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SubjectRe: [PATCH 4/7] tracing: new format for specialized trace points

* KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> Hi Steven,
>
> > TRACE_EVENT(sched_switch,
> >
> > TP_PROTO(struct rq *rq, struct task_struct *prev,
> > struct task_struct *next),
> >
> > TP_ARGS(rq, prev, next),
> >
> > 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 )
> > ),
> >
> > 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),
> >
> > 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;
> > )
> > );
>
> Could you please write the documentation how to make new
> tracepont and use various TP_ macro? Some developer (include
> me) plan to add new tracepoint. but these macro usage is a bit
> difficult.

Sure, here's a commented/annotated variant:

/*
* 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'
*/
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/.

Steve: i flipped around TP_fast_assign() and TP_printk() in the
example above because it makes more sense that way. (we first
assign the record, then we print it out) I suspect we should
flip it around in the code too.

Ingo


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