btrfs: Use trace condition for get_extent tracepoint

Doing an if statement to test some condition to know if we should
trigger a tracepoint is pointless when tracing is disabled. This just
adds overhead and wastes a branch prediction. This is why the
TRACE_EVENT_CONDITION() was created. It places the check inside the jump
label so that the branch does not happen unless tracing is enabled.

That is, instead of doing:

	if (em)
		trace_btrfs_get_extent(root, em);

Which is basically this:

	if (em)
		if (static_key(trace_btrfs_get_extent)) {

Using a TRACE_EVENT_CONDITION() we can just do:

	trace_btrfs_get_extent(root, em);

And the condition trace event will do:

	if (static_key(trace_btrfs_get_extent)) {
		if (em) {
			...

The static key is a non conditional jump (or nop) that is faster than
having to check if em is NULL or not.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index fd67b34..f1a7744 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6187,8 +6187,7 @@ insert:
 	write_unlock(&em_tree->lock);
 out:
 
-	if (em)
-		trace_btrfs_get_extent(root, em);
+	trace_btrfs_get_extent(root, em);
 
 	if (path)
 		btrfs_free_path(path);