From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B1B9F2586C8 for ; Sat, 27 Dec 2025 23:28:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766878119; cv=none; b=E2JoQF4HRojFaJx3bT8KG+onu9z5C8Ft5Ipbqn7Spx5AhLABz3anGGniSbIWRp0pelzswCUWegoELeXZkb1m+DYvUNnRdtoLU1FeuxjqEf7zmI4ndmqxc3DgQyN7AhRXGQSleszG6RHAsAepe5CfHszPYjusQ7QDdTi5WGejvsE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766878119; c=relaxed/simple; bh=AMPTX/7vRNeijtzn3u2mFaYwUezEOsopc4LzzpRzsEA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=fAJyuLtW5YiTKX0TIbY9hu/v0OQEb68rfNRE8MHHDqkuae8Bmf9TEM9xFDsnQnVqp2tAkEtBl8i5SN7kpN8YAg6/173okllvRpz+DwIGLQx4pi46IgFfgI0mAq8BUGHJTF0XkeVG22mcJRJNtcWhuNJDoQYFOMrB/JcVoAmFUak= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=uuEEYmGl; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="uuEEYmGl" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 399B9C4CEF1; Sat, 27 Dec 2025 23:28:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1766878119; bh=AMPTX/7vRNeijtzn3u2mFaYwUezEOsopc4LzzpRzsEA=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=uuEEYmGlRhEuOlmCJRB7XDo4M4tRQH/053hnW5xc4RiEiXhYDCr+l94Dp0HJK9Np1 fASXMHEEyBsvrR4U90AZ9ntd850Jpliyr3iPehEDPzr4qIZtPUpx9n0dP8vioVMR2U fKOwuc26n3TCTRRXtwuW4CTvjPemKBI7GyuJTbGx3IYEPr6WFoeZrUX1mvgnL4Ymxg 9YpM1BB8K+oSGc89ZrVMrKmLqDCXmWdP2fG2x03R+7ALFLqb9Yv4j0K5hM8pP44pgC JJmTdoWNLx6Om0ADffhNnaSxP1ripf5jadYD3tLzWApl2LRMbNj1YLqQ70lTokZ/Gy Mhb05x+54x2yA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id CA616CE0CA7; Sat, 27 Dec 2025 15:28:38 -0800 (PST) Date: Sat, 27 Dec 2025 15:28:38 -0800 From: "Paul E. McKenney" To: Julia Lawall Cc: Theodore Tso , Steven Rostedt , Sasha Levin , Gabriele Paoloni , Kate Stewart , Chuck Wolber , Dmitry Vyukov , Mark Rutland , Thomas Gleixner , Lorenzo Stoakes , Shuah Khan , Chris Mason , linux-kernel@vger.kernel.org Subject: Re: Follow-up on Linux-kernel code accessibility Message-ID: Reply-To: paulmck@kernel.org References: <20251219132812.2a3516df@gandalf.local.home> <20251222104209.15e98861@gandalf.local.home> <89457d2d-153e-4274-b485-2ace983cec4f@paulmck-laptop> <20251224091158.6ab443cb@gandalf.local.home> <20251225150331.GB15088@macsyma.lan> <20251226114830.6bc7a3eb@gandalf.local.home> <20251226192217.GB67711@macsyma.local> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Sat, Dec 27, 2025 at 07:16:28AM +0100, Julia Lawall wrote: > > > On Fri, 26 Dec 2025, Paul E. McKenney wrote: > > > On Fri, Dec 26, 2025 at 02:22:17PM -0500, Theodore Tso wrote: > > > On Fri, Dec 26, 2025 at 11:48:30AM -0500, Steven Rostedt wrote: > > > > Agreed, but knowing what the function is doing should give you some idea of > > > > how it is doing it. > > > > > > > > "Loop doing repeated quiescent-state forcing until the grace period ends." > > > > > > > > Is the only description of "what the function is doing", but it gives you > > > > no clue to why it's using those magic numbers. There should be comments > > > > about how the magic numbers relate to the what. > > > > > > Sure, but rcu_gp_fqs_loop() is a static, internal function. I agree > > > that better documentation would be usefui for people who want to > > > modify the internals of the RCU infrastructure, but it's not something > > > that should be in kernel documentation for newcomers to kernel > > > development. > > > > > > When we talk about making the kernel code more accessible, it's really > > > important to keep in mind that different audiences may have different > > > needs, and too much information can be just as confusing as too > > > little. It seems likely that most newcomers aren't going to be > > > looking to make changes to important systems like RCU. > > > > > > That being said, even though most newcomers aren't probably going to > > > be making changes to file systems, as a file system maintainer I > > > admittedly to have a vested interest in making easier for > > > intermediate-level kernel developers who might take an interest to > > > ext4 development to have an easy path to do so. So I get where Paul > > > is coming from. > > > > > > When we're talking about making the kernel code more accessible, we > > > need need to be very explicit about which target audiences we are > > > targetting, because the strategies might be different for different > > > readers. > > > > Here is how I group them: > > > > 1. RCU users in the Linux kernel. Largish group. Addressed by much > > of the Documentation/RCU contents, the occasional LWN article, > > blog posts, and by RCU experts in various subsystems. > > > > 2. RCU users in userspace. Modest group, but growing. LWN articles, > > blog posts, an academic paper or three, and a number of people > > who have implemented some form or another of RCU. > > > > 3. Userspace RCU implementations. Smallish group, but there are > > quite a few implementations out there. I don't deliberately > > target anything to this group, but it is quite possible that > > some of the writings for the other groups have been helpful. > > > > 4. Linux-kernel RCU implementation. Small group, though a largish > > one if we include all the people who have gotten at least one > > patch accepted. Comments, Linux-kernel RCU Design Documents [1], > > Documentation/RCU/Design, a few LWN articles, a few blog posts, > > and the several regular developers and maintainers. > > > > And I would like to think that Section 9.5 of "Is Parallel Programming > > Hard, And, If So, What Can You Do About It?" [2] has been helpful. > > > > What groups should I be adding? > > In the case of the scheduler, I observe some unexpected behavior, and then > I have the feeling that I backtrack through the code coming from a > direction that was not anticipated by the person who wrote the comments > (sometimes inline comments, more likely the comments in the commit tht > introduced the line I am wondering about). But I'm not sure how to place > RCU. My impression is that it has a more monolithic implementation, and > that I would only ever consider it to be a block box, not something I > would debug my way into. But perhaps people felt that way about the > scheduler functions I am looking at as well. Over time, there have surely > been performance improvements in RCU as well. It seems hard to anticipate > what information people will need. Although RCU does use heuristics, I would say that its operation adapts less to dynamic conditions than does the scheduler. RCU is mostly an observer of state, while the scheduler's job is mostly a controller of state. > Maybe one could consider that division by a constant is sufficiently rare > in the kernel that it should always be documented? If it is (for example) division by 10 in code that converts binary integers to char, or code computing an average, I would not expect a comment to be needed. But I do feel the need to ask what division you are referring to. ;-) Thanx, Paul > julia > > > > > > > Thanx, Paul > > > > [1] https://docs.google.com/document/d/1GCdQC8SDbb54W1shjEXqGZ0Rq8a6kIeYutdSIajfpLA/edit?pli=1&tab=t.0#heading=h.ytgz5i5df43s > > > > [2] https://www.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook.html > >