tracing/events: provide string with undefined size support

This patch provides the support for dynamic size strings on
event tracing.

The key concept is to use a structure with an ending char array field of
undefined size and use such ability to allocate the minimal size on the
ring buffer to make one or more string entries fit inside, as opposite
to a fixed length strings with upper bound.

The strings themselves are represented using fields which have an offset
value from the beginning of the entry.

This patch provides three new macros:

__string(item, src)

This one declares a string to the structure inside TP_STRUCT__entry.
You need to provide the name of the string field and the source that will
be copied inside.
This will also add the dynamic size of the string needed for the ring
buffer entry allocation.
A stack allocated structure is used to temporarily store the offset
of each strings, avoiding double calls to strlen() on each event
insertion.

__get_str(field)

This one will give you a pointer to the string you have created. This
is an abstract helper to resolve the absolute address given the field
name which is a relative address from the beginning of the trace_structure.

__assign_str(dst, src)

Use this macro to automatically perform the string copy from src to
dst. src must be a variable to assign and dst is the name of a __string
field.

Example on how to use it:

TRACE_EVENT(my_event,
	TP_PROTO(char *src1, char *src2),

	TP_ARGS(src1, src2),
	TP_STRUCT__entry(
		__string(str1, src1)
		__string(str2, src2)
	),
	TP_fast_assign(
		__assign_str(str1, src1);
		__assign_str(str2, src2);
	),
	TP_printk("%s %s", __get_str(src1), __get_str(src2))
)

Of course you can mix-up any __field or __array inside this
TRACE_EVENT. The position of the __string or __assign_str
doesn't matter.

Changes in v2:

Address the suggestion of Steven Rostedt: drop the opening_string() macro
and redefine __ending_string() to get the size of the string to be copied
instead of overwritting the whole ring buffer allocation.

Changes in v3:

Address other suggestions of Steven Rostedt and Peter Zijlstra with
some changes: drop the __ending_string and the need to have only one
string field.
Use offsets instead of absolute addresses.

[ Impact: allow more compact memory usage for string tracing ]
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>

include/trace/ftrace.h

@@ -27,6 +27,9 @@
 #undef __field
 #define __field(type, item)		type	item;
 
+#undef __string
+#define __string(item, src)		int	__str_loc_##item;
+
 #undef TP_STRUCT__entry
 #define TP_STRUCT__entry(args...) args
 
@@ -35,14 +38,53 @@
 	struct ftrace_raw_##name {				\
 		struct trace_entry	ent;			\
 		tstruct						\
+		char			__str_data[0];		\
 	};							\
 	static struct ftrace_event_call event_##name
 
 #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
 
+
 /*
  * Stage 2 of the trace events.
  *
+ * Include the following:
+ *
+ * struct ftrace_str_offsets_<call> {
+ *	int				<str1>;
+ *	int				<str2>;
+ *	[...]
+ * };
+ *
+ * The __string() macro will create each int <str>, this is to
+ * keep the offset of each string from the beggining of the event
+ * once we perform the strlen() of the src strings.
+ *
+ */
+
+#undef TRACE_FORMAT
+#define TRACE_FORMAT(call, proto, args, fmt)
+
+#undef __array
+#define __array(type, item, len)
+
+#undef __field
+#define __field(type, item);
+
+#undef __string
+#define __string(item, src)	int item;
+
+#undef TRACE_EVENT
+#define TRACE_EVENT(call, proto, args, tstruct, assign, print)		\
+	struct ftrace_str_offsets_##call {				\
+		tstruct;						\
+	};
+
+#include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
+
+/*
+ * Stage 3 of the trace events.
+ *
  * Override the macros in <trace/trace_events.h> to include the following:
  *
  * enum print_line_t
@@ -80,6 +122,9 @@
 #undef TP_printk
 #define TP_printk(fmt, args...) fmt "\n", args
 
+#undef __get_str
+#define __get_str(field)	(char *)__entry + __entry->__str_loc_##field
+
 #undef TRACE_EVENT
 #define TRACE_EVENT(call, proto, args, tstruct, assign, print)		\
 enum print_line_t							\
@@ -146,6 +191,16 @@ ftrace_raw_output_##call(struct trace_iterator *iter, int flags)	\
 	if (!ret)							\
 		return 0;
 
+#undef __string
+#define __string(item, src)						       \
+	ret = trace_seq_printf(s, "\tfield: __str_loc " #item ";\t"	       \
+			       "offset:%u;tsize:%u;\n",			       \
+			       (unsigned int)offsetof(typeof(field),	       \
+					__str_loc_##item),		       \
+			       (unsigned int)sizeof(field.__str_loc_##item));  \
+	if (!ret)							       \
+		return 0;
+
 #undef __entry
 #define __entry REC
 
@@ -189,6 +244,12 @@ ftrace_format_##call(struct trace_seq *s)				\
 	if (ret)							\
 		return ret;
 
+#undef __string
+#define __string(item, src)						       \
+	ret = trace_define_field(event_call, "__str_loc", #item,	       \
+				offsetof(typeof(field), __str_loc_##item),     \
+				sizeof(field.__str_loc_##item));
+
 #undef TRACE_EVENT
 #define TRACE_EVENT(call, proto, args, tstruct, func, print)		\
 int									\
@@ -212,7 +273,7 @@ ftrace_define_fields_##call(void)					\
 #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
 
 /*
- * Stage 3 of the trace events.
+ * Stage 4 of the trace events.
  *
  * Override the macros in <trace/trace_events.h> to include the following:
  *
@@ -409,6 +470,23 @@ __attribute__((section("_ftrace_events"))) event_##call = {		\
 #undef __entry
 #define __entry entry
 
+#undef __field
+#define __field(type, item)
+
+#undef __array
+#define __array(type, item, len)
+
+#undef __string
+#define __string(item, src)						       \
+	__str_offsets.item = __str_size +				       \
+			     offsetof(typeof(*entry), __str_data);	       \
+	__str_size += strlen(src) + 1;
+
+#undef __assign_str
+#define __assign_str(dst, src)						\
+	__entry->__str_loc_##dst = __str_offsets.dst;			\
+	strcpy(__get_str(dst), src);
+
 #undef TRACE_EVENT
 #define TRACE_EVENT(call, proto, args, tstruct, assign, print)		\
 _TRACE_PROFILE(call, PARAMS(proto), PARAMS(args))			\
@@ -417,17 +495,21 @@ static struct ftrace_event_call event_##call;				\
 									\
 static void ftrace_raw_event_##call(proto)				\
 {									\
+	struct ftrace_str_offsets_##call __maybe_unused __str_offsets;  \
 	struct ftrace_event_call *call = &event_##call;			\
 	struct ring_buffer_event *event;				\
 	struct ftrace_raw_##call *entry;				\
 	unsigned long irq_flags;					\
+	int __str_size = 0;						\
 	int pc;								\
 									\
 	local_save_flags(irq_flags);					\
 	pc = preempt_count();						\
 									\
+	tstruct;							\
+									\
 	event = trace_current_buffer_lock_reserve(event_##call.id,	\
-				  sizeof(struct ftrace_raw_##call),	\
+				 sizeof(struct ftrace_raw_##call) + __str_size,\
 				 irq_flags, pc);			\
 	if (!event)							\
 		return;							\