Re: [PATCH] perf/trace : Fix repetitious traces of perf on tracepoint When i use perf to trace the sched_wakeup_new tracepoint, there is a bug that output the same event repetitiously. It can be reproduced by :

[chengjian (D)]

On 2018/1/15 20:31, Peter Zijlstra wrote:


I'm sorry I gave an inappropriate example to make the phenomenon look 
confusing.

These events are registered per_cpu and attach in the the 
perf_event_ctxp of task too.
So the same event is placed in the process context CPU times.

perf record -e sched:sched_wakeup_new -vv bug_fork
    ......
    sys_perf_event_open: pid 1063  cpu 0  group_fd -1  flags 0x8 = 4
    sys_perf_event_open: pid 1063  cpu 1  group_fd -1  flags 0x8 = 5
    sys_perf_event_open: pid 1063  cpu 2  group_fd -1  flags 0x8 = 6
    sys_perf_event_open: pid 1063  cpu 3  group_fd -1  flags 0x8 = 8
    mmap size 528384B
    perf event ring buffer mmapped per cpu




    /*
     * If we got specified a target task, also iterate its context and
     * deliver this event there too.
     */
    // Here we want to see whether this event can be matched by the 
task not current.
    // task is the child now.(tracing task)
    if (task && task != current) {
        ......
        // the event mmaped per cpu, But
        // task->perf_event_ctxp[perf_sw_context] get all the events 
attach  in this task
        ctx = rcu_dereference(task->perf_event_ctxp[perf_sw_context]);

        list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {
            if (event->attr.type != PERF_TYPE_TRACEPOINT)
                continue;
            if (event->attr.config != entry->type)
                continue;
            //          event->cpu(3) != task_cpu(1)  bug_fork 1063 
[003]   128.001255: sched:sched_wakeup_new: comm=bug_fork pid=1064 
prio=120 target_cpu=002
            //          event->cpu(2) != task_cpu(1)  bug_fork 1063 
[003]   128.001255: sched:sched_wakeup_new: comm=bug_fork pid=1064 
prio=120 target_cpu=002
            //          event->cpu(1) == task_cpu(1)  bug_fork 1063 
[003]   128.001255: sched:sched_wakeup_new: comm=bug_fork pid=1064 
prio=120 target_cpu=002
            //          event->cpu(0) != task_cpu(1)  bug_fork 1063 
[003]   128.001255: sched:sched_wakeup_new: comm=bug_fork pid=1064 
prio=120 target_cpu=002
            //          but we only need one about event->cpu(1) == 
task_cpu(1), the others are repeated
            if (perf_tp_event_match(event, &data, regs))
                perf_swevent_event(event, count, &data, regs);
        }
unlock:
        rcu_read_unlock();
    }
    ```



> > after this patch, perf script(perf-1039, parent-1040, child-1041):
> >
> > 	bug_fork  1040 [002]    36.535963: sched:sched_wakeup_new:
> > comm=bug_fork pid=1041 prio=120 target_cpu=003
> > 	bug_fork  1040 [002]    36.536079: sched:sched_wakeup_new:
> > comm=bug_fork pid=1041 prio=120 target_cpu=003
> >
> > match it twice, an match for tracing current(parent) and an match
> > for task(child).
> So what is the bug? The parent gets one, the child gets one, that's
> correct, no?
>
> .
So the bug is, we match the same event NR_CPU times when tracing the 
waken(child process in my demo).


I will modify my commit, and make it clearer.


Thanks.

CHENG Jian