From 823f9124fb2e33eeb624d139978a52089f8a02ae Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Tue, 10 Mar 2009 12:58:51 -0400 Subject: 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 Signed-off-by: Steven Rostedt --- include/linux/tracepoint.h | 103 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 103 insertions(+) (limited to 'include/linux') diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h index c7b09452514b..119ece224c21 100644 --- a/include/linux/tracepoint.h +++ b/include/linux/tracepoint.h @@ -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)) -- cgit v1.2.2