linux-kernel - Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.

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Date:   Sat, 26 Oct 2019 06:55:28 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andrew Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.

On Tue, Oct 22, 2019 at 6:56 AM Christophe Leroy
<christophe.leroy@....fr> wrote:
>
>
> >>> 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
>

A perf report might be informative.