On 5/12/2026 12:30 PM, Steven Rostedt wrote:
> On Thu, 7 May 2026 21:21:02 -0700
> Boqun Feng <[email protected]> wrote:
>
>> From: Joel Fernandes <[email protected]>
>>
>> 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 <[email protected]>
>> Signed-off-by: Joel Fernandes <[email protected]>
>> Signed-off-by: Lyude Paul <[email protected]>
>> Signed-off-by: Boqun Feng <[email protected]>
>> Link: https://patch.msgid.link/[email protected]
>> ---
>> 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.
