linux-kernel - Re: Follow-up on Linux-kernel code accessibility

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Message-ID: <95007b18-298d-4af1-8a74-d40ee635941a@paulmck-laptop>
Date: Sun, 28 Dec 2025 16:03:37 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: "Dr. David Alan Gilbert" <dave@...blig.org>
Cc: Julia Lawall <julia.lawall@...ia.fr>, 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 Sun, Dec 28, 2025 at 12:46:01PM +0000, Dr. David Alan Gilbert wrote:
> * Paul E. McKenney (paulmck@...nel.org) wrote:
> 
> <snip>
> 
> > > > > 
> > > > > 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?
> 
> Not sure what you mean join in; not really set up for a call etc at the
> moment, but happy to continue a mail chain.

Fair enough.  This question is to be addressed in that walkthrough.
I will hold off discussions of this particular line of code and of this
function until then.  After all, part of the point is to demonstrate
walkthroughs, and answering the question beforehand would prevent this.

In the meantime, please feel free to change your focus to some other
line of code in some unrelated function and I will be happy to continue
*that* discussion via email.

							Thanx, Paul

> But, if we're talking about the understandability of these few lines,
> it looks like this could be something like:
> 
> 
>    // We have 3 ..... for the case when ...
>    const unsigned long something = 3;
> ....
>    j = DIV_ROUND_UP(j, something);
> 
> which would seem to make it more readable with very little effort.
> 
> (Hmm, since j is unsigned long why do we have a test of:
>     if  (j <= 0) 
>       j = 1
> )
> 
> Dave
> 
> > > 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   |_______/
> -- 
>  -----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   |_______/