linux-kernel - Re: [PATCH] doc: Add CPU Isolation documentation

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Message-ID: <8f357de1-72ba-497f-997a-f9a61aec6190@redhat.com>
Date: Mon, 11 Aug 2025 16:38:53 -0400
From: Waiman Long <llong@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>
Cc: Anna-Maria Behnsen <anna-maria@...utronix.de>,
 Gabriele Monaco <gmonaco@...hat.com>, Ingo Molnar <mingo@...nel.org>,
 Jonathan Corbet <corbet@....net>, Marcelo Tosatti <mtosatti@...hat.com>,
 Marco Crivellari <marco.crivellari@...e.com>,
 Michal Hocko <mhocko@...nel.org>, "Paul E . McKenney" <paulmck@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Phil Auld <pauld@...hat.com>,
 Steven Rostedt <rostedt@...dmis.org>, Thomas Gleixner <tglx@...utronix.de>,
 Valentin Schneider <vschneid@...hat.com>, Vlastimil Babka <vbabka@...e.cz>,
 linux-doc@...r.kernel.org
Subject: Re: [PATCH] doc: Add CPU Isolation documentation

On 8/9/25 5:42 AM, Frederic Weisbecker wrote:
> nohz_full was introduced in v3.10 in 2013, which means this
> documentation is overdue for 12 years.
>
> The shoemaker's children always go barefoot. And working on timers
> hasn't made me arriving on time either.
>
> Fortunately Paul wrote a part of the needed documentation a while ago,
> especially concerning nohz_full in Documentation/timers/no_hz.rst and
> also about per-CPU kthreads in
> Documentation/admin-guide/kernel-per-CPU-kthreads.rst
>
> Introduce a new page that gives an overview of CPU isolation in general.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Thanks for working on this document. It will be really useful for those 
customers that need CPU isolation.

> ---
>   Documentation/admin-guide/cpu-isolation.rst | 338 ++++++++++++++++++++
>   Documentation/admin-guide/index.rst         |   1 +
>   2 files changed, 339 insertions(+)
>   create mode 100644 Documentation/admin-guide/cpu-isolation.rst
>
> diff --git a/Documentation/admin-guide/cpu-isolation.rst b/Documentation/admin-guide/cpu-isolation.rst
> new file mode 100644
> index 000000000000..250027acf7b2
> --- /dev/null
> +++ b/Documentation/admin-guide/cpu-isolation.rst
> @@ -0,0 +1,338 @@
> +=============
> +CPU Isolation
> +=============
> +
> +Introduction
> +============
> +
> +"CPU Isolation" means leaving a CPU exclusive to a given userspace
> +workload without any undesired code interference from the kernel.
> +
> +Those interferences, commonly pointed out as "noise", can be triggered
> +by asynchronous events (interrupts, timers, scheduler preemption by
> +workqueues and kthreads, ...) or synchronous events (syscalls and page
> +faults).
> +
> +Such noise usually goes unnoticed. After all synchronous events are a
> +component of the requested kernel service. And asynchronous events are
> +either sufficiently well distributed by the scheduler when executed
> +as tasks or reasonably fast when executed as interrupt. The timer
> +interrupt can even execute 1024 times per seconds without
> +significantly measurable impact most of the time.

"significantly measurable" looks odd to me. Maybe "without a significant 
and measurable impact most of the time".

> +
> +Tradeoffs
> +~~~~~~~~~
> +
> +In terms of cost, this is the most invasive isolation feature. It is
> +assumed to be used when the workload spends most of its time in
> +userspace and doesn't rely on the kernel except for preparatory
> +work because:
> +
> +- RCU is slower due to the locked, offloaded and threaded callbacks
> +  processing (the same that would be obtained with "rcu_nocb=" boot
> +  parameter).
Should be "rcu_nocbs". The '=' is optional.

Cheers,
Longman