linux-kernel - Re: w/r/t "irq & spin_lock: Add counted interrupt disabling/enabling": holes in pcpu

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Message-ID: <874iv1qmgd.ffs@tglx>
Date: Thu, 24 Jul 2025 23:59:46 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Lyude Paul <lyude@...hat.com>, Peter Zijlstra <peterz@...radead.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Dirk Behme <dirk.behme@...il.com>,
 rust-for-linux@...r.kernel.org, Danilo Krummrich <dakr@...hat.com>,
 airlied@...hat.com, Ingo Molnar <mingo@...hat.com>, will@...nel.org,
 Waiman
 Long <longman@...hat.com>, linux-kernel@...r.kernel.org, Miguel Ojeda
 <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 wedsonaf@...il.com, Gary Guo <gary@...yguo.net>, Björn
 Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin
 <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>,
 aliceryhl@...gle.com, Trevor Gross <tmgross@...ch.edu>
Subject: Re: w/r/t "irq & spin_lock: Add counted interrupt
 disabling/enabling": holes in pcpu_hot?

On Thu, Jul 24 2025 at 16:36, Lyude Paul wrote:
> Hey! Sorry to be a bit late here but I just noticed this comment in your email
> that I didn't before
>
> On Wed, 2024-10-23 at 21:51 +0200, Peter Zijlstra wrote:
>> We still have a few holes in pcpu_hot iirc.
>
> I assume you mean we still have space for adding things into pcpu_hot? And if
> so, where exactly are these holes? At some point when working on this series I
> attempted adding a single unsigned long to pcpu_hot and the build failed as a
> result of not having enough space. But if there's somewhere we can add
> something else, that might actually help with a few of the issues in this
> patch series

That's the current lot we have.

ffffffff82f6a000 D __per_cpu_hot_start
ffffffff82f6a000 D hardirq_stack_ptr
ffffffff82f6a008 D __ref_stack_chk_guard
ffffffff82f6a008 D __stack_chk_guard
ffffffff82f6a010 D const_cpu_current_top_of_stack
ffffffff82f6a010 D cpu_current_top_of_stack
ffffffff82f6a018 D const_current_task
ffffffff82f6a018 D current_task
ffffffff82f6a020 D __x86_call_depth
ffffffff82f6a028 D this_cpu_off
ffffffff82f6a030 D __preempt_count
ffffffff82f6a034 D cpu_number
ffffffff82f6a038 D __softirq_pending
ffffffff82f6a03a D hardirq_stack_inuse
ffffffff82f6a03b D __per_cpu_hot_end

So there is a 5 byte hole there at the end, which is obviously too small
for an unsigned long on 64-bit :)