Re: [PATCH 02/11] preempt: Track NMI nesting to separate per-CPU counter

[Steven Rostedt]

On Thu,  7 May 2026 21:21:02 -0700
Boqun Feng <boqun@xxxxxxxxxx> 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.
> 
> Reduce multiple bits in preempt_count for NMI tracking. Reduce NMI_BITS
> from 3 to 1, using it only to detect if we're in an NMI.
> 
> Suggested-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
> Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> Signed-off-by: Boqun Feng <boqun@xxxxxxxxxx>
> Link: https://patch.msgid.link/20260121223933.1568682-3-lyude@xxxxxxxxxx
> ---
>  include/linux/hardirq.h | 16 ++++++++++++----
>  include/linux/preempt.h | 13 +++++++++----
>  kernel/softirq.c        |  2 ++
>  3 files changed, 23 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
> index d57cab4d4c06..cc06bda52c3e 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);
> +
>  extern void synchronize_irq(unsigned int irq);
>  extern bool synchronize_hardirq(unsigned int irq);
>  
> @@ -102,14 +104,16 @@ void irq_exit_rcu(void);
>   */
>  
>  /*
> - * nmi_enter() can nest up to 15 times; see NMI_BITS.
> + * nmi_enter() can nest - nesting is tracked in a per-CPU counter.
>   */
>  #define __nmi_enter()						\
>  	do {							\
>  		lockdep_off();					\
>  		arch_nmi_enter();				\
> -		BUG_ON(in_nmi() == NMI_MASK);			\
> -		__preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
> +		BUG_ON(__this_cpu_read(nmi_nesting) == UINT_MAX);	\

I think we should keep the max nesting fixed to 15. If this doesn't trigger
until UINT_MAX, it may take a long time to see that, and there's no reason
NMIs should nest more than 15 anyway.

Just because the counter allows it, doesn't me the system should allow it.

-- Steve


> +		__this_cpu_inc(nmi_nesting);			\
> +		__preempt_count_add(HARDIRQ_OFFSET);		\
> +		preempt_count_set(preempt_count() | NMI_MASK);	\
>  	} while (0)
>  
>  #define nmi_enter()						\
> @@ -124,8 +128,12 @@ void irq_exit_rcu(void);
>