[PATCH] perf evlist: Print hint for group

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>

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

Suggested-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/ZttgvduaKsVn1r4p@x1/
Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
---
 tools/perf/builtin-evlist.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/perf/builtin-evlist.c b/tools/perf/builtin-evlist.c
index 818ab21c3f73..a9bd7bbef5a9 100644
--- a/tools/perf/builtin-evlist.c
+++ b/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;
 }
-- 
2.38.1