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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 68F5AC43219 for ; Mon, 29 Nov 2021 06:40:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348635AbhK2GoE (ORCPT ); Mon, 29 Nov 2021 01:44:04 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49158 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233836AbhK2GmA (ORCPT ); Mon, 29 Nov 2021 01:42:00 -0500 Received: from mail-qv1-xf2b.google.com (mail-qv1-xf2b.google.com [IPv6:2607:f8b0:4864:20::f2b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D43BEC061748; Sun, 28 Nov 2021 22:38:43 -0800 (PST) Received: by mail-qv1-xf2b.google.com with SMTP id gu12so13619689qvb.6; Sun, 28 Nov 2021 22:38:43 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=GW7wD7qKvROu8z9xZL5o2Gh9JvMEXrXNdc/d8bAaPNs=; b=Ximtyy9D6hF34jeHkx2abhxO2fEDksv5f5qJu4CbvoNWokti3yxmfDL3L7TYPRNI1N nLBExnAtE1kHxfW3UuSIcTZjpfdkrYaWw77PtYlgHprrRWmhg8onSirLlPaay6tbmdGV QOolCDulR6Tk/EKA2Gv8Xlb2VKcWxKtuMIpV9m8bP/Y2OIkRrwh1D8Uy/bj62ViSMN2l ZvGgVYwmHud/XyWXGXVdXp5Qsb6Tf/NpieRKvdWwCGOCaqCRU7xxjxP8pUys/Y0my6pK DI87mJm/+MqXPnLUnP7Gv+O+UoGKFiXftZWbYzOV5qGvrEcopuUsleS20idOc1qycgZd CJ3A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=GW7wD7qKvROu8z9xZL5o2Gh9JvMEXrXNdc/d8bAaPNs=; b=ZJNzQ26Pb9b5MJ1h3uUNqYGv1DaF8OI97Y1jXO35e62sIp4N/U+id1FQbIb1zKi+8n QKlv6MnvQlACW6tq4DYgO2P2XGDXXmwikomWVIzzVHN6aghoX+BwKX6cWG8+e6akI3l+ Ozvclk1Yedo0wVR/WvvQj7PXMQlTOQm48qqzsUfXt5rc+eWaY15RPknM97CG9Xb6fv07 YF5wLyZ6Xm8X5bZ67A2PbR6MN2gsFdGO1DL/GCbYIbfYDGWSERBVVgnvVotH/0QVkt6D DR+Y3vwLW4oPazxAcrRIf0acgfZDdryEtnu6UP2qQ1X+szodvu6Rippkk530utTMvaSg 9fKw== X-Gm-Message-State: AOAM533YI1rNKv+SqHSIcx46h56bqj2sV4uMFtHOzX73HR9Rm2iuz7yU 9217RHcBLVci45zm4BX7KW8= X-Google-Smtp-Source: ABdhPJwSywxmnS7RgaW8ur/GbWm8YSeTFfi4rMgLh/NdfnxtrZBeDnmatO6ciZp/cAThb6LO+btZbw== X-Received: by 2002:a05:6214:984:: with SMTP id dt4mr30031399qvb.120.1638167922796; Sun, 28 Nov 2021 22:38:42 -0800 (PST) Received: from localhost ([66.216.211.25]) by smtp.gmail.com with ESMTPSA id h19sm8495514qth.63.2021.11.28.22.38.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 28 Nov 2021 22:38:42 -0800 (PST) Date: Sun, 28 Nov 2021 22:38:39 -0800 From: Yury Norov To: mirq-test@rere.qmqm.pl Cc: linux-kernel@vger.kernel.org, Yury Norov , "James E.J. Bottomley" , "Martin K. Petersen" , "Paul E. McKenney" , "Rafael J. Wysocki" , Alexander Shishkin , Alexey Klimov , Amitkumar Karwar , Andi Kleen , Andrew Lunn , Andrew Morton , Andy Gross , Andy Lutomirski , Andy Shevchenko , Anup Patel , Ard Biesheuvel , Arnaldo Carvalho de Melo , Arnd Bergmann , Borislav Petkov , Catalin Marinas , Christoph Hellwig , Christoph Lameter , Daniel Vetter , Dave Hansen , David Airlie , David Laight , Dennis Zhou , Dinh Nguyen , Geetha sowjanya , Geert Uytterhoeven , Greg Kroah-Hartman , Guo Ren , Hans de Goede , Heiko Carstens , Ian Rogers , Ingo Molnar , Jakub Kicinski , Jason Wessel , Jens Axboe , Jiri Olsa , Jonathan Cameron , Juri Lelli , Kalle Valo , Kees Cook , Krzysztof Kozlowski , Lee Jones , Marc Zyngier , Marcin Wojtas , Mark Gross , Mark Rutland , Matti Vaittinen , Mauro Carvalho Chehab , Mel Gorman , Michael Ellerman , Mike Marciniszyn , Nicholas Piggin , Palmer Dabbelt , Peter Zijlstra , Petr Mladek , Randy Dunlap , Rasmus Villemoes , Roy