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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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