On Wed, 15 Jul 2015 09:20:42 +0900
AKASHI Takahiro <takahiro.aka...@linaro.org> wrote:
> On 07/14/2015 10:31 PM, Steven Rostedt wrote:
> > On Tue, 14 Jul 2015 21:47:10 +0900
> > Jungseok Lee <jungseokle...@gmail.com> wrote:
> >
> >> Is the below example an unexpected result?
> >> Entry 17 and 18 are ftrace_call and ftrace_ops_no_ops, respectively.
>
> [snip]
>
> > Note, function tracing does not disable interrupts. This looks to be
> > that an interrupt came in while __aloc_skb() was being traced.
>
> Yeah, I think so, too. But if my insight is correct, it's not __alloc_skb()
> but one of functions that it calls. As I said in the commit log message
> of patch[1/3], the exact traced function will not be listed by
> save_stack_trace() because we don't create a stack frame at mcount().
> I think this is a flaw in the current implementation (on x86).
>
> what do you think, Steve?
>
mcount (well ftrace_call actually) does indeed create a stack frame for
itself *and* for what called it. At least on x86_64. See mcount_64.S.
With -pg -mfentry, it creates a stack frame. Without -mfentry, mcount
is called after the current function's frame is made so we don't need
to do much.
Here's what the -mfentry version does:
pushq %rbp
pushq 8*2(%rsp) /* this is the parent pointer */
pushq %rbp
movq %rsp, %rbp
pushq 8*3(%rsp) /* Return address to ftrace_call */
pushq %rbp
movq %rsp, %rbp
Thus the stack looks like this:
<---+
| | |
+------------------------------+ |
| return address for func | |
| return address for func_call | |
| original %rbp | |
+------------------------------+ |
| return address for func | |
| ptr to parent frame (%rbp) | ----+
+------------------------------| <-----+
| return address for func_call | |
| ptr to next frame (%rbp) | ------+
+------------------------------+ <---+
|
|
Current %rbp points to func_call frame -----+
The first box isn't used as a frame, but is used by ftrace_call to save
information to restore everything properly.
Thus, __alloc_skb() is what is currently being traced.
-- Steve
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