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Message-Id: <20230321084650.769212-3-pengdonglin@sangfor.com.cn>
Date: Tue, 21 Mar 2023 01:46:50 -0700
From: Donglin Peng <pengdonglin@...gfor.com.cn>
To: mhiramat@...nel.org, rostedt@...dmis.org, linux@...linux.org.uk,
mark.rutland@....com, will@...nel.org, catalin.marinas@....com,
palmer@...belt.com, paul.walmsley@...ive.com, tglx@...utronix.de,
dave.hansen@...ux.intel.com, x86@...nel.org, mingo@...hat.com,
peterz@...radead.org, xiehuan09@...il.com, dinghui@...gfor.com.cn,
huangcun@...gfor.com.cn, dolinux.peng@...il.com
Cc: linux-trace-kernel@...r.kernel.org,
linux-riscv@...ts.infradead.org,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
Donglin Peng <pengdonglin@...gfor.com.cn>
Subject: [PATCH v6 2/2] tracing: Add documentation for funcgraph-retval and graph_retval_hex
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@...gfor.com.cn>
---
v6:
- Modify the limitations for funcgraph-retval
- Optimize the English expression
v5:
- Describe the limitations of funcgraph-retval
---
Documentation/trace/ftrace.rst | 73 ++++++++++++++++++++++++++++++++++
1 file changed, 73 insertions(+)
diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index b927fb2b94dc..1807f7752037 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 value of each traced function
+ will be printed after an equal sign "=". By default
+ this is off.
+
+ graph_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.
@@ -2673,6 +2686,66 @@ 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 graph_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 nograph_retval_hex > trace_options
+ - hexadecimal always: 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 return values are stored in two registers, only the value of
+ the first return register will be recorded and printed. For example,
+ in x86 architecture, both eax and edx are used to store a 64-bit
+ return value. Eax stores the low 32 bits, and edx stores the high
+ 32 bits. 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
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