Re: [PATCH 35/40] perf record: Synthesize COMM event for a command line workload

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, May 19, 2015 at 04:46:43PM +0900, Namhyung Kim escreveu:
> On Mon, May 18, 2015 at 09:45:35AM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Mon, May 18, 2015 at 09:30:50AM +0900, Namhyung Kim escreveu:
> > > When perf creates a new child to profile, the events are enabled on
> > > exec().  And in this case, it doesn't synthesize any event for the
> > > child since they'll be generated during exec().  But there's an window
> > > between the enabling and the event generation.
> > > 
> > > It used to be overcome since samples are only in kernel (so we always
> > > have the map) and the comm is overridden by a later COMM event.
> > > However it won't work anymore since those samples will go to a missing
> > > thread now but the COMM event will create a (current) thread.  This
> > > leads to those early samples (like native_write_msr_safe) not having a
> > > comm but pid (like ':15328').
> >  
> > > So it needs to synthesize COMM event for the child explicitly before
> > > enabling so that it can have a correct comm.  But at this time, the
> > > comm will be "perf" since it's not exec-ed yet.
> > 
> > This looks reasonable, but I think it probably needs to be done
> > somewhere in perf_evlist__prepare_workload() or
> > perf_evlist__start_workload(), as this affects other tools as well, like
> > 'top', 'trace' and any other that may want to do this start-workload use
> > case.
> 
> Hmm.. I need to look at this again as it only affects on processing
> indexed data files which used to have a separate missing threads tree.

Humm, you're thinking about where you managed to reproduce the problem,
I am thinking outside indexing, etc, i.e. by definition we either enable
the event before we fork, so that we get the PERF_RECORD_FORK/COMM or we
synthesize it either from /proc or directly (preferred) if we decide to
do it after the fork/exec, right?

- Arnaldo

> That's the reason why I didn't put it in a generic place like you
> said.
> 
> However I changed not to use the separate tree - the purpose of the
> tree was to reduce lock acquisition on thread searching but it already
> grabs a rwlock with thread refcounting change.
> 
> Will check whether this is still needed..
> 
> Thanks,
> Namhyung
> 
> 
> > 
> > I also wonder if we can't overcome this without using /proc, i.e.
> > actually moving the "start the workload" to just before the fork, so
> > that the kernel covers that as well.
> > 
> > Or, alternatively, the thread can be created without having to look at
> > /proc at all, but by directly creating the struct thread, with the
> > correct COMM, pid, etc, that we know, since we forked it, etc.
> > 
> > - Arnaldo
> >  
> > > Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> > > ---
> > >  tools/perf/builtin-record.c | 18 +++++++++++++++++-
> > >  tools/perf/util/event.c     |  2 +-
> > >  tools/perf/util/event.h     |  5 +++++
> > >  3 files changed, 23 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> > > index 978ebf648aab..153f38e35555 100644
> > > --- a/tools/perf/builtin-record.c
> > > +++ b/tools/perf/builtin-record.c
> > > @@ -766,8 +766,24 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
> > >  	/*
> > >  	 * Let the child rip
> > >  	 */
> > > -	if (forks)
> > > +	if (forks) {
> > > +		union perf_event *comm_event;
> > > +
> > > +		comm_event = malloc(sizeof(*comm_event) + machine->id_hdr_size);
> > > +		if (comm_event == NULL)
> > > +			goto out_child;
> > > +
> > > +		err = perf_event__synthesize_comm(tool, comm_event,
> > > +						  rec->evlist->workload.pid,
> > > +						  process_synthesized_event,
> > > +						  machine);
> > > +		free(comm_event);
> > > +
> > > +		if (err < 0)
> > > +			goto out_child;
> > > +
> > >  		perf_evlist__start_workload(rec->evlist);
> > > +	}
> > >  
> > >  	if (opts->initial_delay) {
> > >  		usleep(opts->initial_delay * 1000);
> > > diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
> > > index 930d45d5a37a..3adca1302150 100644
> > > --- a/tools/perf/util/event.c
> > > +++ b/tools/perf/util/event.c
> > > @@ -165,7 +165,7 @@ static int perf_event__prepare_comm(union perf_event *event, pid_t pid,
> > >  	return 0;
> > >  }
> > >  
> > > -static pid_t perf_event__synthesize_comm(struct perf_tool *tool,
> > > +pid_t perf_event__synthesize_comm(struct perf_tool *tool,
> > >  					 union perf_event *event, pid_t pid,
> > >  					 perf_event__handler_t process,
> > >  					 struct machine *machine)
> > > diff --git a/tools/perf/util/event.h b/tools/perf/util/event.h
> > > index 40e02544f861..9aacd558ac0f 100644
> > > --- a/tools/perf/util/event.h
> > > +++ b/tools/perf/util/event.h
> > > @@ -452,6 +452,11 @@ int perf_event__synthesize_mmap_events(struct perf_tool *tool,
> > >  				       struct machine *machine,
> > >  				       bool mmap_data);
> > >  
> > > +pid_t perf_event__synthesize_comm(struct perf_tool *tool,
> > > +				  union perf_event *event, pid_t pid,
> > > +				  perf_event__handler_t process,
> > > +				  struct machine *machine);
> > > +
> > >  size_t perf_event__fprintf_comm(union perf_event *event, FILE *fp);
> > >  size_t perf_event__fprintf_mmap(union perf_event *event, FILE *fp);
> > >  size_t perf_event__fprintf_mmap2(union perf_event *event, FILE *fp);
> > > -- 
> > > 2.4.0
> > --
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