Commit-ID: 194ec34184869f0de1cf255c924fc5299e1b3d27 Gitweb: http://git.kernel.org/tip/194ec34184869f0de1cf255c924fc5299e1b3d27 Author: Steven Rostedt <srostedt@xxxxxxxxxx> AuthorDate: Tue, 13 Oct 2009 16:33:50 -0400 Committer: Ingo Molnar <mingo@xxxxxxx> CommitDate: Wed, 14 Oct 2009 08:13:53 +0200 function-graph/x86: Replace unbalanced ret with jmp 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. Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx> LKML-Reference: <20091013203425.042034383@xxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxx> --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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