[RFC - beta][PATCH] tracing: Introduce TRACE_MARKER() no argumenttrace event

[Steven Rostedt]

A while back ago, Wolfgang (and others) have asked about the ability to
add a trace event that recorder no data other than the fact that the
trace event was hit.

I've been reluctant to do so, but I noticed that one already exists in
the iwlwifi tracepoints (iwlwifi_dev_irq) where it does a wasteful:

        /* TP_printk("") doesn't compile */
        TP_printk("%d", 0)

The reason this is wasteful, is that there's a lot of code generated by
the TRACE_EVENT() macros that end up basically being nops.

I figured I would instead create a TRACE_MARKER(name, print) that would
be something like:

Added to tracepoint header:

 TRACE_MARKER(tpname, "Show this message");

Then you have:

	trace_tpname();

in the code.

Notice that the tracepoint function (trace_<name>()) has no arguments.
That's because the message is stored in the tracepoint (in one place)
and is printed when the tracepoint is read. That is, the message isn't
even recorded in the ring buffer.

It still shows up in the tracepoint format file:

name: tpname
ID: 281
format:
	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
	field:int common_pid;	offset:4;	size:4;	signed:1;


print fmt: "Show this message"

The TRACE_MARKER() is basically an optimized version of TRACE_EVENT()
with no arguments. It can be enabled and disabled the same way as any
other event, and stays within the system it was created in.

Is this worth doing?

Attached is a patch that implements this idea, but it is very much in a
beta form. It requires comments, documentation, updates to perf, etc.

-- Steve

diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
index 884d8ad..e464511 100644
--- a/include/linux/ftrace_event.h
+++ b/include/linux/ftrace_event.h
@@ -526,6 +526,12 @@ extern int trace_define_field(struct ftrace_event_call *call, const char *type,
 			      int is_signed, int filter_type);
 extern int trace_add_event_call(struct ftrace_event_call *call);
 extern int trace_remove_event_call(struct ftrace_event_call *call);
+extern void ftrace_raw_event_trace_marker(void *data);
+
+struct ftrace_raw_trace_marker {
+	struct trace_entry	ent;
+};
+extern struct trace_event_functions ftrace_event_type_funcs_trace_marker;
 
 #define is_signed_type(type)	(((type)(-1)) < (type)1)
 
diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index accc497..171e3aa 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -399,6 +399,9 @@ static inline void tracepoint_synchronize_unregister(void)
 	DECLARE_TRACE_CONDITION(name, PARAMS(proto),		\
 				PARAMS(args), PARAMS(cond))
 
+#define TRACE_MARKER(name, print)				\
+	DECLARE_TRACE_NOARGS(name)
+
 #define TRACE_EVENT_FLAGS(event, flag)
 
 #define TRACE_EVENT_PERF_PERM(event, expr...)
diff --git a/include/trace/define_trace.h b/include/trace/define_trace.h
index 02e1003..773d05a 100644
--- a/include/trace/define_trace.h
+++ b/include/trace/define_trace.h
@@ -60,6 +60,10 @@
 #define DECLARE_TRACE(name, proto, args)	\
 	DEFINE_TRACE(name)
 
+#undef TRACE_MARKER
+#define TRACE_MARKER(name, print)		\
+	DEFINE_TRACE(name)
+
 #undef TRACE_INCLUDE
 #undef __TRACE_INCLUDE
 
@@ -93,6 +97,7 @@
 #undef TRACE_EVENT
 #undef TRACE_EVENT_FN
 #undef TRACE_EVENT_CONDITION
+#undef TRACE_MARKER
 #undef DECLARE_EVENT_CLASS
 #undef DEFINE_EVENT
 #undef DEFINE_EVENT_FN
diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 67e1bbf..fab482f 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -8,6 +8,10 @@
 #include <linux/tracepoint.h>
 #include <linux/binfmts.h>
 
+TRACE_MARKER(sched_mark1, "This is a test");
+
+TRACE_MARKER(sched_mark2, "This is another test");
+
 /*
  * Tracepoint for calling kthread_stop, performed to end a kthread:
  */
diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
index 623ddf3..85e2740 100644
--- a/include/trace/ftrace.h
+++ b/include/trace/ftrace.h
@@ -18,6 +18,23 @@
 
 #include <linux/ftrace_event.h>
 
+#ifdef TRACE_SYSTEM
+#define __TRACE_MARKER_CLASS(system) event_trace_marker_class_##system
+#define TRACE_MARKER_CLASS(system) __TRACE_MARKER_CLASS(system)
+/*
+ * Each system has its own trace marker class, but gcc will compile it
+ * out if it is not referenced.
+ */
+static struct ftrace_event_class __maybe_unused
+TRACE_MARKER_CLASS(TRACE_SYSTEM) = {
+	.system		= __stringify(TRACE_SYSTEM),
+	.fields		= LIST_HEAD_INIT(TRACE_MARKER_CLASS(TRACE_SYSTEM).fields),
+	.raw_init	= trace_event_raw_init,
+	.probe		= ftrace_raw_event_trace_marker,
+	.reg		= ftrace_event_reg,
+};
+#endif
+
 /*
  * DECLARE_EVENT_CLASS can be used to add a generic function
  * handlers for events. That is, if all events have the same
@@ -94,6 +111,10 @@
 #define TRACE_EVENT_PERF_PERM(name, expr...)				\
 	__TRACE_EVENT_PERF_PERM(name, expr)
 
