> On May 12, 2026, at 6:16 PM, Boqun Feng <[email protected]> wrote: > > On Tue, May 12, 2026 at 03:22:39PM -0400, Joel Fernandes wrote: >> >> >>> 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? >> > > Something like this on top of your patch? > > diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h > index cc06bda52c3e..a59a33e0f5ca 100644 > --- a/include/linux/hardirq.h > +++ b/include/linux/hardirq.h > @@ -110,7 +110,8 @@ void irq_exit_rcu(void); > do { \ > lockdep_off(); \ > arch_nmi_enter(); \ > - BUG_ON(__this_cpu_read(nmi_nesting) == UINT_MAX); \ > + /* Maximum NMI nesting is 15 */ \ > + BUG_ON(__this_cpu_read(nmi_nesting) == 15); \ > __this_cpu_inc(nmi_nesting); \ > __preempt_count_add(HARDIRQ_OFFSET); \ > preempt_count_set(preempt_count() | NMI_MASK); \ > > I will need to adjust this in patch #10 as well, but shouldn't be hard.
Maybe >= but this sounds good to me. Thanks for adjusting the patches. Thanks! > > Regards, > Boqun > >> Thanks. >>
