Re: [PATCH v18 16/17] Documentation: probes: Update fprobe on function-graph tracer

Steven Rostedt
2024-11-01T14:13:52Z

On Sat, 26 Oct 2024 13:38:46 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
>
> Update fprobe documentation for the new fprobe on function-graph
> tracer. This includes some bahvior changes and pt_regs to
> ftrace_regs interface change.
>
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
> Changes in v2:
> - Update @fregs parameter explanation.
> ---
> Documentation/trace/fprobe.rst | 42 ++++++++++++++++++++++++++--------------
> 1 file changed, 27 insertions(+), 15 deletions(-)
>
> diff --git a/Documentation/trace/fprobe.rst b/Documentation/trace/fprobe.rst
> index 196f52386aaa..f58bdc64504f 100644
> --- a/Documentation/trace/fprobe.rst
> +++ b/Documentation/trace/fprobe.rst
> @@ -9,9 +9,10 @@ Fprobe - Function entry/exit probe
> Introduction
> ============
>
> -Fprobe is a function entry/exit probe mechanism based on ftrace.
> -Instead of using ftrace full feature, if you only want to attach callbacks
> -on function entry and exit, similar to the kprobes and kretprobes, you can
> +Fprobe is a function entry/exit probe mechanism based on the function-graph
> +tracer.

You could still say "ftrace" as function-graph is part of the "ftrace"
infrastructure. But I don't care either way.

> +Instead of tracing all functions, if you want to attach callbacks on specific
> +function entry and exit, similar to the kprobes and kretprobes, you can
> use fprobe. Compared with kprobes and kretprobes, fprobe gives faster
> instrumentation for multiple functions with single handler. This document
> describes how to use fprobe.
> @@ -91,12 +92,14 @@ The prototype of the entry/exit callback function are as follows:
>
> .. code-block:: c
>
> - int entry_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct pt_regs *regs, void *entry_data);
> + int entry_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct ftrace_regs *fregs, void *entry_data);
>
> - void exit_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct pt_regs *regs, void *entry_data);
> + void exit_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct ftrace_regs *fregs, void *entry_data);
>
> -Note that the @entry_ip is saved at function entry and passed to exit handler.
> -If the entry callback function returns !0, the corresponding exit callback will be cancelled.
> +Note that the @entry_ip is saved at function entry and passed to exit
> +handler.
> +If the entry callback function returns !0, the corresponding exit callback
> +will be cancelled.
>
> @fp
> This is the address of `fprobe` data structure related to this handler.
> @@ -112,12 +115,10 @@ If the entry callback function returns !0, the corresponding exit callback will
> This is the return address that the traced function will return to,
> somewhere in the caller. This can be used at both entry and exit.
>
> -@regs
> - This is the `pt_regs` data structure at the entry and exit. Note that
> - the instruction pointer of @regs may be different from the @entry_ip
> - in the entry_handler. If you need traced instruction pointer, you need
> - to use @entry_ip. On the other hand, in the exit_handler, the instruction
> - pointer of @regs is set to the current return address.
> +@fregs
> + This is the `ftrace_regs` data structure at the entry and exit. This
> + includes the function parameters, or the return values. So user can
> + access thos values via appropriate `ftrace_regs_*` APIs.
>
> @entry_data
> This is a local storage to share the data between entry and exit handlers.
> @@ -125,6 +126,17 @@ If the entry callback function returns !0, the corresponding exit callback will
> and `entry_data_size` field when registering the fprobe, the storage is
> allocated and passed to both `entry_handler` and `exit_handler`.
>
> +Entry data size and exit handlers on the same function
> +======================================================
> +
> +Since the entry data is passed via per-task stack and it is has limited size,

"and it has limited size"

> +the entry data size per probe is limited to `15 * sizeof(long)`. You also need
> +to take care that the different fprobes are probing on the same function, this
> +limit becomes smaller. The entry data size is aligned to `sizeof(long)` and
> +each fprobe which has exit handler uses a `sizeof(long)` space on the stack,
> +you should keep the number of fprobes on the same function as small as
> +possible.

-- Steve

> +
> Share the callbacks with kprobes
> ================================
>
> @@ -165,8 +177,8 @@ This counter counts up when;
> - fprobe fails to take ftrace_recursion lock. This usually means that a function
> which is traced by other ftrace users is called from the entry_handler.
>
> - - fprobe fails to setup the function exit because of the shortage of rethook
> - (the shadow stack for hooking the function return.)
> + - fprobe fails to setup the function exit because of failing to allocate the
> + data buffer from the per-task shadow stack.
>
> The `fprobe::nmissed` field counts up in both cases. Therefore, the former
> skips both of entry and exit callback and the latter skips the exit