On 2016/3/23 17:50, Peter Zijlstra wrote:
On Mon, Mar 14, 2016 at 09:59:43AM +0000, Wang Nan wrote:
Convert perf_output_begin to __perf_output_begin and make the later
function able to write records from the end of the ring buffer.
Following commits will utilize the 'backward' flag.
This patch doesn't introduce any extra performance overhead since we
use always_inline.
So while I agree that with __always_inline and constant propagation we
_should_ end up with the same code, we have:
$ size defconfig-build/kernel/events/ring_buffer.o.{pre,post}
text data bss dec hex filename
3785 2 0 3787 ecb
defconfig-build/kernel/events/ring_buffer.o.pre
3673 2 0 3675 e5b
defconfig-build/kernel/events/ring_buffer.o.post
The patch actually makes the file shrink.
So I think we still want to have some actual performance numbers.
In my environment the two objects are nearly idential:
$ objdump -d kernel/events/ring_buffer.o.new > ./out.new.S
$ objdump -d kernel/events/ring_buffer.o.old > ./out.old.S
--- ./out.old.S 2016-03-25 12:18:52.060656423 +0000
+++ ./out.new.S 2016-03-25 12:18:45.376630269 +0000
@@ -1,5 +1,5 @@
-kernel/events/ring_buffer.o.old: file format elf64-x86-64
+kernel/events/ring_buffer.o.new: file format elf64-x86-64
Disassembly of section .text:
@@ -320,7 +320,7 @@
402: 4d 8d 04 0f lea (%r15,%rcx,1),%r8
406: 48 89 c8 mov %rcx,%rax
409: 4c 0f b1 43 40 cmpxchg %r8,0x40(%rbx)
- 40e: 48 39 c8 cmp %rcx,%rax
+ 40e: 48 39 c1 cmp %rax,%rcx
411: 75 b4 jne 3c7 <perf_output_begin+0xc7>
413: 48 8b 73 58 mov 0x58(%rbx),%rsi
417: 48 8b 43 68 mov 0x68(%rbx),%rax
@@ -357,7 +357,7 @@
480: 85 c0 test %eax,%eax
482: 0f 85 02 ff ff ff jne 38a <perf_output_begin+0x8a>
488: 48 c7 c2 00 00 00 00 mov $0x0,%rdx
- 48f: be 7c 00 00 00 mov $0x7c,%esi
+ 48f: be 89 00 00 00 mov $0x89,%esi
494: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
49b: c6 05 00 00 00 00 01 movb $0x1,0x0(%rip) # 4a2
<perf_output_begin+0x1a2>
4a2: e8 00 00 00 00 callq 4a7 <perf_output_begin+0x1a7>
@@ -874,7 +874,7 @@
c39: eb e7 jmp c22 <perf_aux_output_begin+0x172>
c3b: 80 3d 00 00 00 00 00 cmpb $0x0,0x0(%rip) # c42
<perf_aux_output_begin+0x192>
c42: 75 93 jne bd7 <perf_aux_output_begin+0x127>
- c44: be 2b 01 00 00 mov $0x12b,%esi
+ c44: be 49 01 00 00 mov $0x149,%esi
c49: 48 c7 c7 00 00 00 00 mov $0x0,%rdi
c50: e8 00 00 00 00 callq c55 <perf_aux_output_begin+0x1a5>
c55: c6 05 00 00 00 00 01 movb $0x1,0x0(%rip) # c5c
<perf_aux_output_begin+0x1ac>
I think you enabled some unusual config options?
Thank you.
