Message-ID: <435c7085-34e-16e9-5711-e53aa54cf4fc@inria.fr>
Date: Sun, 28 Dec 2025 10:36:39 +0100 (CET)
From: Julia Lawall <julia.lawall@...ia.fr>
To: "Paul E. McKenney" <paulmck@...nel.org>
cc: "Dr. David Alan Gilbert" <dave@...blig.org>, Theodore Tso <tytso@....edu>, 
    Steven Rostedt <rostedt@...dmis.org>, Sasha Levin <sashal@...nel.org>, 
    Gabriele Paoloni <gpaoloni@...hat.com>, 
    Kate Stewart <kstewart@...uxfoundation.org>, 
    Chuck Wolber <chuckwolber@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
    Mark Rutland <mark.rutland@....com>, Thomas Gleixner <tglx@...utronix.de>, 
    Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
    Shuah Khan <skhan@...uxfoundation.org>, Chris Mason <clm@...a.com>, 
    linux-kernel@...r.kernel.org
Subject: Re: Follow-up on Linux-kernel code accessibility



On Sat, 27 Dec 2025, Paul E. McKenney wrote:

> On Sun, Dec 28, 2025 at 01:48:37AM +0000, Dr. David Alan Gilbert wrote:
> > * Paul E. McKenney (paulmck@...nel.org) wrote:
> > > On Sun, Dec 28, 2025 at 12:32:50AM +0100, Julia Lawall wrote:
> > > >
> > > >
> > > > On Sat, 27 Dec 2025, Paul E. McKenney wrote:
> > > >
> > > > > 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.  ;-)
> > > >
> > > > j = (j + 2) / 3;
> > >
> > > "Divide by three rounding up."
> >
> > That's not *that* obvious, but ok, but then why 3?
> >
> > Then later there is:
> >          WRITE_ONCE(rcu_state.jiffies_kick_kthreads,
> >                    jiffies + (j ? 3 * j : 2));
> >
> > Which I assume is related - but then why the 2?
>
> Would the two of you (Julia and Dave) like to join in with Steve and
> myself walking through this function and its caller?

Sure.  The question was indeed why divide by 3.

julia

>
> > Curiosly my local tame Qwen3 LLM explained the rounding up:
> >
> > > I see some code doing 'j = (j + 2) / 3'  - what's it trying to do?
> > The expression `j = (j + 2) / 3` is a clever way to **round up an
> > integer division by 3** — that is, it computes the ceiling of `j / 3`
> > for non-negative integers.
>
> Good to see!
>
> 							Thanx, Paul
>
> > Dave
> >
> > >
> > > 							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
> > > > > > >
> > > > >
> > >
> > --
> >  -----Open up your eyes, open up your mind, open up your code -------
> > / Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \
> > \        dave @ treblig.org |                               | In Hex /
> >  \ _________________________|_____ http://www.treblig.org   |_______/
>

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