Commit 823f9124 authored by Steven Rostedt's avatar Steven Rostedt
Browse files

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: default avatarKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: default avatarSteven Rostedt <srostedt@redhat.com>
parent 30a8fecc
......@@ -157,6 +157,109 @@ static inline void tracepoint_synchronize_unregister(void)
#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))
......
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