linux-kernel - Re: [PATCH v17 02/16] preempt: Track NMI nesting to separate per-CPU counter

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Message-ID: <cd3e4e70-a007-47e4-8313-ae786e363a34@nvidia.com>
Date: Fri, 6 Feb 2026 11:00:16 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: Boqun Feng <boqun@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Lyude Paul <lyude@...hat.com>,
 rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
 Thomas Gleixner <tglx@...utronix.de>, Boqun Feng <boqun.feng@...il.com>,
 Daniel Almeida <daniel.almeida@...labora.com>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Gary Guo <gary@...yguo.net>, Björn 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>, Danilo Krummrich <dakr@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>, Ingo Molnar <mingo@...hat.com>,
 Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v17 02/16] preempt: Track NMI nesting to separate per-CPU
 counter



On 2/6/2026 10:28 AM, Boqun Feng wrote:
>> I was also coming from the goal of long term kernel code maintainability. If
>> we decide to have additional preempt count flags in the future, does special
>> casing 32 bit add even more complexity? (not rhetorical, really asking)
>>
> First, given what preempt count is, I don't think that'll happen
> frequently.

Not sure I buy the argument of not happening frequently. I don't think any of us
have a crystal ball. There are cases in the future that can come up IMO.

> Also I think the reality is that we care about 64bit> performance more than
32bit, in that sense, if this "conditional 32 bit
> preempt count case" becomes an issue, the reasonable action to me is
> just making all preempt count 64bit 

You might be missing something here. You can't make all of preempt count 64 bit,
that's the point, it doesn't work. That's why Peter did what he did to
special-case 32 bit. See:
https://lore.kernel.org/all/20251020204421.GA197647@joelbox2/

That said, I am ok with the approach now that Peter mentions 32-bit x86 is
"deprecated". :-)

-- 
Joel Fernandes