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[96.27.162.190]) by smtp.gmail.com with ESMTPSA id r17sm620719qta.78.2021.02.23.21.27.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 23 Feb 2021 21:27:45 -0800 (PST) Date: Tue, 23 Feb 2021 21:27:44 -0800 From: Yury Norov To: Will Deacon Cc: Catalin Marinas , linux-arm-kernel@lists.infradead.org, Linux Kernel Mailing List , linux-arch@vger.kernel.org, Alexey Klimov Subject: Re: [PATCH] arm64: enable GENERIC_FIND_FIRST_BIT Message-ID: <20210224052744.GA1168363@yury-ThinkPad> References: <20201205165406.108990-1-yury.norov@gmail.com> <20201207112530.GB4379@willie-the-truck> <20201208103549.GA5887@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201208103549.GA5887@willie-the-truck> Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org 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 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?