+#undef TRACE_MARKER
+#define TRACE_MARKER(name, print)				\
+	DEFINE_EVENT(unused, name, unsued, unused)
+
 #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
 
 
@@ -147,6 +168,9 @@
 #undef TRACE_EVENT_PERF_PERM
 #define TRACE_EVENT_PERF_PERM(event, expr...)
 
+#undef TRACE_MARKER
+#define TRACE_MARKER(name, print)
+
 #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
 
 /*
@@ -626,6 +650,22 @@ static struct ftrace_event_call __used event_##call = {			\
 static struct ftrace_event_call __used					\
 __attribute__((section("_ftrace_events"))) *__event_##call = &event_##call
 
+
+#undef TRACE_MARKER
+#define TRACE_MARKER(call, print)					\
+									\
+static const char print_fmt_##call[] = "\"" print "\"";			\
+									\
+static struct ftrace_event_call __used event_##call = {			\
+	.name			= #call,				\
+	.class			= &TRACE_MARKER_CLASS(TRACE_SYSTEM),	\
+	.event.funcs		= &ftrace_event_type_funcs_trace_marker,\
+	.print_fmt		= print_fmt_##call,			\
+};									\
+static struct ftrace_event_call __used					\
+__attribute__((section("_ftrace_events"))) *__event_##call = &event_##call
+
+
 #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
 
 
@@ -650,6 +690,9 @@ __attribute__((section("_ftrace_events"))) *__event_##call = &event_##call
 #undef __perf_task
 #define __perf_task(t)	(__pe.task = (t))
 
+#undef TRACE_MARKER
+#define TRACE_MARKER(name, print)
+
 #undef DECLARE_EVENT_CLASS
 #define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print)	\
 static notrace void							\
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b46131e..00bcf93 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2656,6 +2656,7 @@ static void __sched __schedule(void)
 	struct rq *rq;
 	int cpu;
 
+	trace_sched_mark1();
 need_resched:
 	preempt_disable();
 	cpu = smp_processor_id();
@@ -2848,6 +2849,7 @@ asmlinkage void __sched preempt_schedule_irq(void)
 int default_wake_function(wait_queue_t *curr, unsigned mode, int wake_flags,
 			  void *key)
 {
+	trace_sched_mark2();
 	return try_to_wake_up(curr->private, mode, wake_flags);
 }
 EXPORT_SYMBOL(default_wake_function);
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 22826c7..f82c936 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -86,6 +86,51 @@ __find_event_field(struct list_head *head, char *name)
 	return NULL;
 }
 
+/* TRACE_MARKER() class */
+
+enum print_line_t
+ftrace_raw_output_trace_marker(struct trace_iterator *iter, int flags,
+			       struct trace_event *trace_event)
+{
+	struct ftrace_event_call *event_call;
+	struct trace_seq *s = &iter->seq;
+	int ret;
+
+	ret = ftrace_raw_output_prep(iter, trace_event);
+	if (ret)
+		return ret;
+
+	event_call = container_of(trace_event, typeof(*event_call), event);
+
+	ret = trace_seq_printf(s, "%s\n", event_call->print_fmt);
+	if (!ret)
+		return TRACE_TYPE_PARTIAL_LINE;
+
+	return TRACE_TYPE_HANDLED;
+}
+
+struct trace_event_functions ftrace_event_type_funcs_trace_marker = {
+	.trace			= ftrace_raw_output_trace_marker,
+};
+
+void ftrace_raw_event_trace_marker(void *data)
+{
+	struct ftrace_event_file *ftrace_file = data;
+	struct ftrace_event_call *event_call = ftrace_file->event_call;
+	struct ftrace_event_buffer fbuffer;
+	struct ftrace_raw_trace_marker *entry;
+
+	if (ftrace_trigger_soft_disabled(ftrace_file))
+		return;
+
+	entry = ftrace_event_buffer_reserve(&fbuffer, ftrace_file,
+				event_call->event.type, sizeof(*entry));
+	if (!entry)
+		return;
+	ftrace_event_buffer_commit(&fbuffer, event_call);
+}
+
+
 struct ftrace_event_field *
 trace_find_event_field(struct ftrace_event_call *call, char *name)
 {
@@ -1533,7 +1578,7 @@ event_create_dir(struct dentry *parent, struct ftrace_event_file *file)
 	 * the fields if they are not already defined.
 	 */
 	head = trace_get_fields(call);
-	if (list_empty(head)) {
+	if (list_empty(head) && call->class->define_fields) {
 		ret = call->class->define_fields(call);
 		if (ret < 0) {
 			pr_warning("Could not initialize trace point"
--
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