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    Subject[PATCH 1/5] [PATCH 1/5] function-graph/x86: replace unbalanced ret with jmp
    From: Steven Rostedt <>

    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

    Cc: Mathieu Desnoyers <>
    Cc: Frederic Weisbecker <>
    Signed-off-by: Steven Rostedt <>
    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
    - 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

    .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


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