[PATCH v3 9/9] Documentation: tracing: Update fprobe event example with BTF field

Masami Hiramatsu (Google)
2023-07-26T13:01:51Z

Update fprobe event example with BTF data structure field specification.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
---
Changes in v2:
- Remove 'retval' and use '$retval'.
Changes in v3:
- Add description about mixture of '.' and '->' usage.
---
Documentation/trace/fprobetrace.rst | 64 +++++++++++++++++++++++++----------
1 file changed, 46 insertions(+), 18 deletions(-)

diff --git a/Documentation/trace/fprobetrace.rst b/Documentation/trace/fprobetrace.rst
index 7297f9478459..8e9bebcf0a2e 100644
--- a/Documentation/trace/fprobetrace.rst
+++ b/Documentation/trace/fprobetrace.rst
@@ -79,9 +79,9 @@ automatically set by the given name. ::
f:fprobes/myprobe vfs_read count=count pos=pos

It also chooses the fetch type from BTF information. For example, in the above
-example, the ``count`` is unsigned long, and the ``pos`` is a pointer. Thus, both
-are converted to 64bit unsigned long, but only ``pos`` has "%Lx" print-format as
-below ::
+example, the ``count`` is unsigned long, and the ``pos`` is a pointer. Thus,
+both are converted to 64bit unsigned long, but only ``pos`` has "%Lx"
+print-format as below ::

# cat events/fprobes/myprobe/format
name: myprobe
@@ -105,9 +105,47 @@ is expanded to all function arguments of the function or the tracepoint. ::
# cat dynamic_events
f:fprobes/myprobe vfs_read file=file buf=buf count=count pos=pos

-BTF also affects the ``$retval``. If user doesn't set any type, the retval type is
-automatically picked from the BTF. If the function returns ``void``, ``$retval``
-is rejected.
+BTF also affects the ``$retval``. If user doesn't set any type, the retval
+type is automatically picked from the BTF. If the function returns ``void``,
+``$retval`` is rejected.
+
+You can access the data fields of a data structure using allow operator ``->``
+(for pointer type) and dot operator ``.`` (for data structure type.)::
+
+# echo 't sched_switch preempt prev_pid=prev->pid next_pid=next->pid' >> dynamic_events
+
+The field access operators, ``->`` and ``.`` can be combined for accessing deeper
+members and other structure members pointed by the member. e.g. ``foo->bar.baz->qux``
+If there is non-name union member, you can directly access it as the C code does.
+For example::
+
+ struct {
+ union {
+ int a;
+ int b;
+ };
+ } *foo;
+
+To access ``a`` and ``b``, use ``foo->a`` and ``foo->b`` in this case.
+
+This data field access is available for the return value via ``$retval``,
+e.g. ``$retval->name``.
+
+For these BTF arguments and fields, ``:string`` and ``:ustring`` change the
+behavior. If these are used for BTF argument or field, it checks whether
+the BTF type of the argument or the data field is ``char *`` or ``char []``,
+or not. If not, it rejects applying the string types. Also, with the BTF
+support, you don't need a memory dereference operator (``+0(PTR)``) for
+accessing the string pointed by a ``PTR``. It automatically adds the memory
+dereference operator according to the BTF type. e.g. ::
+
+# echo 't sched_switch prev->comm:string' >> dynamic_events
+# echo 'f getname_flags%return $retval->name:string' >> dynamic_events
+
+The ``prev->comm`` is an embedded char array in the data structure, and
+``$retval->name`` is a char pointer in the data structure. But in both
+cases, you can use ``:string`` type to get the string.
+

Usage examples
--------------
@@ -161,10 +199,10 @@ parameters. This means you can access any field values in the task
structure pointed by the ``prev`` and ``next`` arguments.

For example, usually ``task_struct::start_time`` is not traced, but with this
-traceprobe event, you can trace it as below.
+traceprobe event, you can trace that field as below.
::

- # echo 't sched_switch comm=+1896(next):string start_time=+1728(next):u64' > dynamic_events
+ # echo 't sched_switch comm=next->comm:string next->start_time' > dynamic_events
# head -n 20 trace | tail
# TASK-PID CPU# ||||| TIMESTAMP FUNCTION
# | | | ||||| | |
@@ -176,13 +214,3 @@ traceprobe event, you can trace it as below.
<idle>-0 [000] d..3. 5606.690317: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="kworker/0:1" usage=1 start_time=137000000
kworker/0:1-14 [000] d..3. 5606.690339: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="swapper/0" usage=2 start_time=0
<idle>-0 [000] d..3. 5606.692368: sched_switch: (__probestub_sched_switch+0x4/0x10) comm="kworker/0:1" usage=1 start_time=137000000
-
-Currently, to find the offset of a specific field in the data structure,
-you need to build kernel with debuginfo and run `perf probe` command with
-`-D` option. e.g.
-::
-
- # perf probe -D "__probestub_sched_switch next->comm:string next->start_time"
- p:probe/__probestub_sched_switch __probestub_sched_switch+0 comm=+1896(%cx):string start_time=+1728(%cx):u64
-
-And replace the ``%cx`` with the ``next``.