From: Dave Hansen <dave.han...@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.han...@linux.intel.com>
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: Jan Kara <j...@suse.cz>
Cc: Al Viro <v...@zeniv.linux.org.uk>
Cc: Eric Paris <epa...@redhat.com>
Cc: John McCutchan <j...@johnmccutchan.com>
Cc: Robert Love <rl...@rlove.org>
Cc: Tim Chen <tim.c.c...@linux.intel.com>
Cc: Andi Kleen <a...@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);
_
--
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