The patch titled
Subject: fs/notify: optimize inotify/fsnotify code for unwatched files
has been removed from the -mm tree. Its filename was
fs-optimize-inotify-fsnotify-code-for-unwatched-files.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Subject: 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 <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Eric Paris <eparis@xxxxxxxxxx>
Cc: John McCutchan <john@xxxxxxxxxxxxxxxxx>
Cc: Robert Love <rlove@xxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/notify/fsnotify.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff -puN fs/notify/fsnotify.c~fs-optimize-inotify-fsnotify-code-for-unwatched-files fs/notify/fsnotify.c
--- a/fs/notify/fsnotify.c~fs-optimize-inotify-fsnotify-code-for-unwatched-files
+++ a/fs/notify/fsnotify.c
@@ -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.
_
Patches currently in -mm which might be from dave.hansen@xxxxxxxxxxxxxxx are
fs-do-not-prefault-sys_write-user-buffer-pages.patch
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