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>

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index fd67b34e220d271c5e570b4960540468b92c029b..f1a77449d032b1fbd481eb16cd232653e5bfbefa 100644 (file)
--- 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);

diff --git a/include/trace/events/btrfs.h b/include/trace/events/btrfs.h
index f18b3b76e01e22e00c00ee133b2ebdc1013bcc62..4832d75dcbaedb888a2751303fe7d5a4e62b8010 100644 (file)
--- 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             )