tracing: explain why stack tracer is empty

If the stack tracing is disabled (by default) the stack_trace file
will only contain the header:

 # cat /debug/tracing/stack_trace
        Depth    Size      Location    (0 entries)
        -----    ----      --------

This can be frustrating to a developer that does not realize that the
stack tracer is disabled. This patch adds the following text:

  # cat /debug/tracing/stack_trace
        Depth    Size      Location    (0 entries)
        -----    ----      --------
 #
 #  Stack tracer disabled
 #
 # To enable the stack tracer, either add 'stacktrace' to the
 # kernel command line
 # or 'echo 1 > /proc/sys/kernel/stack_tracer_enabled'
 #

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index 4564fd9..91ccbf3 100644 (file)
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -245,6 +245,17 @@ static int trace_lookup_stack(struct seq_file *m, long i)
 #endif
 }
 
+static void print_disabled(struct seq_file *m)
+{
+       seq_puts(m, "#\n"
+                "#  Stack tracer disabled\n"
+                "#\n"
+                "# To enable the stack tracer, either add 'stacktrace' to the\n"
+                "# kernel command line\n"
+                "# or 'echo 1 > /proc/sys/kernel/stack_tracer_enabled'\n"
+                "#\n");
+}
+
 static int t_show(struct seq_file *m, void *v)
 {
        long i;
@@ -255,6 +266,10 @@ static int t_show(struct seq_file *m, void *v)
                           "    (%d entries)\n"
                           "        -----    ----      --------\n",
                           max_stack_trace.nr_entries);
+
+               if (!stack_tracer_enabled && !max_stack_size)
+                       print_disabled(m);
+
                return 0;
        }