linux-kernel - Re: [PATCH v2 2/5] man/man7/sched.7: Update the real-time section

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Message-ID: <l3u4eon4paxymk4zxre7trz5jxkrc7vr3ohkdmmvvkw2n6lofl@5m7lz332hyr4>
Date: Wed, 15 Oct 2025 23:40:29 +0200
From: Alejandro Colomar <alx@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, linux-man@...r.kernel.org, 
	André Almeida <andrealmeid@...lia.com>, Darren Hart <dvhart@...radead.org>, 
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>, 
	Juri Lelli <juri.lelli@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Valentin Schneider <vschneid@...hat.com>, 
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v2 2/5] man/man7/sched.7: Update the real-time section

Hi Sebastian,

On Mon, Sep 15, 2025 at 04:13:02PM +0200, Sebastian Andrzej Siewior wrote:
> Update the outdated information:
> - The PREEMPT_RT patch is merged. The patch continues to exist (as of
>   now) but is not mandatory.
> - The patch can be still downloaded but most people use the git tree
>   these days. Add a reference to it.
> - CONFIG_PREEMPT_DESKTOP was never thing as far as I remember. It was
>   always CONFIG_PREEMPT and its description referred to "low latency
>   desktop".
> - Within the PREEMPT-RT patch there was a brief window which introduced
>   PREEMPT_RT_BASE and PREEMPT_RT_FULL. I am going to ignore this.
> - The introduction of PREEMPT_LAZY in 6.13 moved PREEMPT_RT from a
>   preemption model to an option.
> - The mentioned wiki is deprecated. Update the URL to the new one.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Thanks!  I've applied the patch, with the following amendment:

	diff --git i/man/man7/sched.7 w/man/man7/sched.7
	index 51300d611..9622c1824 100644
	--- i/man/man7/sched.7
	+++ w/man/man7/sched.7
	@@ -960,18 +960,18 @@ .SS Real-time features in the mainline Linux kernel
	 or cloned from a git tree
	 .UR https://git.kernel.org\:/pub\:/scm\:/linux\:/kernel\:/git\:/rt\:/linux-stable-rt.git
	 .UE .
	-The individual releases are maintained as long as
	-the matching LTS kernel is maintained.
	+The individual releases are maintained
	+as long as the matching LTS kernel is maintained.
	 .P
	-Since 6.12 it possible to enable the real-time preemption
	+Since Linux 6.12 it possible to enable the real-time preemption
	 without the need for any patches.
	 The
	 .I realtime\-patch
	 continues to exist and contains support
	-for not yet integrated architectures,
	-drivers and features that are in development.
	+for not-yet-integrated architectures,
	+drivers, and features that are in development.
	 .P
	-Since 6.13 real-time becomes an option
	+Since Linux 6.13 real-time became an option
	 and not a preemption model.
	 With this change,
	 the following preemption models are available:
	@@ -982,13 +982,13 @@ .SS Real-time features in the mainline Linux kernel
	 .BR CONFIG_PREEMPT_LAZY .
	 The real-time capabilities can be enabled with the option
	 .B CONFIG_PREEMPT_RT
	-and the preemption model be set to either
	+and the preemption model can be set to either
	 .B CONFIG_PREEMPT
	 or
	 .BR CONFIG_PREEMPT_LAZY .
	 The latter model is less eager to preempt
	 .B SCHED_NORMAL
	-tasks in an attempt to reduce lock holder preemption.
	+tasks in an attempt to reduce lock-holder preemption.
	 It does not affect real-time tasks.
	 .P
	 With


Cheers,
Alex

> ---
>  man/man7/sched.7 | 58 ++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 41 insertions(+), 17 deletions(-)
> 
> diff --git a/man/man7/sched.7 b/man/man7/sched.7
> index 3299c21dc1ad8..db7753c9e912c 100644
> --- a/man/man7/sched.7
> +++ b/man/man7/sched.7
> @@ -933,16 +933,12 @@ the following to modify the autogroup nice value for
>  .EE
>  .in
>  .SS Real-time features in the mainline Linux kernel
> -.\" FIXME . Probably this text will need some minor tweaking
> -.\" ask Carsten Emde about this.
> +.\" The archaeologist knows about v2.6.9-mm1-V0.1-realtime-preempt
>  Since Linux 2.6.18, Linux is gradually
>  becoming equipped with real-time capabilities,
>  most of which are derived from the former
>  .I realtime\-preempt
>  patch set.
> -Until the patches have been completely merged into the
> -mainline kernel,
> -they must be installed to achieve the best real-time performance.
>  These patches are named:
>  .P
>  .in +4n
> @@ -953,24 +949,52 @@ they must be installed to achieve the best real-time performance.
>  .P
>  and can be downloaded from
>  .UR http://www.kernel.org\:/pub\:/linux\:/kernel\:/projects\:/rt/
> +.UE
> +or cloned from a git tree
> +.UR https://git.kernel.org\:/pub\:/scm\:/linux\:/kernel\:/git\:/rt\:/linux-stable-rt.git
>  .UE .
> +The individual releases are maintained as long as
> +the matching LTS kernel is maintained.
>  .P
> -Without the patches and prior to their full inclusion into the mainline
> -kernel, the kernel configuration offers only the three preemption classes
> +Since 6.12 it possible to enable the real-time preemption
> +without the need for any patches.
> +The
> +.I realtime\-patch
> +continues to exist and contains support
> +for not yet integrated architectures,
> +drivers and features that are in development.
> +.P
> +Since 6.13 real-time becomes an option
> +and not a preemption model.
> +With this change,
> +the following preemption models are available:
>  .BR CONFIG_PREEMPT_NONE ,
>  .BR CONFIG_PREEMPT_VOLUNTARY ,
> +.BR CONFIG_PREEMPT ,
>  and
> -.B CONFIG_PREEMPT_DESKTOP
> -which respectively provide no, some, and considerable
> -reduction of the worst-case scheduling latency.
> -.P
> -With the patches applied or after their full inclusion into the mainline
> -kernel, the additional configuration item
> +.BR CONFIG_PREEMPT_LAZY .
> +The real-time capabilities can be enabled with the option
>  .B CONFIG_PREEMPT_RT
> -becomes available.
> -If this is selected, Linux is transformed into a regular
> +and the preemption model be set to either
> +.B CONFIG_PREEMPT
> +or
> +.BR CONFIG_PREEMPT_LAZY .
> +The latter model is less eager to preempt
> +.B SCHED_NORMAL
> +tasks in an attempt to reduce lock holder preemption.
> +It does not affect real-time tasks.
> +.P
> +With
> +.B CONFIG_PREEMPT_RT
> +enabled,
> +Linux is transformed into a regular
>  real-time operating system.
> -The FIFO and RR scheduling policies are then used to run a thread
> +The
> +.BR SCHED_FIFO ,
> +.BR SCHED_RR ,
> +and
> +.B SCHED_DEADLINE
> +scheduling policies are then used to run a thread
>  with true real-time priority and a minimum worst-case scheduling latency.
>  .SH NOTES
>  The
> @@ -1032,5 +1056,5 @@ and
>  .I \%Documentation/\:scheduler/\:sched\-nice\-design\:.txt
>  .P
>  Worth looking at:
> -.UR http://rt.wiki.kernel.org/\:index.php
> +.UR https://wiki.linuxfoundation.org/\:realtime/\:start
>  .UE
> -- 
> 2.51.0
> 
> 

-- 
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).

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