Pledge , Russell King , Saeed Mahameed , Sagi Grimberg , Sergey Senozhatsky , Solomon Peachy , Stephen Boyd , Stephen Rothwell , Steven Rostedt , Subbaraya Sundeep , Sudeep Holla , Sunil Goutham , Tariq Toukan , Tejun Heo , Thomas Bogendoerfer , Thomas Gleixner , Ulf Hansson , Vincent Guittot , Vineet Gupta , Viresh Kumar , Vivien Didelot , Vlastimil Babka , Will Deacon , bcm-kernel-feedback-list@broadcom.com, kvm@vger.kernel.org, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-crypto@vger.kernel.org, linux-csky@vger.kernel.org, linux-ia64@vger.kernel.org, linux-mips@vger.kernel.org, linux-mm@kvack.org, linux-perf-users@vger.kernel.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-snps-arc@lists.infradead.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH 0/9] lib/bitmap: optimize bitmap_weight() usage Message-ID: <20211129063839.GA338729@lapt> References: <20211128035704.270739-1-yury.norov@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Nov 28, 2021 at 07:03:41PM +0100, mirq-test@rere.qmqm.pl wrote: > On Sat, Nov 27, 2021 at 07:56:55PM -0800, Yury Norov wrote: > > In many cases people use bitmap_weight()-based functions like this: > > > > if (num_present_cpus() > 1) > > do_something(); > > > > This may take considerable amount of time on many-cpus machines because > > num_present_cpus() will traverse every word of underlying cpumask > > unconditionally. > > > > We can significantly improve on it for many real cases if stop traversing > > the mask as soon as we count present cpus to any number greater than 1: > > > > if (num_present_cpus_gt(1)) > > do_something(); > > > > To implement this idea, the series adds bitmap_weight_{eq,gt,le} > > functions together with corresponding wrappers in cpumask and nodemask. > > Having slept on it I have more structured thoughts: > > First, I like substituting bitmap_empty/full where possible - I think > the change stands on its own, so could be split and sent as is. Ok, I can do it. > I don't like the proposed API very much. One problem is that it hides > the comparison operator and makes call sites less readable: > > bitmap_weight(...) > N > > becomes: > > bitmap_weight_gt(..., N) > > and: > bitmap_weight(...) <= N > > becomes: > > bitmap_weight_lt(..., N+1) > or: > !bitmap_weight_gt(..., N) > > I'd rather see something resembling memcmp() API that's known enough > to be easier to grasp. For above examples: > > bitmap_weight_cmp(..., N) > 0 > bitmap_weight_cmp(..., N) <= 0 > ... bitmap_weight_cmp() cannot be efficient. Consider this example: bitmap_weight_lt(1000 0000 0000 0000, 1) == false ^ stop here bitmap_weight_cmp(1000 0000 0000 0000, 1) == 0 ^ stop here I agree that '_gt' is less verbose than '>', but the advantage of '_gt' over '>' is proportional to length of bitmap, and it means that this API should exist. > This would also make the implementation easier in not having to > copy and paste the code three times. Could also use a simple > optimization reducing code size: In the next version I'll reduce code duplication like this: bool bitmap_eq(..., N); bool bitmap_ge(..., N); #define bitmap_weight_gt(..., N) bitmap_weight_ge(..., N + 1) #define bitmap_weight_lt(..., N) !bitmap_weight_ge(..., N) #define bitmap_weight_le(..., N) !bitmap_weight_gt(..., N) Thanks, Yury