From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753057AbbF3WFE (ORCPT ); Tue, 30 Jun 2015 18:05:04 -0400 Received: from mga02.intel.com ([134.134.136.20]:7990 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751544AbbF3WE4 (ORCPT ); Tue, 30 Jun 2015 18:04:56 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.15,380,1432623600"; d="scan'208";a="516823441" Subject: [PATCH 1/3] fs: optimize inotify/fsnotify code for unwatched files To: dave@sr71.net Cc: dave.hansen@linux.intel.com, akpm@linux-foundation.org, jack@suse.cz, viro@zeniv.linux.org.uk, eparis@redhat.com, john@johnmccutchan.com, rlove@rlove.org, ak@linux.intel.com, linux-kernel@vger.kernel.org From: Dave Hansen Date: Tue, 30 Jun 2015 15:04:55 -0700 Message-Id: <20150630220455.7DE1D236@viggo.jf.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Dave Hansen I have a _tiny_ microbenchmark that sits in a loop and writes single bytes to a file. Writing one byte to a tmpfs file is around 2x slower than reading one byte from a file, which is a _bit_ more than I expecte. This is a dumb benchmark, but I think it's hard to deny that write() is a hot path and we should avoid unnecessary overhead there. I did a 'perf record' of 30-second samples of read and write. The top item in a diffprofile is srcu_read_lock() from fsnotify(). There are active inotify fd's from systemd, but nothing is actually listening to the file or its part of the filesystem. I *think* we can avoid taking the srcu_read_lock() for the common case where there are no actual marks on the file. This means that there will both be nothing to notify for *and* implies that there is no need for clearing the ignore mask. This patch gave a 13.1% speedup in writes/second on my test, which is an improvement from the 10.8% that I saw with the last version. Signed-off-by: Dave Hansen Cc: Andrew Morton Cc: Jan Kara Cc: Al Viro Cc: Eric Paris Cc: John McCutchan Cc: Robert Love Cc: Andi Kleen Cc: linux-kernel@vger.kernel.org --- b/fs/notify/fsnotify.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff -puN fs/notify/fsnotify.c~optimize-fsnotify fs/notify/fsnotify.c --- a/fs/notify/fsnotify.c~optimize-fsnotify 2015-06-30 15:03:56.819399401 -0700 +++ b/fs/notify/fsnotify.c 2015-06-30 15:03:56.822399536 -0700 @@ -205,6 +205,16 @@ int fsnotify(struct inode *to_tell, __u3 mnt = NULL; /* + * Optimization: srcu_read_lock() has a memory barrier which can + * be expensive. It protects walking the *_fsnotify_marks lists. + * However, if we do not walk the lists, we do not have to do + * SRCU because we have no references to any objects and do not + * need SRCU to keep them "alive". + */ + if (hlist_empty(&to_tell->i_fsnotify_marks) && + (!mnt || hlist_empty(&mnt->mnt_fsnotify_marks))) + return 0; + /* * if this is a modify event we may need to clear the ignored masks * otherwise return if neither the inode nor the vfsmount care about * this type of event. _