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 B2E2EC4167B for ; Thu, 2 Nov 2023 18:11:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1346018AbjKBSL3 (ORCPT ); Thu, 2 Nov 2023 14:11:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53344 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232280AbjKBSL0 (ORCPT ); Thu, 2 Nov 2023 14:11:26 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 99C1219AF for ; Thu, 2 Nov 2023 11:09:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698948541; 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=8atlEFdKhiLltidUbIHo3yQFF4oBkvMX6luoN2eXWEM=; b=da/J6PYy2Mr2P+MYHHHUYtIJVPhmkSwgsOxzKnT07C58Djy1+JojSR5NenK/dW6e8RZqUk BSfHKJYxP1tz8h3EMnIBXHxol/M4odPvU1UBpjSoatbB4eq8Q6TgL51K9Wn9fgPeUH5b9G Tz5cbmMqRszJEcFxp41GmfB1ZoQIzP4= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-183-DXrnkakaPfqgQGb3kAYGdQ-1; Thu, 02 Nov 2023 14:08:56 -0400 X-MC-Unique: DXrnkakaPfqgQGb3kAYGdQ-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (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 F14C11C29EA4; Thu, 2 Nov 2023 18:08:55 +0000 (UTC) Received: from [10.22.17.8] (unknown [10.22.17.8]) by smtp.corp.redhat.com (Postfix) with ESMTP id BBC05502D; Thu, 2 Nov 2023 18:08:54 +0000 (UTC) Message-ID: Date: Thu, 2 Nov 2023 14:08:54 -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 From: Waiman Long 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> <63726aac-2a9b-11f2-6c24-9f33ced68706@redhat.com> In-Reply-To: <63726aac-2a9b-11f2-6c24-9f33ced68706@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.5 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/2/23 09:01, Waiman Long wrote: > > 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. Just curious. Does the deadline code remove the deadline quota of an exiting task? Regards, Longman