perf header: Display message if BPF/BTF info is empty

The perf.data file may contain a bpf_prog_info or bpf_btf feature. If
the contents of these are empty then nothing is displayed. Rather than
display nothing and not account for the file space, display an empty
message.

Signed-off-by: Ian Rogers <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Namhyung Kim <[email protected]>

Author: captain5050

tools/perf/util/header.c

@@ -1814,6 +1814,9 @@ static void print_bpf_prog_info(struct feat_fd *ff, FILE *fp)
 	root = &env->bpf_progs.infos;
 	next = rb_first(root);
 
+	if (!next)
+		printf("# bpf_prog_info empty\n");
+
 	while (next) {
 		struct bpf_prog_info_node *node;
 
@@ -1838,6 +1841,9 @@ static void print_bpf_btf(struct feat_fd *ff, FILE *fp)
 	root = &env->bpf_progs.btfs;
 	next = rb_first(root);
 
+	if (!next)
+		printf("# btf info empty\n");
+
 	while (next) {
 		struct btf_node *node;