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

Masami Hiramatsu
2024-11-12T23:49:00Z

On Fri, 1 Nov 2024 10:14:48 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> 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"

Ah, I missed updating this document patch in v19. Need to fix that.

Thank you!

>
> > +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
>

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>