[PATCH] [1/2] Optimization for touch_atime

[Andi Kleen <andi@firstfloor.org>]

Some benchmark testing shows touch_atime to be high up in profile
logs for IO intensive workloads. Most likely that's due to the lock
in mnt_want_write().  Unfortunately touch_atime first takes the lock,
and then does all the other tests that could avoid atime updates (like
noatime or relatime).

Do it the other way round -- first try to avoid the update and only
then if that didn't succeed take the lock. That works because none of
the atime avoidance tests rely on locking.

This also eliminates a goto.

Signed-off-by: Andi Kleen <ak@linux.intel.com>

---
 fs/inode.c |   20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

Index: linux-2.6.30-ak/fs/inode.c
===================================================================
--- linux-2.6.30-ak.orig/fs/inode.c
+++ linux-2.6.30-ak/fs/inode.c
@@ -1361,31 +1361,31 @@ void touch_atime(struct vfsmount *mnt, s
 	struct inode *inode = dentry->d_inode;
 	struct timespec now;
 
-	if (mnt_want_write(mnt))
-		return;
 	if (inode->i_flags & S_NOATIME)
-		goto out;
+		return;
 	if (IS_NOATIME(inode))
-		goto out;
+		return;
 	if ((inode->i_sb->s_flags & MS_NODIRATIME) && S_ISDIR(inode->i_mode))
-		goto out;
+		return;
 
 	if (mnt->mnt_flags & MNT_NOATIME)
-		goto out;
+		return;
 	if ((mnt->mnt_flags & MNT_NODIRATIME) && S_ISDIR(inode->i_mode))
-		goto out;
+		return;
 
 	now = current_fs_time(inode->i_sb);
 
 	if (!relatime_need_update(mnt, inode, now))
-		goto out;
+		return;
 
 	if (timespec_equal(&inode->i_atime, &now))
-		goto out;
+		return;
+
+	if (mnt_want_write(mnt))
+		return;
 
 	inode->i_atime = now;
 	mark_inode_dirty_sync(inode);
-out:
 	mnt_drop_write(mnt);
 }
 EXPORT_SYMBOL(touch_atime);