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

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Message-ID: <20251014082506.GO3245006@noisy.programming.kicks-ass.net>
Date: Tue, 14 Oct 2025 10:25:06 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: Lyude Paul <lyude@...hat.com>, 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>,
	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 05:27:32PM -0400, Joel Fernandes wrote:
> 
> 
> On 10/13/2025 4:00 PM, Peter Zijlstra wrote:
> > 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 :/
> 
> Great point. I will move this to DECLARE_PER_CPU_CACHE_HOT()
> so it's co-located with preempt_count and run some tests. Let me know if that
> works for you, thanks!

Well, I hate how on entry we then end up incrementing both. How terrible
would it be to make __preempt_count u64 instead?