linux-kernel - Re: [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation.

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Date:   Fri, 17 Jan 2020 10:26:56 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, nathanl@...ux.ibm.com,
        arnd@...db.de, tglx@...utronix.de, vincenzo.frascino@....com,
        luto@...nel.org, x86@...nel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mips@...r.kernel.org
Subject: Re: [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation.



Le 17/01/2020 à 09:58, Segher Boessenkool a écrit :
> Hi!
> 
> On Thu, Jan 16, 2020 at 05:58:24PM +0000, Christophe Leroy wrote:
>> On a powerpc8xx, with current powerpc/32 ASM VDSO:
>>
>> gettimeofday:    vdso: 907 nsec/call
>> clock-getres-realtime:    vdso: 484 nsec/call
>> clock-gettime-realtime:    vdso: 899 nsec/call
>>
>> The first patch adds VDSO generic C support without any changes to common code.
>> Performance is as follows:
>>
>> gettimeofday:    vdso: 1211 nsec/call
>> clock-getres-realtime:    vdso: 722 nsec/call
>> clock-gettime-realtime:    vdso: 1216 nsec/call
>>
>> Then a few changes in the common code have allowed performance improvement. At
>> the end of the series we have:
>>
>> gettimeofday:    vdso: 974 nsec/call
>> clock-getres-realtime:    vdso: 545 nsec/call
>> clock-gettime-realtime:    vdso: 941 nsec/call
>>
>> The final result is rather close to pure ASM VDSO:
>> * 7% more on gettimeofday (9 cycles)
>> * 5% more on clock-gettime-realtime (6 cycles)
>> * 12% more on clock-getres-realtime (8 cycles)
> 
> Nice!  Much better.
> 
> It should be tested on more representative hardware, too, but this looks
> promising alright :-)
> 

Yes.

Now the challenge is to get VDSO32 buildable on PPC64. The big issue is 
that in most powerpc/include/asm/*.h , CONFIG_PPC64 is used to know if 
the build is a 64 bits build or a 32 bits build, so VDSO32 build fails.

I don't know how this could be easily fixed.

Christophe