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

Peter Zijlstra
2025-10-14T08:25:24Z

On Mon, Oct 13, 2025 at 05:27:32PM -0400, Joel Fernandes wrote:
>
>
> 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@nvidia.com>
> >>
> >> 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@gmail.com>
> >> Signed-off-by: Joel Fernandes <joelaf@google.com>
> >> Signed-off-by: Lyude Paul <lyude@redhat.com>
> >> ---
> >> 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!

Well, I hate how on entry we then end up incrementing both. How terrible
would it be to make __preempt_count u64 instead?