Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.

Christophe Leroy
2019-10-22T13:56:38Z

Le 22/10/2019 à 11:01, Christophe Leroy a écrit :
>
>
> Le 21/10/2019 à 23:29, Thomas Gleixner a écrit :
>> On Mon, 21 Oct 2019, Christophe Leroy wrote:
>>
>>> This is a tentative to switch powerpc/32 vdso to generic C
>>> implementation.
>>> It will likely not work on 64 bits or even build properly at the moment.
>>>
>>> powerpc is a bit special for VDSO as well as system calls in the
>>> way that it requires setting CR SO bit which cannot be done in C.
>>> Therefore, entry/exit and fallback needs to be performed in ASM.
>>>
>>> To allow that, C fallbacks just return -1 and the ASM entry point
>>> performs the system call when the C function returns -1.
>>>
>>> The performance is rather disappoiting. That's most likely all
>>> calculation in the C implementation are based on 64 bits math and
>>> converted to 32 bits at the very end. I guess C implementation should
>>> use 32 bits math like the assembly VDSO does as of today.
>>
>>> gettimeofday:    vdso: 750 nsec/call
>>>
>>> gettimeofday:    vdso: 1533 nsec/call
>
> Small improvement (3%) with the proposed change:
>
> gettimeofday:    vdso: 1485 nsec/call

By inlining do_hres() I get the following:

gettimeofday: vdso: 1072 nsec/call

Christophe

>
> Though still some way to go.
>
> Christophe
>
>>
>> The only real 64bit math which can matter is the 64bit * 32bit multiply,
>> i.e.
>>
>> static __always_inline
>> u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
>> {
>>          return ((cycles - last) & mask) * mult;
>> }
>>
>> Everything else is trivial add/sub/shift, which should be roughly the
>> same
>> in ASM.
>>
>> Can you try to replace that with:
>>
>> static __always_inline
>> u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
>> {
>>          u64 ret, delta = ((cycles - last) & mask);
>>          u32 dh, dl;
>>
>>          dl = delta;
>>          dh = delta >> 32;
>>
>>          res = mul_u32_u32(al, mul);
>>          if (ah)
>>                  res += mul_u32_u32(ah, mul) << 32;
>>
>>          return res;
>> }
>>
>> That's pretty much what __do_get_tspec does in ASM.
>>
>> Thanks,
>>
>>     tglx
>>