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: Sat, 26 Oct 2019 18:06:52 +0200 [thread overview]
Message-ID: <439bce37-9c2c-2afe-9c9e-2f500472f9f8@c-s.fr> (raw)
In-Reply-To: <alpine.DEB.2.21.1910261751140.10190@nanos.tec.linutronix.de>
Le 26/10/2019 à 17:53, Thomas Gleixner a écrit :
> On Tue, 22 Oct 2019, Christophe Leroy wrote:
>> 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
>
> What's the effect for clock_gettime()?
>
> gettimeofday() is suboptimal vs. the PPC ASM variant due to an extra
> division, but clock_gettime() should be 1:1 comparable.
>
Original PPC asm:
clock-gettime-realtime: vdso: 928 nsec/call
My original RFC:
clock-gettime-realtime: vdso: 1570 nsec/call
With your suggested vdso_calc_delta():
clock-gettime-realtime: vdso: 1512 nsec/call
With your vdso_calc_delta() and inlined do_hres():
clock-gettime-realtime: vdso: 1302 nsec/call
Christophe
next prev parent reply other threads:[~2019-10-26 16:08 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
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 [this message]
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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