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btrfs: Use trace condition for get_extent tracepoint

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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>
This commit is contained in:
Steven Rostedt 2013-11-14 22:57:29 -05:00 committed by Chris Mason
parent 52a1575921
commit 4cd8587ce8
2 changed files with 4 additions and 3 deletions
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3
fs/btrfs/inode.c
View file

@ -6187,8 +6187,7 @@ 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);
trace_btrfs_get_extent(root, em);
if (path)
btrfs_free_path(path);

4
include/trace/events/btrfs.h
View file

@ -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 )