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

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Message-ID: <d481f196-703e-4ed0-8db1-dbc3822c349e@nvidia.com>
Date: Mon, 13 Oct 2025 17:27:32 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Peter Zijlstra <peterz@...radead.org>, 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>,
 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 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!

 - Joel