[PATCH v3] fsnotify: optimize the case of no parent watcher

[Amir Goldstein <amir73il@gmail.com>]

If parent inode is not watching, check for the event in masks of
sb/mount/inode masks early to optimize out most of the code in
__fsnotify_parent() and avoid calling fsnotify().

Jens has reported that this optimization improves BW and IOPS in an
io_uring benchmark by more than 10% and reduces perf reported CPU usage.

before:

+    4.51%  io_uring  [kernel.vmlinux]  [k] fsnotify
+    3.67%  io_uring  [kernel.vmlinux]  [k] __fsnotify_parent

after:

+    2.37%  io_uring  [kernel.vmlinux]  [k] __fsnotify_parent

Reported-and-tested-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/linux-fsdevel/b45bd8ff-5654-4e67-90a6-aad5e6759e0b@kernel.dk/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Jan,

Considering that this change looks like a clear win and it actually
the change that you suggested, I cleaned it up a bit and posting for
your consideration.

I've kept the wrappers fsnotify_path() and fsnotify_sb_has_watchers()
although they are not directly related to the optimization, because they
makes the code a bit nicer IMO.

Jens,

I took the liberty to add Reported-and-tested-by to attribute for your
help even though the patch that you tested was slightly different than
this cleaned up version, because logically it should be identical.

Thanks,
Amir.

Changes since v2:
- Add helper fsnotify_object_watched()
- Move the optimization from inlined fsnotify_path() to
  extern __fsnotify_parent()
- Optimize for any event - not only content events
- Drop the SB_I_CONTENT_WATCHED flag

 fs/notify/fsnotify.c     | 28 +++++++++++++++++-----------
 include/linux/fsnotify.h | 22 +++++++++++++++-------
 2 files changed, 32 insertions(+), 18 deletions(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 8bfd690e9f10..6582204cca70 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -141,7 +141,7 @@ void __fsnotify_update_child_dentry_flags(struct inode *inode)
 }
 
 /* Are inode/sb/mount interested in parent and name info with this event? */
-static bool fsnotify_event_needs_parent(struct inode *inode, struct mount *mnt,
+static bool fsnotify_event_needs_parent(struct inode *inode, __u32 mnt_mask,
 					__u32 mask)
 {
 	__u32 marks_mask = 0;
@@ -160,13 +160,22 @@ static bool fsnotify_event_needs_parent(struct inode *inode, struct mount *mnt,
 	/* Did either inode/sb/mount subscribe for events with parent/name? */
 	marks_mask |= fsnotify_parent_needed_mask(inode->i_fsnotify_mask);
 	marks_mask |= fsnotify_parent_needed_mask(inode->i_sb->s_fsnotify_mask);
-	if (mnt)
-		marks_mask |= fsnotify_parent_needed_mask(mnt->mnt_fsnotify_mask);
+	marks_mask |= fsnotify_parent_needed_mask(mnt_mask);
 
 	/* Did they subscribe for this event with parent/name info? */
 	return mask & marks_mask;
 }
 
+/* Are there any inode/mount/sb objects that are interested in this event? */
+static inline bool fsnotify_object_watched(struct inode *inode, __u32 mnt_mask,
+					   __u32 mask)
+{
+	__u32 marks_mask = inode->i_fsnotify_mask | mnt_mask |
+			   inode->i_sb->s_fsnotify_mask;
+
+	return mask & marks_mask & ALL_FSNOTIFY_EVENTS;
+}
+
 /*
  * Notify this dentry's parent about a child's events with child name info
  * if parent is watching or if inode/sb/mount are interested in events with
@@ -179,7 +188,7 @@ int __fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data,
 		      int data_type)
 {
 	const struct path *path = fsnotify_data_path(data, data_type);
-	struct mount *mnt = path ? real_mount(path->mnt) : NULL;
+	__u32 mnt_mask = path ? real_mount(path->mnt)->mnt_fsnotify_mask : 0;
 	struct inode *inode = d_inode(dentry);
 	struct dentry *parent;
 	bool parent_watched = dentry->d_flags & DCACHE_FSNOTIFY_PARENT_WATCHED;
@@ -190,16 +199,13 @@ int __fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data,
 	struct qstr *file_name = NULL;
 	int ret = 0;
 
