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, 3 insertions, 1 deletions

diff --git a/include/trace/events/btrfs.h b/include/trace/events/btrfs.h
index f18b3b76e01e..4832d75dcbae 100644
--- a/include/trace/events/btrfs.h
+++ b/include/trace/events/btrfs.h
@@ -162,12 +162,14 @@ DEFINE_EVENT(btrfs__inode, btrfs_inode_evict,
                 { EXTENT_FLAG_LOGGING,          "LOGGING"       },      \
                 { EXTENT_FLAG_FILLING,          "FILLING"       })
 
-TRACE_EVENT(btrfs_get_extent,
+TRACE_EVENT_CONDITION(btrfs_get_extent,
 
         TP_PROTO(struct btrfs_root *root, struct extent_map *map),
 
         TP_ARGS(root, map),
 
+        TP_CONDITION(map),
+
         TP_STRUCT__entry(
                 __field(        u64,  root_objectid     )
                 __field(        u64,  start             )