RFC ftrace: Use -mfentry when supported (this is for x86_64 right now)

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.

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(-)