-	/*
-	 * Do inode/sb/mount care about parent and name info on non-dir?
-	 * Do they care about any event at all?
-	 */
-	if (!inode->i_fsnotify_marks && !inode->i_sb->s_fsnotify_marks &&
-	    (!mnt || !mnt->mnt_fsnotify_marks) && !parent_watched)
+	/* Optimize the likely case of nobody watching this path */
+	if (likely(!parent_watched) &&
+	    likely(!fsnotify_object_watched(inode, mnt_mask, mask)))
 		return 0;
 
 	parent = NULL;
-	parent_needed = fsnotify_event_needs_parent(inode, mnt, mask);
+	parent_needed = fsnotify_event_needs_parent(inode, mnt_mask, mask);
 	if (!parent_watched && !parent_needed)
 		goto notify;
 
diff --git a/include/linux/fsnotify.h b/include/linux/fsnotify.h
index 8300a5286988..2ddfd7d1eca8 100644
--- a/include/linux/fsnotify.h
+++ b/include/linux/fsnotify.h
@@ -17,6 +17,12 @@
 #include <linux/slab.h>
 #include <linux/bug.h>
 
+/* Are there any inode/mount/sb objects that are being watched at all? */
+static inline bool fsnotify_sb_has_watchers(struct super_block *sb)
+{
+	return atomic_long_read(&sb->s_fsnotify_connectors);
+}
+
 /*
  * Notify this @dir inode about a change in a child directory entry.
  * The directory entry may have turned positive or negative or its inode may
@@ -30,7 +36,7 @@ static inline int fsnotify_name(__u32 mask, const void *data, int data_type,
 				struct inode *dir, const struct qstr *name,
 				u32 cookie)
 {
-	if (atomic_long_read(&dir->i_sb->s_fsnotify_connectors) == 0)
+	if (!fsnotify_sb_has_watchers(dir->i_sb))
 		return 0;
 
 	return fsnotify(mask, data, data_type, dir, name, NULL, cookie);
@@ -44,7 +50,7 @@ static inline void fsnotify_dirent(struct inode *dir, struct dentry *dentry,
 
 static inline void fsnotify_inode(struct inode *inode, __u32 mask)
 {
-	if (atomic_long_read(&inode->i_sb->s_fsnotify_connectors) == 0)
+	if (!fsnotify_sb_has_watchers(inode->i_sb))
 		return;
 
 	if (S_ISDIR(inode->i_mode))
@@ -59,7 +65,7 @@ static inline int fsnotify_parent(struct dentry *dentry, __u32 mask,
 {
 	struct inode *inode = d_inode(dentry);
 
-	if (atomic_long_read(&inode->i_sb->s_fsnotify_connectors) == 0)
+	if (!fsnotify_sb_has_watchers(inode->i_sb))
 		return 0;
 
 	if (S_ISDIR(inode->i_mode)) {
@@ -89,15 +95,17 @@ static inline void fsnotify_dentry(struct dentry *dentry, __u32 mask)
 	fsnotify_parent(dentry, mask, dentry, FSNOTIFY_EVENT_DENTRY);
 }
 
-static inline int fsnotify_file(struct file *file, __u32 mask)
+static inline int fsnotify_path(const struct path *path, __u32 mask)
 {
-	const struct path *path;
+	return fsnotify_parent(path->dentry, mask, path, FSNOTIFY_EVENT_PATH);
+}
 
+static inline int fsnotify_file(struct file *file, __u32 mask)
+{
 	if (file->f_mode & FMODE_NONOTIFY)
 		return 0;
 
-	path = &file->f_path;
-	return fsnotify_parent(path->dentry, mask, path, FSNOTIFY_EVENT_PATH);
+	return fsnotify_path(&file->f_path, mask);
 }
 
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
-- 
2.34.1