Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this isfor x86_64 right now)

[Masami Hiramatsu]

(2011/02/10 5:02), Steven Rostedt wrote:
> Thanks to Andi Kleen gcc 4.6.0 now supports -mfentry with the -pg option
> to place a call to __fentry__ at the very beginning of the function
> instead of after the fact.
> 
> The old way:
> 
> 00000000000000c4 <atomic_long_add>:
>       c4:       55                      push   %rbp
>       c5:       48 89 e5                mov    %rsp,%rbp
>       c8:       e8 00 00 00 00          callq  cd <atomic_long_add+0x9>
>                         c9: R_X86_64_PC32       mcount-0x4
>       cd:       f0 48 01 3e             lock add %rdi,(%rsi)
>       d1:       c9                      leaveq 
>       d2:       c3                      retq   
> 
> The new way:
> 
> 000000000000009e <atomic_long_add>:
>       9e:       e8 00 00 00 00          callq  a3 <atomic_long_add+0x5>
>                         9f: R_X86_64_PC32       __fentry__-0x4
>       a3:       55                      push   %rbp
>       a4:       48 89 e5                mov    %rsp,%rbp
>       a7:       f0 48 01 3e             lock add %rdi,(%rsi)
>       ab:       5d                      pop    %rbp
>       ac:       c3                      retq   
> 
> Note, with -mfentry, frame pointers is no longer required
> by the function tracer. But this patch series still requires
> FRAME_POINTER to be set, since I need to figure out a good way to
> enable FRAME_POINTER only if gcc doesn't support this. But that can
> come later.
> 
> With the new __fentry__, we could possible record the parameters
> of a function call. This may take some work, and perhaps be
> a little like kprobes. But it is doable.

Hm, very interesting. With this feature and dynamic-ftrace,
it may be possible to use it automatically instead of kprobes
via kprobe-events when user probes function entry.

Thanks,

> This is still just RFC. I only wrote the code to support x86_64
> even though gcc 4.6.0 also supports i386. I figured I would post
> this first to get peoples reactions before converting
> i386 too. Other archs can soon follow.
> 
> -- Steve
> 
> The following patches are in:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-trace.git
> 
>     branch: rfc/tracing/fentry
> 
> 
> Steven Rostedt (4):
>       ftrace: Make recordmcount.c handle __fentry__
>       ftrace: Add -mfentry to Makefile on function tracer
>       ftrace: Do not test frame pointers if -mfentry is used
>       ftrace/x86: Add support for -mfentry to x86_64
> 
> ----
>  Makefile                             |    6 +++++-
>  arch/x86/Kconfig                     |    1 +
>  arch/x86/include/asm/ftrace.h        |    7 ++++++-
>  arch/x86/kernel/entry_64.S           |   17 ++++++++++++++++-
>  arch/x86/kernel/x8664_ksyms_64.c     |    6 +++++-
>  kernel/trace/Kconfig                 |    5 +++++
>  kernel/trace/trace_functions_graph.c |    5 ++++-
>  scripts/recordmcount.h               |    4 +++-
>  8 files changed, 45 insertions(+), 6 deletions(-)


-- 
Masami HIRAMATSU
2nd Dept. Linux Technology Center
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
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