[PATCH v5 2/2] tracing: Add documentation for funcgraph-retval and graph_retval_hex

[Donglin Peng]

Add documentation for the two newly introduced options for the
function_graph tracer. The new option funcgraph-retval is used to
control whether or not to display the return value. The new option
graph_retval_hex is used to control the display format of the return
value.

Signed-off-by: Donglin Peng <pengdonglin@xxxxxxxxxxxxxx>
---
v5:
 - Describe the limitations of funcgraph-retval
---
 Documentation/trace/ftrace.rst | 75 ++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index b927fb2b94dc..797841d15da9 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -1328,6 +1328,19 @@ Options for function_graph tracer:
 	only a closing curly bracket "}" is displayed for
 	the return of a function.
 
+  funcgraph-retval
+	When set, the return event will include the function and
+	its return value. Note that even if the function return
+	type is void, a return value will also be printed, you
+	should ignore it. This option is off by default.
+
+  graph_retval_hex
+	Depend on function-retval. When set, the function return
+	value will be printed in hexadecimal format. If this is not
+	set and the return value looks like an error code, it will
+	be printed in signed decimal format, else in hexadecimal
+	format. This option is off by default.
+
   sleep-time
 	When running function graph tracer, to include
 	the time a task schedules out in its function.
@@ -2673,6 +2686,68 @@ It is default disabled.
     0)   1.757 us    |        } /* kmem_cache_free() */
     0)   2.861 us    |      } /* putname() */
 
+The return value of each traced function can be displayed. This
+feature can be very useful when you encounter system call failures
+and want to locate the function that return errors firstly.
+
+	- hide: echo nofuncgraph-retval > trace_options
+	- show: echo funcgraph-retval > trace_options
+
+  Example with funcgraph-retval::
+
+    1)               |    cgroup_migrate() {
+    1)   0.651 us    |      cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
+    1)               |      cgroup_migrate_execute() {
+    1)               |        cpu_cgroup_can_attach() {
+    1)               |          cgroup_taskset_first() {
+    1)   0.732 us    |            cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
+    1)   1.232 us    |          } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
+    1)   0.380 us    |          sched_rt_can_attach(); /* = 0x0 */
+    1)   2.335 us    |        } /* cpu_cgroup_can_attach = -22 */
+    1)   4.369 us    |      } /* cgroup_migrate_execute = -22 */
+    1)   7.143 us    |    } /* cgroup_migrate = -22 */
+
+The above example shows that the cgroup function cpu_cgroup_can_attach
+returned the error code -22 firstly, then we can read the code of this
+function to get the root cause.
+
+If the option graph_retval_hex is not set and the return value looks
+like an error code, it will be printed in signed decimal format. In
+other cases, it will be printed in hexadecimal format.
+
+	- hexadecimal or signed decimal: echo nograph_retval_hex > trace_options
+	- hexadecimal: echo graph_retval_hex > trace_options
+
+  Example with graph_retval_hex::
+
+    1)               |      cgroup_migrate() {
+    1)   0.651 us    |        cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
+    1)               |        cgroup_migrate_execute() {
+    1)               |          cpu_cgroup_can_attach() {
+    1)               |            cgroup_taskset_first() {
+    1)   0.732 us    |              cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
+    1)   1.232 us    |            } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
+    1)   0.380 us    |            sched_rt_can_attach(); /* = 0x0 */
+    1)   2.335 us    |          } /* cpu_cgroup_can_attach = 0xffffffea */
+    1)   4.369 us    |        } /* cgroup_migrate_execute = 0xffffffea */
+    1)   7.143 us    |      } /* cgroup_migrate = 0xffffffea */
+
+There are some limitations when using the funcgraph-retval currently:
+
+- Even if the function return type is void, a return value will still
+  be printed, and you can just ignore it.
+
+- Even if the return value is not an error code actually, it may be
+  displayed as an error code. You should read the code to check.
+  For example, both 0xfe and 0xfffe are be interpreted as -2.
+
+- Only the value of the first return register will be recorded and
+  printed even if the return values may be stored in two registers
+  actually. For example, both the eax and edx are used to store a
+  64 bit return value in the x86 architecture, and the eax stores
+  the low 32 bit, the edx stores the high 32 bit, however only the
+  value stored in eax will be recorded and printed.
+
 You can put some comments on specific functions by using
 trace_printk() For example, if you want to put a comment inside
 the __might_sleep() function, you just have to include
-- 
2.25.1