From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-12.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A3260C433FE for ; Tue, 8 Dec 2020 10:36:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5454923A5A for ; Tue, 8 Dec 2020 10:36:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728699AbgLHKgf (ORCPT ); Tue, 8 Dec 2020 05:36:35 -0500 Received: from mail.kernel.org ([198.145.29.99]:44474 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727965AbgLHKgf (ORCPT ); Tue, 8 Dec 2020 05:36:35 -0500 Date: Tue, 8 Dec 2020 10:35:50 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1607423754; bh=ffXh/ffsFEu14yYkVpY1W+3Tb6JWOvb9jWcp7gmAz2g=; h=From:To:Cc:Subject:References:In-Reply-To:From; b=jv1ty0fx2BX6Qj72fdbIYdZax8z6degjujXihkSyIV3UDb97UyieX87rWb9JnOawd pXhwuacdG+oxlOhtVN1nycBQGosStF8DwNj07kppxrJ/n2SKKvQORxiMA19Mb+xBJs xz71tgAnVkOVazSgi0d+kxvDN5kC6lqWXpiBAtVnBZmobANuUmpOrTEtMaANVSB3PQ gwZO1zItEGR8VUcUdPpVyXosXmXS7/zzf5wYI64aJrLaJl0Qf6RG0/jONM3jY+8OW7 Y0oeCYv2buhqif9br8mchpVwsELCVxhQigueqioBdr8BhNLL7Na0L7A9OlPMlJqJSq KfGD6MFcN2qew== From: Will Deacon To: Yury Norov 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: <20201208103549.GA5887@willie-the-truck> References: <20201205165406.108990-1-yury.norov@gmail.com> <20201207112530.GB4379@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org 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. Will