From 7c49b8616460ebb12ee56d80d1abfbc20b6f3cbb Mon Sep 17 00:00:00 2001 From: Dave Hansen Date: Fri, 4 Sep 2015 15:43:01 -0700 Subject: [PATCH] fs/notify: optimize inotify/fsnotify code for unwatched files 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 Reviewed-by: Jan Kara Cc: Al Viro Cc: Eric Paris Cc: John McCutchan Cc: Robert Love Cc: Andi Kleen Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/notify/fsnotify.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c index dd3fb0b17be7..d675e76251d3 100644 --- a/fs/notify/fsnotify.c +++ b/fs/notify/fsnotify.c @@ -204,6 +204,16 @@ int fsnotify(struct inode *to_tell, __u32 mask, void *data, int data_is, else 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 -- 2.20.1