[PATCH v2] tracing: Add saved_tgids file to show cached pid to tgid mappings

[Michael Sartain <mikesart@fastmail.com>]

Export the cached pid / tgid mappings added by Joel Fernandes' patch [1]
in debugfs tracing saved_tgids file. This allows user apps to translate
the pids from a trace to their respective thread group.

Example saved_tgids file with pid / tgid values separated by ' ':

  # cat saved_tgids
  1048 1048
  1047 1047
  7 7
  1049 1047
  1054 1047
  1053 1047

Let userspace apps reading binary buffer know tgid's.

[1] https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1434654.html

Reviewed-by: Joel Fernandes <joelaf@google.com>
Signed-off-by: Michael Sartain <mikesart@fastmail.com>
---
[v2] Don't reuse cmdlines code for saved_tgids file.

 kernel/trace/trace.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c72c36c..e962fb3 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4701,6 +4701,76 @@ static const struct file_operations tracing_readme_fops = {
 	.llseek		= generic_file_llseek,
 };
 
+static void *saved_tgids_next(struct seq_file *m, void *v, loff_t *pos)
+{
+	int *ptr = v;
+
+	if (*pos || m->count)
+		ptr++;
+
+	(*pos)++;
+
+	for (; ptr <= &tgid_map[PID_MAX_DEFAULT]; ptr++) {
+		if (trace_find_tgid(*ptr))
+			return ptr;
+	}
+
+	return NULL;
+}
+
+static void *saved_tgids_start(struct seq_file *m, loff_t *pos)
+{
+	void *v;
+	loff_t l = 0;
+
+	if (!tgid_map)
+		return NULL;
+
+	v = &tgid_map[0];
+	while (l <= *pos) {
+		v = saved_tgids_next(m, v, &l);
+		if (!v)
+			return NULL;
+	}
+
+	return v;
+}
+
+static void saved_tgids_stop(struct seq_file *m, void *v)
+{
+}
+
+static int saved_tgids_show(struct seq_file *m, void *v)
+{
+	int pid = (int *)v - tgid_map;
+
+	seq_printf(m, "%d %d\n", pid, trace_find_tgid(pid));
+	return 0;
+}
+
+static const struct seq_operations tracing_saved_tgids_seq_ops = {
+	.start		= saved_tgids_start,
+	.stop		= saved_tgids_stop,
+	.next		= saved_tgids_next,
+	.show		= saved_tgids_show,
+};
+
+static int tracing_saved_tgids_open(struct inode *inode, struct file *filp)
+{
+	if (tracing_disabled)
+		return -ENODEV;
+
+	return seq_open(filp, &tracing_saved_tgids_seq_ops);
+}
+
+
+static const struct file_operations tracing_saved_tgids_fops = {
+	.open		= tracing_saved_tgids_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= seq_release,
+};
+
 static void *saved_cmdlines_next(struct seq_file *m, void *v, loff_t *pos)
 {
 	unsigned int *ptr = v;
@@ -7945,6 +8015,9 @@ static __init int tracer_init_tracefs(void)
 	trace_create_file("saved_cmdlines_size", 0644, d_tracer,
 			  NULL, &tracing_saved_cmdlines_size_fops);
 
+	trace_create_file("saved_tgids", 0444, d_tracer,
+			NULL, &tracing_saved_tgids_fops);
+
 	trace_eval_init();
 
 	trace_create_eval_file(d_tracer);
-- 
2.13.2