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

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Message-ID: <aVL7G55hAhyplgQz@gallifrey>
Date: Mon, 29 Dec 2025 22:05:15 +0000
From: "Dr. David Alan Gilbert" <dave@...blig.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
	Julia Lawall <julia.lawall@...ia.fr>, Theodore Tso <tytso@....edu>,
	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

* Steven Rostedt (rostedt@...dmis.org) wrote:

> There's nothing there that says "3" is needed. Why divide at all? Why not
> just add 1?

Now while Paul obviously doesn't want to tell us, I decided I should
do what someone who actually cared would do, and looked at the git history,
To my surprise, I see I made a mistake - the three magic constants
in this function were all added separately, and the two uses of '3'
are not necessarily the same.

8c7c4829a8 - from 2016 
  rcu: Awaken grace-period kthread if too long since FQS

  says it's a crude hack but doesn't mention why it chose 3 in the commit
  message; among other stuff it adds:

+                               WRITE_ONCE(rsp->jiffies_kick_kthreads,
+                                          jiffies + 3 * j);

9cf422a8 - from 2018
  rcu: Accommodate zero jiffies_till_first_fqs and kthread kicking

  replaces that:
-                                  jiffies + 3 * j);
+                                  jiffies + (j ? 3 * j : 2));

  to stop using too much cpu and says 'at least two jiffies'

fb77dcc - from 2022
  rcu: Decrease FQS scan wait time in case of callback overloading

  introduces the:
  j = (j + 2) / 3;
and commit message says:
    This commit therefore divides the normal time between rescans by three,
    rounding up.  Thus a small system running at HZ=1000 that is suffering
    from callback overload will wait only one jiffy instead of the normal
    three between rescans.

So that one does have a commit message explanation at least; although
I'm not sure I know where the 'normal three' comes from - but I guess
that's followable eventually.

But heck, this is making it hard to find them.

Dave

-- 
 -----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   |_______/