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    SubjectRe: [PATCH 1/5] [PATCH 1/5] function-graph/x86: replace unbalanced ret with jmp
    * Steven Rostedt (rostedt@goodmis.org) wrote:
    > From: Steven Rostedt <srostedt@redhat.com>
    >
    > The function graph tracer replaces the return address with a hook to
    > trace the exit of the function call. This hook will finish by returning
    > to the real location the function should return to.
    >
    > But the current implementation uses a ret to jump to the real return
    > location. This causes a imbalance between calls and ret. That is
    > the original function does a call, the ret goes to the handler
    > and then the handler does a ret without a matching call.
    >
    > Although the function graph tracer itself still breaks the branch
    > predictor by replacing the original ret, by using a second ret and
    > causing an imbalance, it breaks the predictor even more.
    >
    > This patch replaces the ret with a jmp to keep the calls and ret
    > balanced. I tested this on one box and it showed a 1.7% increase in
    > performance. Another box only showed a small 0.3% increase. But no
    > box that I tested this on showed a decrease in performance by making this
    > change.

    This sounds exactly like what I proposed at LPC. I'm glad it shows
    actual improvements.

    Just to make sure I understand, the old sequence was:

    call fct
    call ftrace_entry
    ret to fct
    ret to ftrace_exit
    ret to caller

    and you now have:

    call fct
    call ftrace_entry
    ret to fct
    ret to ftrace_exit
    jmp to caller

    Am I correct ?

    Mathieu

    >
    > Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
    > Cc: Frederic Weisbecker <fweisbec@gmail.com>
    > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    > ---
    > arch/x86/kernel/entry_32.S | 7 ++-----
    > arch/x86/kernel/entry_64.S | 6 +++---
    > 2 files changed, 5 insertions(+), 8 deletions(-)
    >
    > diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
    > index c097e7d..7d52e9d 100644
    > --- a/arch/x86/kernel/entry_32.S
    > +++ b/arch/x86/kernel/entry_32.S
    > @@ -1185,17 +1185,14 @@ END(ftrace_graph_caller)
    >
    > .globl return_to_handler
    > return_to_handler:
    > - pushl $0
    > pushl %eax
    > - pushl %ecx
    > pushl %edx
    > movl %ebp, %eax
    > call ftrace_return_to_handler
    > - movl %eax, 0xc(%esp)
    > + movl %eax, %ecx
    > popl %edx
    > - popl %ecx
    > popl %eax
    > - ret
    > + jmp *%ecx
    > #endif
    >
    > .section .rodata,"a"
    > diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
    > index b5c061f..bd5bbdd 100644
    > --- a/arch/x86/kernel/entry_64.S
    > +++ b/arch/x86/kernel/entry_64.S
    > @@ -155,11 +155,11 @@ GLOBAL(return_to_handler)
    >
    > call ftrace_return_to_handler
    >
    > - movq %rax, 16(%rsp)
    > + movq %rax, %rdi
    > movq 8(%rsp), %rdx
    > movq (%rsp), %rax
    > - addq $16, %rsp
    > - retq
    > + addq $24, %rsp
    > + jmp *%rdi
    > #endif
    >
    >
    > --
    > 1.6.3.3
    >
    >

    --
    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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