From patchwork Wed May 12 00:38:35 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ian Kent X-Patchwork-Id: 12252439 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=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=unavailable 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 9D0FDC433B4 for ; Wed, 12 May 2021 00:38:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7AA0261628 for ; Wed, 12 May 2021 00:38:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229637AbhELAjy convert rfc822-to-8bit (ORCPT ); Tue, 11 May 2021 20:39:54 -0400 Received: from us-smtp-delivery-44.mimecast.com ([205.139.111.44]:56415 "EHLO us-smtp-delivery-44.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229932AbhELAjx (ORCPT ); Tue, 11 May 2021 20:39:53 -0400 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-478-fRxEsnuBNqm26hnjJgzNNQ-1; Tue, 11 May 2021 20:38:43 -0400 X-MC-Unique: fRxEsnuBNqm26hnjJgzNNQ-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 211FA8030CF; Wed, 12 May 2021 00:38:41 +0000 (UTC) Received: from web.messagingengine.com (ovpn-116-20.sin2.redhat.com [10.67.116.20]) by smtp.corp.redhat.com (Postfix) with ESMTP id 103AC9CA0; Wed, 12 May 2021 00:38:36 +0000 (UTC) Subject: [PATCH v4 0/5] kernfs: proposed locking and concurrency improvement From: Ian Kent To: Greg Kroah-Hartman , Tejun Heo Cc: Al Viro , Eric Sandeen , Fox Chen , Brice Goglin , Rick Lindsley , David Howells , Miklos Szeredi , Marcelo Tosatti , linux-fsdevel , Kernel Mailing List Date: Wed, 12 May 2021 08:38:35 +0800 Message-ID: <162077975380.14498.11347675368470436331.stgit@web.messagingengine.com> User-Agent: StGit/0.23 MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=raven@themaw.net X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: themaw.net Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org There have been a few instances of contention on the kernfs_mutex during path walks, a case on very large IBM systems seen by myself, a report by Brice Goglin and followed up by Fox Chen, and I've since seen a couple of other reports by CoreOS users. The common thread is a large number of kernfs path walks leading to slowness of path walks due to kernfs_mutex contention. The problem being that changes to the VFS over some time have increased it's concurrency capabilities to an extent that kernfs's use of a mutex is no longer appropriate. There's also an issue of walks for non-existent paths causing contention if there are quite a few of them which is a less common problem. This patch series is relatively straight forward. All it does is add the ability to take advantage of VFS negative dentry caching to avoid needless dentry alloc/free cycles for lookups of paths that don't exit and change the kernfs_mutex to a read/write semaphore. The patch that tried to stay in VFS rcu-walk mode during path walks has been dropped for two reasons. First, it doesn't actually give very much improvement and, second, if there's a place where mistakes could go unnoticed it would be in that path. This makes the patch series simpler to review and reduces the likelihood of problems going unnoticed and popping up later. The patch to use a revision to identify if a directory has changed has also been dropped. If the directory has changed the dentry revision needs to be updated to avoid subsequent rb tree searches and after changing to use a read/write semaphore the update also requires a lock. But the d_lock is the only lock available at this point which might itself be contended. Changes since v3: - remove unneeded indirection when referencing the super block. - check if inode attribute update is actually needed. Changes since v2: - actually fix the inode attribute update locking. - drop the patch that tried to stay in rcu-walk mode. - drop the use a revision to identify if a directory has changed patch. Changes since v1: - fix locking in .permission() and .getattr() by re-factoring the attribute handling code. --- Ian Kent (5): kernfs: move revalidate to be near lookup kernfs: use VFS negative dentry caching kernfs: switch kernfs to use an rwsem kernfs: use i_lock to protect concurrent inode updates kernfs: add kernfs_need_inode_refresh() fs/kernfs/dir.c | 170 ++++++++++++++++++++---------------- fs/kernfs/file.c | 4 +- fs/kernfs/inode.c | 45 ++++++++-- fs/kernfs/kernfs-internal.h | 5 +- fs/kernfs/mount.c | 12 +-- fs/kernfs/symlink.c | 4 +- include/linux/kernfs.h | 2 +- 7 files changed, 147 insertions(+), 95 deletions(-) -- Ian