tracing: Add saved_tgids file to show cached pid to tgid mappings

Export the cached pid / tgid mappings 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

Link: http://lkml.kernel.org/r/20170630004023.064965233@goodmis.org
Link: http://lkml.kernel.org/r/20170706040713.unwkumbta5menygi@mikesart-cos

Reviewed-by: Joel Fernandes <joelaf@google.com>
Signed-off-by: Michael Sartain <mikesart@fastmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

1 files changed, 73 insertions, 0 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 00e2e4169b1e..f079a8ca1117 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4688,6 +4688,76 @@ 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;
@@ -7920,6 +7990,9 @@ 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);