Re: [PATCH v1] perf cs-etm: Pass -1 as pid value for machine__set_current_tid()

[Leo Yan <leo.yan@linaro.org>]

On Thu, Nov 18, 2021 at 10:14:12AM -0700, Mathieu Poirier wrote:
> Good morning Leo,
> 
> On Sat, Nov 13, 2021 at 10:35:40PM +0800, Leo Yan wrote:
> > Currently, cs-etm passes the tid value for both tid and pid parameters
> > when calling machine__set_current_tid(), this can lead to confusion for
> > thread handling.  E.g. we arbitrarily pass the same value for pid and
> > tid, perf tool will be misled to consider it is a main thread (see
> > thread__main_thread()).
> > 
> > On the other hand, Perf tool only can retrieve tid from Arm CoreSight
> > context packet, and we have no chance to know pid (it maps to kernel's
> > task_struct::tgid) from hardware tracing data.  For this reason, this
> > patch passes -1 as pid for function machine__set_current_tid().
> > 
> > Signed-off-by: Leo Yan <leo.yan@linaro.org>
> > ---
> >  tools/perf/util/cs-etm.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> > index f323adb1af85..eed1a5930072 100644
> > --- a/tools/perf/util/cs-etm.c
> > +++ b/tools/perf/util/cs-etm.c
> > @@ -1118,7 +1118,7 @@ int cs_etm__etmq_set_tid(struct cs_etm_queue *etmq,
> >  	if (cs_etm__get_cpu(trace_chan_id, &cpu) < 0)
> >  		return err;
> >  
> > -	err = machine__set_current_tid(etm->machine, cpu, tid, tid);
> > +	err = machine__set_current_tid(etm->machine, cpu, -1, tid);
> 
> I remember wondering about what to do with the pid parameter when I wrote this
> patch... 

Some updates after I digged into the pid parameter for
machine__set_current_tid().

During the recording phase, the perf tool will capture events
PERF_RECORD_COMM and PERF_RECORD_FORK; these events contain pid/tid
for profiled program.  Below is an example for RECORD_FORK/RECORD_COMM
events in perf data file:

  0x89f0 [0x40]: event: 7
  .
  . ... raw event: size 64 bytes
  .  0000:  07 00 00 00 00 20 40 00 59 6d 00 00 59 6d 00 00  ..... @.Ym..Ym..
  .  0010:  5a 6d 00 00 59 6d 00 00 00 00 00 00 00 00 00 00  Zm..Ym..........
  .  0020:  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  .  0030:  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................

  0 0 0x89f0 [0x40]: PERF_RECORD_FORK(27993:27994):(27993:27993)

  0x8a30 [0x38]: event: 3
  .
  . ... raw event: size 56 bytes
  .  0000:  03 00 00 00 00 00 38 00 59 6d 00 00 5a 6d 00 00  ......8.Ym..Zm..
  .  0010:  6d 61 69 6e 00 00 00 00 00 00 00 00 00 00 00 00  main............
  .  0020:  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  .  0030:  00 00 00 00 00 00 00 00                          ........

  0 0 0x8a30 [0x38]: PERF_RECORD_COMM: main:27993/27994

In the reporting phase, perf tool will setup threads structure based on
the RECORD_FORK and RECORD_COMM events.  This means perf tool will set
the pid/tid for every thread, e.g. in up case, it allocates thread
context for 'main' program, and its one child thread is setup to
thread->pid_ as '27993' and thread->tid as '27994'.

Afterwards, when perf tool decodes CoreSight trace data and handles
context packet, at the end, machine__update_thread_pid() is invoked
for updating thread's pid:

  machine__update_thread_pid(struct machine *machine,
                             struct thread *th, pid_t pid)
  {
      if (pid == th->pid_ || pid == -1 || th->pid_ != -1)
          return;

      ...
  }

Whatever we pass the pid parameter as tid or '-1' from the caller
function machine__set_current_tid(), it doesn't change anything for the
thread context.  Since th->pid_ has been initialized and its value is
not '-1', no matter what's the pid value is passed via argument,
machine__update_thread_pid() will directly bail out.  This is why
before we pass 'tid' value rather than '-1' for pid, it doesn't cause
any error.

For this reason, this patch doesn't improve anything.  After discussed
with Mathieu offline, I decided to drop this change.  So update the
info in case someone is interested in the relevant info.

Thanks,
Leo

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