Re: [PATCH] perf bench: add --prefault option for causing page faults before benchmark

[Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>]

On 2010年11月10日 18:29, Ingo Molnar wrote:
>
> * Hitoshi Mitake<mitake@dcl.info.waseda.ac.jp>  wrote:
>
>> This patch adds --prefault option to perf bench mem memcpy.
>> If user specify this option to perf bench mem memcpy, overhead of
>> page faults will be removed from the score of memcpy().
>>
>> Example of usage:
>> | % ./perf bench mem memcpy -l 500MB
>> | # Running mem/memcpy benchmark...
>> | # Copying 500MB Bytes from 0x7fc036749010 to 0x7fc055b4a010 ...
>> |
>> |      628.526821 MB/Sec
>> | mitake@X201i:~/linux/.../tools/perf% ./perf bench mem memcpy -l 500MB --prefault
>> | # Running mem/memcpy benchmark...
>> | # Copying 500MB Bytes from 0x7ff1b45e2010 to 0x7ff1d39e3010 ...
>> |
>> |        4.849256 GB/Sec
>
> Ok, looks rather useful.
>
> We are rather close to being able to apply these bits. We need a resolution for the
> arch/x86/lib/memcpy_64.S details. The ugliest are these kinds of #ifdefs:
>
> +#ifndef PERF_BENCH
>   .Lmemcpy_e:
>          .previous
> +#endif
>
> What happens if we keep that label in place?

This is the part of objdump -D arch/x86/lib/memcpy_64.o,

Disassembly of section .altinstr_replacement:

0000000000000000 <.altinstr_replacement>:
    0:   48 89 f8                mov    %rdi,%rax
    3:   89 d1                   mov    %edx,%ecx
    5:   c1 e9 03                shr    $0x3,%ecx
    8:   83 e2 07                and    $0x7,%edx
    b:   f3 48 a5                rep movsq %ds:(%rsi),%es:(%rdi)
    e:   89 d1                   mov    %edx,%ecx
   10:   f3 a4                   rep movsb %ds:(%rsi),%es:(%rdi)
   12:   c3                      retq

I didn't know that we can use the symbol name which start with '.',
and it seems that such a symbol is eliminated from object file.

We can know the start address of .Lmemcpy_c, the rep version of memcpy()
because the start address is stored in another section,
.altinstructions like this.

These information can be exploited for our purose, I'll try it.

>
> This:
>
> +#ifndef PERF_BENCH
>   ENTRY(__memcpy)
>   ENTRY(memcpy)
>          CFI_STARTPROC
> +#else
> +	.globl  memcpy_x86_64_unrolled
> +memcpy_x86_64_unrolled:
> +#endif
>
> Could be removed if you defined an ENTRY() macro in perf, right?
>
> This:
>
> +#ifndef PERF_BENCH
> +
>          CFI_ENDPROC
>   ENDPROC(memcpy)
>   ENDPROC(__memcpy)
>
> Could be solved by defining ENDPROC()/etc. macros in perf, right?
>
> We could remove this #ifdef:
>
> +#ifndef PERF_BENCH
> +
>   #include<linux/linkage.h>
>
>   #include<asm/cpufeature.h>
>   #include<asm/dwarf2.h>
>
> +#endif /* PERF_BENCH */
>
> if you added empty linkage.h, cpufeature.h and dwarf2.h files as
> tools/perf/util/include/linux/linkage.h, tools/perf/util/include/asm/cpufeature.h.
>
> That linkage.h file could even contain a short perf version of the ENTRY() macro,
> etc.
>
> That way we can avoid having to touch arch/x86/lib/memcpy_64.S altogether.

Thanks for your advice. adding empty headers and macros
will be the smart way to include memcpy_64.S without modification.

Thanks,