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

[Joel Fernandes]

On 5/12/2026 12:30 PM, Steven Rostedt wrote:
> 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.

That's fine with me. Boqun, do you want to make the one-line change to the patch?

Thanks.