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

[Peter Zijlstra]

On Mon, Oct 13, 2025 at 11:48:03AM -0400, Lyude Paul wrote:
> From: Joel Fernandes <joelagnelf@xxxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
> Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> ---
>  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 :/