Re: [PATCH V2 1/1] perf/core: don't find side-band event from all pmus

[Arnaldo Carvalho de Melo]

Em Tue, Mar 29, 2016 at 02:06:09PM +0200, Peter Zijlstra escreveu:
> On Wed, Mar 23, 2016 at 11:24:37AM -0700, kan.liang@xxxxxxxxx wrote:
> > The V2 patch is mainly based on Peter's suggestion. But I didn't rename
> > perf_event_aux to perf_event_sb. Because it looks there are many aux things
> > in the codes, e.g. AUX area in ring buffer. I'm not sure if we need to change
> > all aux to sb. We may do the rename later in separate patch.
> 
> Right.. no problem doing that in a separate patch.
> 
> > +static void perf_event_sb_mask(unsigned int sb_mask,
> > +			       perf_event_aux_output_cb output,
> > +			       void *data)
> > +{
> > +	int sb;
> > +
> > +	for (sb = 0; sb < sb_nr; sb++) {
> > +		if (!(sb_mask & (1 << sb)))
> > +			continue;
> > +		perf_event_sb_iterate(sb, output, data);
> > +	}
> > +}
> 
> > @@ -5852,7 +5910,8 @@ static void perf_event_task(struct task_struct *task,
> >  
> >  	perf_event_aux(perf_event_task_output,
> >  		       &task_event,
> > -		       task_ctx);
> > +		       task_ctx,
> > +		       (1 << sb_task) | (1 << sb_mmap) | (1 << sb_comm));
> >  }
> 
> So one side-effect of this change is that the above event can be
> delivered 3 times if you're 'lucky'.
> 
> Acme; does userspace care?

So, when processing a PERF_RECORD_FORK there is some sanity checks in
machine__process_fork_event() (tools/perf/util/machine.c), and one that
is affected by copies is:

        struct thread *thread = machine__find_thread(machine,
                                                     event->fork.pid,
                                                     event->fork.tid);
<SNIP>
        /* if a thread currently exists for the thread id remove it */
        if (thread != NULL) {
                machine__remove_thread(machine, thread);
                thread__put(thread);
        }

        thread = machine__findnew_thread(machine, event->fork.pid,
                                         event->fork.tid);
<SNIP>

So we conceivably may end up with multiple 'struct thread' pointing to
the same kernel thread, being held as references somewhere (in a
hist_entry, for instance, if a PERF_RECORD_SAMPLE happens mid sentence).

It probably will cope, but can't we just emit one single record?

PERF_RECORD_EXIT are ok:

int machine__process_exit_event(struct machine *machine, union perf_event *event,
                                struct perf_sample *sample __maybe_unused)
{
        struct thread *thread = machine__find_thread(machine,
                                                     event->fork.pid,
                                                     event->fork.tid);

        if (dump_trace)
                perf_event__fprintf_task(event, stdout);

        if (thread != NULL) {
                thread__exited(thread);
                thread__put(thread);
        }

        return 0;
}

PERF_RECORD_COMM will unecessarily add a new COMM to its list, leading tooling
to think that there was a prctl(PR_SET_NAME). This could conceivably be annoyed
away by checking if the "new" COMM is the current one, again, can't the kernel
emit just one record?

- Arnaldo