Re: [PATCH v2 9/9] Documentation: tracing: Update fprobe event example with BTF field

[Alan Maguire <alan.maguire@oracle.com>]

On 17/07/2023 16:24, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Update fprobe event example with BTF data structure field specification.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

One suggestion below, but

Reviewed-by: Alan Maguire <alan.maguire@oracle.com>

> ---
>  Changes in v2:
>   - Remove 'retval' and use '$retval'.
> ---
>  Documentation/trace/fprobetrace.rst |   50 ++++++++++++++++++++++-------------
>  1 file changed, 32 insertions(+), 18 deletions(-)
> 
> diff --git a/Documentation/trace/fprobetrace.rst b/Documentation/trace/fprobetrace.rst
> index 7297f9478459..e9e764fadf14 100644
> --- a/Documentation/trace/fprobetrace.rst
> +++ b/Documentation/trace/fprobetrace.rst
> @@ -79,9 +79,9 @@ automatically set by the given name. ::
>   f:fprobes/myprobe vfs_read count=count pos=pos
>  
>  It also chooses the fetch type from BTF information. For example, in the above
> -example, the ``count`` is unsigned long, and the ``pos`` is a pointer. Thus, both
> -are converted to 64bit unsigned long, but only ``pos`` has "%Lx" print-format as
> -below ::
> +example, the ``count`` is unsigned long, and the ``pos`` is a pointer. Thus,
> +both are converted to 64bit unsigned long, but only ``pos`` has "%Lx"
> +print-format as below ::
>  
>   # cat events/fprobes/myprobe/format
>   name: myprobe
> @@ -105,9 +105,33 @@ is expanded to all function arguments of the function or the tracepoint. ::
>   # cat dynamic_events
>   f:fprobes/myprobe vfs_read file=file buf=buf count=count pos=pos
>  
> -BTF also affects the ``$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
> +

Could you describe what field access combinations are supported here;
i.e. foo->bar[.baz]?

> +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 +185,10 @@ 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 +200,3 @@ 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``.
> 
>