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=-5.3 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 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 251D6C433E0 for ; Fri, 19 Jun 2020 00:28:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ED25E208B8 for ; Fri, 19 Jun 2020 00:27:59 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="YuKX4BOp" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728948AbgFSA17 (ORCPT ); Thu, 18 Jun 2020 20:27:59 -0400 Received: from userp2120.oracle.com ([156.151.31.85]:37290 "EHLO userp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726926AbgFSA15 (ORCPT ); Thu, 18 Jun 2020 20:27:57 -0400 Received: from pps.filterd (userp2120.oracle.com [127.0.0.1]) by userp2120.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 05J0M2u0004756; Fri, 19 Jun 2020 00:27:46 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=subject : to : cc : references : from : message-id : date : mime-version : in-reply-to : content-type : content-transfer-encoding; s=corp-2020-01-29; bh=mZHm+L5Ei1oR1XqCOi3sReF6y81eXJTuWYq1i07VHxc=; b=YuKX4BOpzlRKsrGXHMuPk/Yi06HFsG8RxziPXHMVeobCjvE6vkITeNYsd2g7VHDWuXqG dUJdhwrfVho+sWi/FpqTcnGKQtEiZuAa7LQosMRTjZ3qZFrdtUf0CSMnpBaoc36Wqqhf OUTQX5lxW6qNFMM6MydbVHVK3rrZuOkD46a2v7NfYoUnTpSIq99xrRpe3wxaNDAzm/3W Vw9O3QFU43p5dX+JFqTxt8EAtLrdAjQ4sdp9fePBkVIac9hQdi/7woRFQHekSXYXXCyZ zhrYZsf3CK6pQxI2X9ehtCfsTve3+gngerXudw3wSagIEzsX3NTNCuyGqj3UqM4Ejdga cQ== Received: from aserp3030.oracle.com (aserp3030.oracle.com [141.146.126.71]) by userp2120.oracle.com with ESMTP id 31qg35a2m0-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Fri, 19 Jun 2020 00:27:46 +0000 Received: from pps.filterd (aserp3030.oracle.com [127.0.0.1]) by aserp3030.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 05J0JEVO167553; Fri, 19 Jun 2020 00:27:46 GMT Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by aserp3030.oracle.com with ESMTP id 31q66qm8u9-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 19 Jun 2020 00:27:45 +0000 Received: from abhmp0002.oracle.com (abhmp0002.oracle.com [141.146.116.8]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id 05J0Rj3G017571; Fri, 19 Jun 2020 00:27:45 GMT Received: from dhcp-10-159-251-35.vpn.oracle.com (/10.159.251.35) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Thu, 18 Jun 2020 17:27:45 -0700 Subject: Re: severe proc dentry lock contention To: "Eric W. Biederman" , Matthew Wilcox Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Matthew Wilcox , Srinivas Eeda , "joe.jin@oracle.com" References: <54091fc0-ca46-2186-97a8-d1f3c4f3877b@oracle.com> <20200618233958.GV8681@bombadil.infradead.org> <877dw3apn8.fsf@x220.int.ebiederm.org> From: Junxiao Bi Message-ID: <2cf6af59-e86b-f6cc-06d3-84309425bd1d@oracle.com> Date: Thu, 18 Jun 2020 17:27:43 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <877dw3apn8.fsf@x220.int.ebiederm.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9656 signatures=668680 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 spamscore=0 adultscore=0 phishscore=0 mlxscore=0 bulkscore=0 malwarescore=0 mlxlogscore=999 suspectscore=3 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2006190000 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9656 signatures=668680 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 bulkscore=0 phishscore=0 mlxscore=0 clxscore=1011 malwarescore=0 impostorscore=0 adultscore=0 cotscore=-2147483648 lowpriorityscore=0 mlxlogscore=999 spamscore=0 suspectscore=3 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2006190000 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/18/20 5:02 PM, ebiederm@xmission.com wrote: > Matthew Wilcox writes: > >> On Thu, Jun 18, 2020 at 03:17:33PM -0700, Junxiao Bi wrote: >>> When debugging some performance issue, i found that thousands of threads >>> exit around same time could cause a severe spin lock contention on proc >>> dentry "/proc/$parent_process_pid/task/", that's because threads needs to >>> clean up their pid file from that dir when exit. Check the following >>> standalone test case that simulated the case and perf top result on v5.7 >>> kernel. Any idea on how to fix this? >> Thanks, Junxiao. >> >> We've looked at a few different ways of fixing this problem. >> >> Even though the contention is within the dcache, it seems like a usecase >> that the dcache shouldn't be optimised for -- generally we do not have >> hundreds of CPUs removing dentries from a single directory in parallel. >> >> We could fix this within procfs. We don't have a great patch yet, but >> the current approach we're looking at allows only one thread at a time >> to call dput() on any /proc/*/task directory. >> >> We could also look at fixing this within the scheduler. Only allowing >> one CPU to run the threads of an exiting process would fix this particular >> problem, but might have other consequences. >> >> I was hoping that 7bc3e6e55acf would fix this, but that patch is in 5.7, >> so that hope is ruled out. > Does anyone know if problem new in v5.7? I am wondering if I introduced > this problem when I refactored the code or if I simply churned the code > but the issue remains effectively the same. It's not new issue, we see it in old kernel like v4.14 > > Can you try only flushing entries when the last thread of the process is > reaped? I think in practice we would want to be a little more > sophisticated but it is a good test case to see if it solves the issue. Thank you. i will try and let you know. Thanks, Junxiao. > > diff --git a/kernel/exit.c b/kernel/exit.c > index cebae77a9664..d56e4eb60bdd 100644 > --- a/kernel/exit.c > +++ b/kernel/exit.c > @@ -152,7 +152,7 @@ void put_task_struct_rcu_user(struct task_struct *task) > void release_task(struct task_struct *p) > { > struct task_struct *leader; > - struct pid *thread_pid; > + struct pid *thread_pid = NULL; > int zap_leader; > repeat: > /* don't need to get the RCU readlock here - the process is dead and > @@ -165,7 +165,8 @@ void release_task(struct task_struct *p) > > write_lock_irq(&tasklist_lock); > ptrace_release_task(p); > - thread_pid = get_pid(p->thread_pid); > + if (p == p->group_leader) > + thread_pid = get_pid(p->thread_pid); > __exit_signal(p); > > /* > @@ -188,8 +189,10 @@ void release_task(struct task_struct *p) > } > > write_unlock_irq(&tasklist_lock); > - proc_flush_pid(thread_pid); > - put_pid(thread_pid); > + if (thread_pid) { > + proc_flush_pid(thread_pid); > + put_pid(thread_pid); > + } > release_thread(p); > put_task_struct_rcu_user(p); >