tracing: document TRACE_EVENT macro in tracepoint.h
Impact: clean up / comments
Kosaki Motohiro asked about an explanation to the TRACE_EVENT macro.
Ingo Molnar replied with a nice description.
This patch takes the description that Ingo wrote (with some slight
modifications) and adds it to the tracepoint.h file.
Reported-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index c7b0945..119ece22 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -157,6 +157,109 @@
#define TRACE_FORMAT(name, proto, args, fmt) \
DECLARE_TRACE(name, PARAMS(proto), PARAMS(args))
+
+/*
+ * For use with the TRACE_EVENT macro:
+ *
+ * We define a tracepoint, its arguments, its printk format
+ * and its 'fast binay record' layout.
+ *
+ * Firstly, name your tracepoint via TRACE_EVENT(name : the
+ * 'subsystem_event' notation is fine.
+ *
+ * Think about this whole construct as the
+ * 'trace_sched_switch() function' from now on.
+ *
+ *
+ * TRACE_EVENT(sched_switch,
+ *
+ * *
+ * * A function has a regular function arguments
+ * * prototype, declare it via TP_PROTO():
+ * *
+ *
+ * TP_PROTO(struct rq *rq, struct task_struct *prev,
+ * struct task_struct *next),
+ *
+ * *
+ * * Define the call signature of the 'function'.
+ * * (Design sidenote: we use this instead of a
+ * * TP_PROTO1/TP_PROTO2/TP_PROTO3 ugliness.)
+ * *
+ *
+ * TP_ARGS(rq, prev, next),
+ *
+ * *
+ * * Fast binary tracing: define the trace record via
+ * * TP_STRUCT__entry(). You can think about it like a
+ * * regular C structure local variable definition.
+ * *
+ * * This is how the trace record is structured and will
+ * * be saved into the ring buffer. These are the fields
+ * * that will be exposed to user-space in
+ * * /debug/tracing/events/<*>/format.
+ * *
+ * * The declared 'local variable' is called '__entry'
+ * *
+ * * __field(pid_t, prev_prid) is equivalent to a standard declariton:
+ * *
+ * * pid_t prev_pid;
+ * *
+ * * __array(char, prev_comm, TASK_COMM_LEN) is equivalent to:
+ * *
+ * * char prev_comm[TASK_COMM_LEN];
+ * *
+ *
+ * TP_STRUCT__entry(
+ * __array( char, prev_comm, TASK_COMM_LEN )
+ * __field( pid_t, prev_pid )
+ * __field( int, prev_prio )
+ * __array( char, next_comm, TASK_COMM_LEN )
+ * __field( pid_t, next_pid )
+ * __field( int, next_prio )
+ * ),
+ *
+ * *
+ * * Assign the entry into the trace record, by embedding
+ * * a full C statement block into TP_fast_assign(). You
+ * * can refer to the trace record as '__entry' -
+ * * otherwise you can put arbitrary C code in here.
+ * *
+ * * Note: this C code will execute every time a trace event
+ * * happens, on an active tracepoint.
+ * *
+ *
+ * TP_fast_assign(
+ * memcpy(__entry->next_comm, next->comm, TASK_COMM_LEN);
+ * __entry->prev_pid = prev->pid;
+ * __entry->prev_prio = prev->prio;
+ * memcpy(__entry->prev_comm, prev->comm, TASK_COMM_LEN);
+ * __entry->next_pid = next->pid;
+ * __entry->next_prio = next->prio;
+ * )
+ *
+ * *
+ * * Formatted output of a trace record via TP_printk().
+ * * This is how the tracepoint will appear under ftrace
+ * * plugins that make use of this tracepoint.
+ * *
+ * * (raw-binary tracing wont actually perform this step.)
+ * *
+ *
+ * TP_printk("task %s:%d [%d] ==> %s:%d [%d]",
+ * __entry->prev_comm, __entry->prev_pid, __entry->prev_prio,
+ * __entry->next_comm, __entry->next_pid, __entry->next_prio),
+ *
+ * );
+ *
+ * This macro construct is thus used for the regular printk format
+ * tracing setup, it is used to construct a function pointer based
+ * tracepoint callback (this is used by programmatic plugins and
+ * can also by used by generic instrumentation like SystemTap), and
+ * it is also used to expose a structured trace record in
+ * /debug/tracing/events/.
+ */
+
#define TRACE_EVENT(name, proto, args, struct, assign, print) \
DECLARE_TRACE(name, PARAMS(proto), PARAMS(args))