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SubjectRe: {PATCH] Re: wasting time on page fault
On Wed, 04 Oct 2000, Brian Gerst wrote:

>Mikulas Patocka wrote:
>>
>> Hi.
>>
>> arch/i386/kernel/entry.S
>> xchgl %eax, ORIG_EAX(%esp) # orig_eax (get the error code. )
>> movl %esp,%edx
>> xchgl %ecx, ES(%esp) # get the address and save es.
>> pushl %eax # push the error code
>>
>> xchg with memory operand has implicit lock prefix and is slooooooow. It is
>
>Here is a patch that removes the xchg instructions. As an interesting
>note, I tested the difference in cycles between one xchgl and two movl
>instructions on 3 different processors (a Celeron, a K6-2, and an
>Athlon, all UP). The celeron and K6-2 showed noticable improvements.
>The real surprise was the the Athlon, where the times were identical.
>This means that either the Athlon has a really fast lock cycle or it is
>ignoring it.

I also did some testing on the performance of xchgl reg1,(reg2) vs the
the equivalent (but non-locked) sequence using 3 moves and a scratch reg:

Pentium II/III xchgl takes 19 cycles
3 moves takes just under 3 cycles
-> xchgl is more than 6 times slower
K6-III xchgl takes 77 (!!) cycles
3 moves takes just under 2 cycles
-> xchgl is more than 38 times slower

We could also do the following:

1. Move the error_code block from divide_error to page_fault;
this removes one jump from the page_fault path.
2. Apply Brian's patch to replace xchgl with ordinary moves;
this is an improvement for all faults.
3. Move the page_fault/error_code block to immediately precede
ret_from_exception; this removes one jump from the return path
and is an improvement for all faults.

The patch below against 2.4.0-test9 implements this. I'm been running
with this in place for a while now, with no observable negative effects.
I also disassembled arch/i386/kernel/entry.o before and after the patch
and verified that it didn't cause any short-offset jumps in frequently
executed paths to become long-offset jumps.

/Mikael

--- linux-2.4.0-test9/arch/i386/kernel/entry.S.~1~ Wed Oct 4 16:03:19 2000
+++ linux-2.4.0-test9/arch/i386/kernel/entry.S Mon Oct 9 16:30:16 2000
@@ -256,7 +256,35 @@
movl $-ENOSYS,EAX(%esp)
jmp ret_from_sys_call

- ALIGN
+ENTRY(page_fault)
+ pushl $ SYMBOL_NAME(do_page_fault)
+ ALIGN /* for the benefit of the other exception paths */
+error_code:
+ pushl %ds
+ pushl %eax
+ xorl %eax,%eax
+ pushl %ebp
+ pushl %edi
+ pushl %esi
+ pushl %edx
+ decl %eax # eax = -1
+ pushl %ecx
+ pushl %ebx
+ cld
+ movl %es,%ecx
+ movl ORIG_EAX(%esp), %esi # get the error code
+ movl ES(%esp), %edi # get the function address
+ movl %eax, ORIG_EAX(%esp)
+ movl %ecx, ES(%esp)
+ movl %esp,%edx
+ pushl %esi # push the error code
+ pushl %edx # push the pt_regs pointer
+ movl $(__KERNEL_DS),%edx
+ movl %edx,%ds
+ movl %edx,%es
+ GET_CURRENT(%ebx)
+ call *%edi
+ addl $8,%esp
ret_from_exception:
#ifdef CONFIG_SMP
GET_CURRENT(%ebx)
@@ -291,32 +319,7 @@
ENTRY(divide_error)
pushl $0 # no error code
pushl $ SYMBOL_NAME(do_divide_error)
- ALIGN
-error_code:
- pushl %ds
- pushl %eax
- xorl %eax,%eax
- pushl %ebp
- pushl %edi
- pushl %esi
- pushl %edx
- decl %eax # eax = -1
- pushl %ecx
- pushl %ebx
- cld
- movl %es,%ecx
- xchgl %eax, ORIG_EAX(%esp) # orig_eax (get the error code. )
- movl %esp,%edx
- xchgl %ecx, ES(%esp) # get the address and save es.
- pushl %eax # push the error code
- pushl %edx
- movl $(__KERNEL_DS),%edx
- movl %edx,%ds
- movl %edx,%es
- GET_CURRENT(%ebx)
- call *%ecx
- addl $8,%esp
- jmp ret_from_exception
+ jmp error_code

ENTRY(coprocessor_error)
pushl $0
@@ -408,10 +411,6 @@

ENTRY(alignment_check)
pushl $ SYMBOL_NAME(do_alignment_check)
- jmp error_code
-
-ENTRY(page_fault)
- pushl $ SYMBOL_NAME(do_page_fault)
jmp error_code

ENTRY(machine_check)
-
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