheduling. Due to > > this, if the rbtree is empty, the 'left' variable is set to NULL within > > the function. This can cause crashes within the function. > > > > This is not an issue if put_prev_task() is invoked on the currently > > running task before calling pick_next_entity(). However, in core > > scheduling, it is possible that a sibling CPU picks for another RQ in > > the core, via pick_task_fair(). This remote sibling would not get any > > opportunities to do a put_prev_task(). > > > > Fix it by refactoring pick_task_fair() such that pick_next_entity() is > > called with the cfs_rq->curr. This will prevent pick_next_entity() from > > crashing if its rbtree is empty. > > > > Also this fixes another possible bug where update_curr() would not be > > called on the cfs_rq hierarchy if the rbtree is empty. This could effect > > cross-cpu comparison of vruntime. > > > > It is not clear from the changelog as to what does put_prev_task() do to prevent > the crash from occuring? Why did we pass NULL as curr in the first place to > pick_next_entity? > > The patch looks functionally correct as in, it passes curr as the reference > to pick_next_entity() for left and entity_before comparisons. This patch should not exist; it should be smashed into the previous patch. There is no point in preserving a crash.