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SubjectRe: [PATCH 4/7] tracing: new format for specialized trace points
On Tue, Mar 10, 2009 at 12:57:14AM -0400, Steven Rostedt wrote:
> Here's the example. The only updated macro in this patch is the
> sched_switch trace point.

Note that we shouldn't keep two variants around long-term, that's
just going to cause confusion.

> The old method looked like this:
>
> TRACE_EVENT_FORMAT(sched_switch,
> TP_PROTO(struct rq *rq, struct task_struct *prev,
> struct task_struct *next),
> TP_ARGS(rq, prev, next),
> TP_FMT("task %s:%d ==> %s:%d",
> prev->comm, prev->pid, next->comm, next->pid),
> TRACE_STRUCT(
> TRACE_FIELD(pid_t, prev_pid, prev->pid)
> TRACE_FIELD(int, prev_prio, prev->prio)
> TRACE_FIELD_SPECIAL(char next_comm[TASK_COMM_LEN],
> next_comm,
> TP_CMD(memcpy(TRACE_ENTRY->next_comm,
> next->comm,
> TASK_COMM_LEN)))
> TRACE_FIELD(pid_t, next_pid, next->pid)
> TRACE_FIELD(int, next_prio, next->prio)
> ),
> TP_RAW_FMT("prev %d:%d ==> next %s:%d:%d")
> );
>
> The above method is hard to read and requires two format fields.
>
> The new method:
>
> /*
> * Tracepoint for task switches, performed by the scheduler:
> *
> * (NOTE: the 'rq' argument is not used by generic trace events,
> * but used by the latency tracer plugin. )
> */
> 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;
> )
> );

While the idea behing it seems like an improvement to me, the
implementation feel actually worse than the old one too me. I would
expect this to look more like:

struct trace_sched_switch {
char prev_comm[TASK_COMM_LEN],
pid_t prev_pid,
int prev_prio,
char next_comm[TASK_COMM_LEN],
pid_t next_pid,
int next_prio,
}
static void trace_sched_assign(struct trace_sched_switch *dst, struct rq *rq,
struct task_struct *prev, struct task_struct *next)
{
memcpy(dst->next_comm, next->comm, TASK_COMM_LEN);
dst->prev_pid = prev->pid;
dst->prev_prio = prev->prio;
memcpy(dst->prev_comm, prev->comm, TASK_COMM_LEN);
dst->next_pid = next->pid;
dst->next_prio = next->prio;
};

TRACE_EVENT(sched_switch,
trace_proto(struct rq *rq, struct task_struct *prev,
struct task_struct *next),
trace_args(rq, prev, next),
trace_struct(struct trace_sched_switch),
trace_assign(trace_sched_assign);

trace_pretty_print("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),
);



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