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Message-ID: <20210224093343.GA11306@willie-the-truck> Date: Wed, 24 Feb 2021 09:33:44 +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 Tue, Feb 23, 2021 at 09:27:44PM -0800, Yury Norov wrote: > On Tue, Dec 08, 2020 at 10:35:50AM +0000, Will Deacon wrote: > > 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. > > Ping? Not sure what happened to this. Maybe resend at -rc1? Will
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