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Date:   Sat, 26 Oct 2019 17:53:34 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Christophe Leroy <christophe.leroy@c-s.fr>
cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        vincenzo.frascino@arm.com, luto@kernel.org,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.
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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.

Thanks,

	tglx
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