From: Thomas Gleixner <tglx@linutronix.de>
To: Christophe Leroy <christophe.leroy@c-s.fr>
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 17:53:34 +0200 (CEST) [thread overview]
Message-ID: <alpine.DEB.2.21.1910261751140.10190@nanos.tec.linutronix.de> (raw)
In-Reply-To: <95bd2367-8edc-29db-faa3-7729661e05f2@c-s.fr>
[-- Attachment #1: Type: text/plain, Size: 1554 bytes --]
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
next prev parent reply other threads:[~2019-10-26 15:56 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 [this message]
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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