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Subject[PATCH 1/5] [PATCH 1/5] function-graph/x86: replace unbalanced ret with jmp
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.

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



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