perf evlist: Print hint for group

An event group is a critical relationship. There is a -g option that can
display the relationship. But it's hard for a user to know when should
this option be applied.

If there is an event group in the perf record, print a hint to suggest
the user apply the -g to display the group information.

With the patch,

  $ perf record -e "{cycles,instructions},instructions" sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.024 MB perf.data (4 samples) ]
  $

  $ perf evlist
  cycles
  instructions
  instructions
  # Tip: use 'perf evlist -g' to show group information

  $ perf evlist -g
  {cycles,instructions}
  instructions
  $

Committer testing:

So for a perf.data file _with_ a group:

  root@number:~# perf evlist -g
  {cpu_core/branch-instructions/pp,cpu_core/branches/}
  dummy:u
  root@number:~# perf evlist
  cpu_core/branch-instructions/pp
  cpu_core/branches/
  dummy:u
  # Tip: use 'perf evlist -g' to show group information
  root@number:~#

Then for something _without_ a group, no hint:

  root@number:~# perf record ls
  <SNIP>
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.035 MB perf.data (7 samples) ]
  root@number:~# perf evlist
  cpu_atom/cycles/P
  cpu_core/cycles/P
  dummy:u
  root@number:~#

No suggestion, good.

Suggested-by: Arnaldo Carvalho de Melo <[email protected]>
Reviewed-by: Ian Rogers <[email protected]>
Signed-off-by: Kan Liang <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Closes: https://lore.kernel.org/lkml/ZttgvduaKsVn1r4p@x1/
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: Kan Liang

tools/perf/builtin-evlist.c

@@ -36,7 +36,7 @@ static int __cmd_evlist(const char *file_name, struct perf_attr_details *details
 		.force     = details->force,
 	};
 	struct perf_tool tool;
-	bool has_tracepoint = false;
+	bool has_tracepoint = false, has_group = false;
 
 	perf_tool__init(&tool, /*ordered_events=*/false);
 	/* only needed for pipe mode */
@@ -54,11 +54,17 @@ static int __cmd_evlist(const char *file_name, struct perf_attr_details *details
 
 		if (pos->core.attr.type == PERF_TYPE_TRACEPOINT)
 			has_tracepoint = true;
+
+		if (!evsel__is_group_leader(pos))
+			has_group = true;
 	}
 
 	if (has_tracepoint && !details->trace_fields)
 		printf("# Tip: use 'perf evlist --trace-fields' to show fields for tracepoint events\n");
 
+	if (has_group && !details->event_group)
+		printf("# Tip: use 'perf evlist -g' to show group information\n");
+
 	perf_session__delete(session);
 	return 0;
 }