[PATCH v2 2/2] tracing: Shrink the size of struct ftrace_event_field

[Li Zefan <lizefan@huawei.com>]

Use bit fields, and the size of struct ftrace_event_field can be
shrunk from 48 bytes to 40 bytes on 64bit kernel.

slab_name active_obj nr_obj size obj_per_slab
---------------------------------------------
ftrace_event_field   1105   1105     48   85  (before)
ftrace_event_field   1224   1224     40  102  (after)

This saves a few Kbytes: (1224 * 40) - (1105 * 48) = 4080

v2:
- use !!is_signed, and nuke the check on this field.
- use a different way to detect overflow.
(both suggested by Steven)

Signed-off-by: Li Zefan <lizefan@huawei.com>
---
 kernel/trace/trace.h        |  8 ++++----
 kernel/trace/trace_events.c | 14 ++++++++++----
 2 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 4a4f6e1..3e8c97f 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -904,10 +904,10 @@ struct ftrace_event_field {
 	struct list_head	link;
 	const char		*name;
 	const char		*type;
-	int			filter_type;
-	int			offset;
-	int			size;
-	int			is_signed;
+	unsigned int		filter_type:4;
+	unsigned int		offset:12;
+	unsigned int		size:12;
+	unsigned int		is_signed:1;
 };
 
 struct event_filter {
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 7d85429..d72694d 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -106,6 +106,9 @@ trace_find_event_field(struct ftrace_event_call *call, char *name)
 	return __find_event_field(head, name);
 }
 
+/* detect bit-field overflow */
+#define VERIFY_SIZE(type) WARN_ON(type > field->type)
+
 static int __trace_define_field(struct list_head *head, const char *type,
 				const char *name, int offset, int size,
 				int is_signed, int filter_type)
@@ -120,13 +123,16 @@ static int __trace_define_field(struct list_head *head, const char *type,
 	field->type = type;
 
 	if (filter_type == FILTER_OTHER)
-		field->filter_type = filter_assign_type(type);
-	else
-		field->filter_type = filter_type;
+		filter_type = filter_assign_type(type);
 
+	field->filter_type = filter_type;
 	field->offset = offset;
 	field->size = size;
-	field->is_signed = is_signed;
+	field->is_signed = !!is_signed;
+
+	VERIFY_SIZE(filter_type);
+	VERIFY_SIZE(offset);
+	VERIFY_SIZE(size);
 
 	list_add(&field->link, head);
 
-- 
1.8.0.2