From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Paul Mackerras <paulus@samba.org>,
luto@kernel.org, vincenzo.frascino@arm.com,
linuxppc-dev@lists.ozlabs.org
Subject: Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.
Date: Tue, 22 Oct 2019 15:56:33 +0200 [thread overview]
Message-ID: <95bd2367-8edc-29db-faa3-7729661e05f2@c-s.fr> (raw)
In-Reply-To: <f4486e86-3c0c-0eec-1639-0e5956cdb8f1@c-s.fr>
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
>>
next prev parent reply other threads:[~2019-10-22 13:59 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-21 12:53 [RFC PATCH] powerpc/32: Switch VDSO to C implementation Christophe Leroy
2019-10-21 21:29 ` Thomas Gleixner
2019-10-22 9:01 ` Christophe Leroy
2019-10-22 13:56 ` Christophe Leroy [this message]
2019-10-26 13:55 ` Andy Lutomirski
2019-10-26 15:54 ` Christophe Leroy
2019-10-26 15:53 ` Thomas Gleixner
2019-10-26 16:06 ` Christophe Leroy
2019-10-26 18:48 ` Thomas Gleixner
2019-10-26 23:06 ` Segher Boessenkool
2019-10-27 9:21 ` Christophe Leroy
2019-10-27 19:07 ` Segher Boessenkool
2019-12-20 18:24 ` Christophe Leroy
2020-01-09 14:05 ` Thomas Gleixner
2020-01-09 15:21 ` Christophe Leroy
2020-01-10 22:42 ` Thomas Gleixner
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