lkml.org 
[lkml]   [2009]   [Oct]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
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


\
 
 \ /
  Last update: 2009-10-13 23:15    [from the cache]
©2003-2014 Jasper Spaans. Advertise on this site