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

[Joel Fernandes]

On 10/13/2025 4:00 PM, Peter Zijlstra wrote:
> 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 :/

Great point. I will move this to DECLARE_PER_CPU_CACHE_HOT()
so it's co-located with preempt_count and run some tests. Let me know if that
works for you, thanks!

 - Joel