Re: [PATCH 2/2] perf record: Synthesize COMM event for a command line workload

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

Em Mon, Sep 21, 2015 at 09:26:49AM +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 if events are processed and displayed before the
> COMM event overrides like in 'perf script'.  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.
> 
> Acked-by: Jiri Olsa <jolsa@redhat.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  tools/perf/builtin-record.c | 41 ++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 142eeb341b29..b83373adb9f8 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -469,6 +469,43 @@ static void workload_exec_failed_signal(int signo __maybe_unused,
>  	child_finished = 1;
>  }
>  
> +static int synthesize_workload_comm_event(struct perf_evlist *evlist, void *arg)
> +{
> +	union perf_event *event;
> +	struct record *rec = arg;
> +	struct machine *machine = &rec->session->machines.host;
> +	int pid = evlist->workload.pid;
> +	const char *comm_str = program_invocation_short_name;
> +	size_t comm_size, total_size;
> +	int ret;
> +
> +	comm_size = PERF_ALIGN(strlen(comm_str) + 1, sizeof(u64));
> +	total_size = sizeof(event->comm) + machine->id_hdr_size;
> +	/*
> +	 * (aligned) comm size might be smaller than expected size
> +	 * (i.e.  size of event->comm.comm[]), in that case it needs
> +	 * to shrink the total size.
> +	 */
> +	if (comm_size < sizeof(event->comm.comm))
> +		total_size -= sizeof(event->comm.comm) - comm_size;
> +
> +	event = zalloc(total_size);
> +	if (event == NULL)
> +		return -ENOMEM;
> +
> +	event->comm.header.type = PERF_RECORD_COMM;
> +	event->comm.header.size = total_size;
> +
> +	event->comm.pid = pid;
> +	event->comm.tid = pid;
> +	strncpy(event->comm.comm, comm_str, comm_size);
> +
> +	ret = record__write(rec, event, total_size);
> +
> +	free(event);
> +	return ret;
> +}
> +
>  static void snapshot_sig_handler(int sig);
>  
>  static int __cmd_record(struct record *rec, int argc, const char **argv)
> @@ -637,7 +674,9 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
>  	 * Let the child rip
>  	 */
>  	if (forks)
> -		perf_evlist__start_workload(rec->evlist);
> +		perf_evlist__start_workload_ex(rec->evlist,
> +					       synthesize_workload_comm_event,
> +					       rec);

Why not call it directly? I.e.:

	if (forks) {
		err = synthesize_workload_comm_event(evlist, rec);
		if (!err)
			err = perf_evlist__start_workload(rec->evlist);
	}

Since, from what I saw, the very first thing that
perf_evlist__start_workload_ex() does is to call the callback?

Also, don't we have already a synthesize_comm routine? I.e. can't
perf_event__prepare_comm() be used here?

Something like:

	union perf_event event;
	pid_t tgid, ppid;

	err = perf_event__prepare_comm(&event, evlist->workload.pid,
				       machine, &tgid, &ppid);
	if (!err)
		err = record__write(rec, &event, sizeof(event.comm));

- Arnaldo

>  
>  	if (opts->initial_delay) {
>  		usleep(opts->initial_delay * 1000);
> -- 
> 2.5.0