[RFCv2][PATCH 1/7] fs: optimize inotify/fsnotify code for unwatched files

[Dave Hansen <dave@sr71.net>]

From: Dave Hansen <dave.hansen@linux.intel.com>

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 13.8% 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 <dave.hansen@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Paris <eparis@redhat.com>
Cc: John McCutchan <john@johnmccutchan.com>
Cc: Robert Love <rlove@rlove.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
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	2015-06-24 17:14:34.573109264 -0700
+++ b/fs/notify/fsnotify.c	2015-06-24 17:14:34.576109399 -0700
@@ -213,6 +213,16 @@ int fsnotify(struct inode *to_tell, __u3
 	    !(test_mask & to_tell->i_fsnotify_mask) &&
 	    !(mnt && test_mask & mnt->mnt_fsnotify_mask))
 		return 0;
+	/*
+	 * 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 (!to_tell->i_fsnotify_marks.first &&
+	    (!mnt || !mnt->mnt_fsnotify_marks.first))
+		return 0;
 
 	idx = srcu_read_lock(&fsnotify_mark_srcu);
 
_