linux-kernel - RE: [BUG] perf: can not resolve symbols for forked threads

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Date:	Tue, 24 Jun 2014 06:47:38 +0000
From:	Tony Lu <zlu@...era.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...hat.com>,
	"Arnaldo Carvalho de Melo" <acme@...stprotocols.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...era.com>
Subject: RE: [BUG] perf: can not resolve symbols for forked threads

Thanks for your reply.

>-----Original Message-----
>From: Peter Zijlstra [mailto:peterz@...radead.org]
>Sent: Monday, June 23, 2014 5:13 PM
>To: Tony Lu
>Cc: Paul Mackerras; Ingo Molnar; Arnaldo Carvalho de Melo;
>linux-kernel@...r.kernel.org; Chris Metcalf
>Subject: Re: [BUG] perf: can not resolve symbols for forked threads
>
>On Thu, Jun 19, 2014 at 07:39:42AM +0000, Tony Lu wrote:
>> Hi
>>
>> I got the below output that shows perf can not resolve symbols for
>> forked threads. I did a system-wide collection from all CPUs after the
>> application hello run.
>
>There's no fork() in... :-)

Yes, but perf regards pthread_created threads as forked threads, and a PERF_RECORD_FORK event will be delivered when a thread is forked or cloned.

>
>> #include <pthread.h>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <unistd.h>
>> #define NUM_THREADS     5
>>
>> void foo(void)
>> {
>>   long i = 1000000000;
>>   while (i--) {
>>     ;
>>   }
>> }
>>
>> void *PrintHello(void *threadid)
>> {
>>    long tid;
>>    tid = (long)threadid;
>>    printf("Hello World! It's me, thread #%ld!\n", tid);
>>    foo();
>>    pthread_exit(NULL);
>> }
>>
>> int main(int argc, char *argv[])
>> {
>>    pthread_t threads[NUM_THREADS];
>>    int rc;
>>    long t;
>>    for(t=0;t<NUM_THREADS;t++){
>>      printf("In main: creating thread %ld\n", t);
>>      rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
>>      if (rc){
>>        printf("ERROR; return code from pthread_create() is %d\n", rc);
>>        exit(-1);
>>        }
>>      }
>>
>>    /* Last thing that main() should do */
>>    pthread_exit(NULL);
>> }
>
>That pthread_exit() is the problem; this results in:
>
>29456 pts/23   Zl     0:00          |               \_ [hello] <defunct>
>
>You want to wait for the threads to complete using pthread_join().
>
>I suspect the defunct state hides the process.

Yes, using pthread_join() can workaround this problem.
Does that mean the parent thread can not exit before the child threads? It still seems like a perf bug.

-Thanks
-Tony
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