tracing: Add documentation for funcgraph-retval and funcgraph-retval-hex

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

Link: https://lkml.kernel.org/r/2b5635f05146161b54c9ea6307e25efe5ccebdad.1680954589.git.pengdonglin@sangfor.com.cn
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Donglin Peng <pengdonglin@sangfor.com.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 027437b..df2d3e5 100644 (file)
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -1359,6 +1359,19 @@ Options for function_graph tracer:
        only a closing curly bracket "}" is displayed for
        the return of a function.
 
+  funcgraph-retval
+       When set, the return value of each traced function
+       will be printed after an equal sign "=". By default
+       this is off.
+
+  funcgraph-retval-hex
+       When set, the return value will always be printed
+       in hexadecimal format. If the option is not set and
+       the return value is an error code, it will be printed
+       in signed decimal format; otherwise it will also be
+       printed in hexadecimal format. By default, this option
+       is off.
+
   sleep-time
        When running function graph tracer, to include
        the time a task schedules out in its function.
@@ -2704,6 +2717,119 @@ 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 after
+an equal sign "=". When encountering system call failures, it
+can be verfy helpful to quickly locate the function that first
+returns an error code.
+
+       - 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 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.
+
+When the option funcgraph-retval-hex is not set, the return value can
+be displayed in a smart way. Specifically, if it is an error code,
+it will be printed in signed decimal format, otherwise it will
+printed in hexadecimal format.
+
+       - smart: echo nofuncgraph-retval-hex > trace_options
+       - hexadecimal: echo funcgraph-retval-hex > trace_options
+
+  Example with funcgraph-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 */
+
+At present, there are some limitations when using the funcgraph-retval
+option, and these limitations will be eliminated in the future:
+
+- Even if the function return type is void, a return value will still
+  be printed, and you can just ignore it.
+
+- Even if return values are stored in multiple registers, only the
+  value contained in the first register will be recorded and printed.
+  To illustrate, in the x86 architecture, eax and edx are used to store
+  a 64-bit return value, with the lower 32 bits saved in eax and the
+  upper 32 bits saved in edx. However, only the value stored in eax
+  will be recorded and printed.
+
+- In certain procedure call standards, such as arm64's AAPCS64, when a
+  type is smaller than a GPR, it is the responsibility of the consumer
+  to perform the narrowing, and the upper bits may contain UNKNOWN values.
+  Therefore, it is advisable to check the code for such cases. For instance,
+  when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values,
+  especially when larger types are truncated, whether explicitly or implicitly.
+  Here are some specific cases to illustrate this point:
+
+  **Case One**::
+
+  The function narrow_to_u8 is defined as follows::
+
+       u8 narrow_to_u8(u64 val)
+       {
+               // implicitly truncated
+               return val;
+       }
+
+  It may be compiled to::
+
+       narrow_to_u8:
+               < ... ftrace instrumentation ... >
+               RET
+
+  If you pass 0x123456789abcdef to this function and want to narrow it,
+  it may be recorded as 0x123456789abcdef instead of 0xef.
+
+  **Case Two**::
+
+  The function error_if_not_4g_aligned is defined as follows::
+
+       int error_if_not_4g_aligned(u64 val)
+       {
+               if (val & GENMASK(31, 0))
+                       return -EINVAL;
+
+               return 0;
+       }
+
+  It could be compiled to::
+
+       error_if_not_4g_aligned:
+               CBNZ    w0, .Lnot_aligned
+               RET                     // bits [31:0] are zero, bits
+                                       // [63:32] are UNKNOWN
+       .Lnot_aligned:
+               MOV    x0, #-EINVAL
+               RET
+
+  When passing 0x2_0000_0000 to it, the return value may be recorded as
+  0x2_0000_0000 instead of 0.
+
 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