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

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Date:   Tue, 22 Oct 2019 15:56:33 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        vincenzo.frascino@....com, luto@...nel.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.



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
>>