linux-kernel - Re: [PATCH v13 01/17] preempt: Track NMI nesting to separate per-CPU counter

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Message-ID: <20251013200035.GD2734756@noisy.programming.kicks-ass.net>
Date: Mon, 13 Oct 2025 22:00:35 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Lyude Paul <lyude@...hat.com>
Cc: rust-for-linux@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
	Daniel Almeida <daniel.almeida@...labora.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Danilo Krummrich <dakr@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
	Bj??rn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, Ryo Takakura <ryotkkr98@...il.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	"open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v13 01/17] preempt: Track NMI nesting to separate per-CPU
 counter

On Mon, Oct 13, 2025 at 11:48:03AM -0400, Lyude Paul wrote:
> From: Joel Fernandes <joelagnelf@...dia.com>
> 
> Move NMI nesting tracking from the preempt_count bits to a separate per-CPU
> counter (nmi_nesting). This is to free up the NMI bits in the preempt_count,
> allowing those bits to be repurposed for other uses.  This also has the benefit
> of tracking more than 16-levels deep if there is ever a need.
> 
> Suggested-by: Boqun Feng <boqun.feng@...il.com>
> Signed-off-by: Joel Fernandes <joelaf@...gle.com>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
>  include/linux/hardirq.h   | 17 +++++++++++++----
>  kernel/softirq.c          |  2 ++
>  rust/kernel/alloc/kvec.rs |  5 +----
>  rust/kernel/cpufreq.rs    |  3 +--
>  4 files changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
> index d57cab4d4c06f..177eed1de35cc 100644
> --- a/include/linux/hardirq.h
> +++ b/include/linux/hardirq.h
> @@ -10,6 +10,8 @@
>  #include <linux/vtime.h>
>  #include <asm/hardirq.h>
>  
> +DECLARE_PER_CPU(unsigned int, nmi_nesting);

Urgh, and it isn't even in the same cacheline as the preempt_count :/