Commit 4cd8587c authored Nov 14, 2013 by Steven Rostedt Committed by Chris Mason Nov 20, 2013

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>

parent 52a15759

fs/btrfs/inode.c

+1 −2

Original line number	Diff line number	Diff line
		@@ -6187,7 +6187,6 @@ struct extent_map btrfs_get_extent(struct inode inode, struct page *page,
		write_unlock(&em_tree->lock);
		out:

		if (em)
		trace_btrfs_get_extent(root, em);

		if (path)

include/trace/events/btrfs.h

+3 −1

Original line number	Diff line number	Diff line
		@@ -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 )