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Message-ID: <20230621162944.916952813@goodmis.org>
Date: Wed, 21 Jun 2023 12:29:26 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: linux-kernel@...r.kernel.org
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
Donglin Peng <pengdonglin@...gfor.com.cn>
Subject: [for-next][PATCH 03/13] tracing: Add documentation for funcgraph-retval and
funcgraph-retval-hex
From: Donglin Peng <pengdonglin@...gfor.com.cn>
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@...nel.org>
Signed-off-by: Donglin Peng <pengdonglin@...gfor.com.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
Documentation/trace/ftrace.rst | 126 +++++++++++++++++++++++++++++++++
1 file changed, 126 insertions(+)
diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 027437b745a0..df2d3e57a83f 100644
--- 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
--
2.39.2
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