linux-kernel - Re: [PATCH] arm64: enable GENERIC_FIND_FIRST

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Date:   Tue, 8 Dec 2020 10:35:50 +0000
From:   Will Deacon <will@...nel.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, Alexey Klimov <aklimov@...hat.com>
Subject: Re: [PATCH] arm64: enable GENERIC_FIND_FIRST_BIT

On Mon, Dec 07, 2020 at 05:59:16PM -0800, Yury Norov wrote:
> (CC: Alexey Klimov)
> 
> On Mon, Dec 7, 2020 at 3:25 AM Will Deacon <will@...nel.org> wrote:
> >
> > On Sat, Dec 05, 2020 at 08:54:06AM -0800, Yury Norov wrote:
> > > ARM64 doesn't implement find_first_{zero}_bit in arch code and doesn't
> > > enable it in config. It leads to using find_next_bit() which is less
> > > efficient:
> >
> > [...]
> >
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index 1515f6f153a0..2b90ef1f548e 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -106,6 +106,7 @@ config ARM64
> > >       select GENERIC_CPU_AUTOPROBE
> > >       select GENERIC_CPU_VULNERABILITIES
> > >       select GENERIC_EARLY_IOREMAP
> > > +     select GENERIC_FIND_FIRST_BIT
> >
> > Does this actually make any measurable difference? The disassembly with
> > or without this is _very_ similar for me (clang 11).
> >
> > Will
> 
> On A-53 find_first_bit() is almost twice faster than find_next_bit(),
> according to
> lib/find_bit_benchmark. (Thanks to Alexey for testing.)

I guess it's more compiler dependent than anything else, and it's a pity
that find_next_bit() isn't implemented in terms of the generic
find_first_bit() tbh, but if the numbers are as you suggest then I don't
have a problem selecting this on arm64.

Will