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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C5013C4332F for ; Thu, 2 Nov 2023 13:02:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1376388AbjKBNCd (ORCPT ); Thu, 2 Nov 2023 09:02:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54110 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1347433AbjKBNCb (ORCPT ); Thu, 2 Nov 2023 09:02:31 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2FDA2136 for ; Thu, 2 Nov 2023 06:01:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698930103; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ASeLFDp8swxfgAuKjSU2H8QvmbgML9pw1swrs3fA+jY=; b=ApeMecNjw8ij/dQh6cj0qairK3b4BgJkCIhS3hNr283hZzDMikd6iKM1/PBtKNewXIQYcz iZPzdNEvFqkRifqSjvXht4YxDAHcEkRypWu3nclOsT0Rup1DvQY4UJkWQIos3Md/ByltEr XRx6ACy+4kA2tNS4dQxwbSvTftryTvY= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-156-RpxDaBupOw2Get0BiU7CWQ-1; Thu, 02 Nov 2023 09:01:40 -0400 X-MC-Unique: RpxDaBupOw2Get0BiU7CWQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 2647785A5B5; Thu, 2 Nov 2023 13:01:40 +0000 (UTC) Received: from [10.22.17.8] (unknown [10.22.17.8]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9F3322026D4C; Thu, 2 Nov 2023 13:01:38 +0000 (UTC) Message-ID: <63726aac-2a9b-11f2-6c24-9f33ced68706@redhat.com> Date: Thu, 2 Nov 2023 09:01:38 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.14.0 Subject: Re: [PATCH v2] cgroup/cpuset: Change nr_deadline_tasks to an atomic_t value Content-Language: en-US To: Juri Lelli Cc: =?UTF-8?Q?Michal_Koutn=c3=bd?= , Tejun Heo , Zefan Li , Johannes Weiner , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Peter Zijlstra , Ingo Molnar , Qais Yousef , Hao Luo , Dietmar Eggemann , Steven Rostedt , Xia Fukun References: <20231024141834.4073262-1-longman@redhat.com> <8e1b5497-d4ca-50a0-7cb1-ffa098e0a1c2@redhat.com> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.4 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/2/23 06:26, Juri Lelli wrote: > Hi Waiman, > > On 01/11/23 13:59, Waiman Long wrote: >> On 11/1/23 12:34, Michal Koutný wrote: >>> On Tue, Oct 24, 2023 at 10:18:34AM -0400, Waiman Long wrote: >>>> The nr_deadline_tasks field in cpuset structure was introduced by >>>> commit 6c24849f5515 ("sched/cpuset: Keep track of SCHED_DEADLINE task >>>> in cpusets"). Unlike nr_migrate_dl_tasks which is only modified under >>>> cpuset_mutex, nr_deadline_tasks can be updated under two different >>>> locks - cpuset_mutex in most cases or css_set_lock in cgroup_exit(). As >>>> a result, data races can happen leading to incorrect nr_deadline_tasks >>>> value. >>> The effect is that dl_update_tasks_root_domain() processes tasks >>> unnecessarily or that it incorrectly skips dl_add_task_root_domain()? >> The effect is that dl_update_tasks_root_domain() may return incorrectly or >> it is doing unnecessary work. Will update the commit log to reflect that. >>>> Since it is not practical to somehow take cpuset_mutex in cgroup_exit(), >>>> the easy way out to avoid this possible race condition is by making >>>> nr_deadline_tasks an atomic_t value. >>> If css_set_lock is useless for this fields and it's going to be atomic, >>> could you please add (presumably) a cleanup that moves dec_dl_tasks_cs() >>> from under css_set_lock in cgroup_exit() to a (new but specific) >>> cpuset_cgrp_subsys.exit() handler? >> But css_set_lock is needed for updating other css data. It is true that we >> can move dec_dl_tasks_cs() outside of the lock. I can do that in the next >> version. > Not sure if you had a chance to check my last question/comment on your > previous posting? > > https://lore.kernel.org/lkml/ZSjfBWgZf15TchA5@localhost.localdomain/ Thanks for the reminder. I look at your comment again. Even though dl_rebuild_rd_accounting() operates on css(es) via css_task_iter_start() and css_task_iter_next(), the css_set_lock is released at the end of it. So it is still possible that a task can call cgroup_exit() after css_task_iter_next() and is being processed by dl_add_task_root_domain(). Is there a helper in the do_exit() path to nullify the dl_task() check. Or maybe we can also check for PF_EXITING in dl_add_task_root_domain() under the pi_lock and do the dl_task() check the under pi_lock to synchronize with dl_add_task_root_domain(). What do you think? I still believe that it doesn't really matter if we call dec_dl_tasks_cs() inside or outside the css_set_lock. Cheers, Longman Cheers, Longman