Re: [PATCH x86/nmi 1/2] x86/nmi: Accumulate NMI-progress evidence in exc_nmi()

From: Paul E. McKenney
Date: Mon Jan 09 2023 - 12:06:33 EST


On Mon, Jan 09, 2023 at 05:19:59PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 04, 2023 at 05:15:36PM -0800, Paul E. McKenney wrote:
>
> > diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
> > index cec0bfa3bc04f..4f1651dc65b3a 100644
> > --- a/arch/x86/kernel/nmi.c
> > +++ b/arch/x86/kernel/nmi.c
> > @@ -69,6 +69,15 @@ struct nmi_stats {
> > unsigned int unknown;
> > unsigned int external;
> > unsigned int swallow;
> > + unsigned long recv_jiffies;
> > + unsigned long idt_seq;
> > + unsigned long idt_nmi_seq;
> > + unsigned long idt_ignored;
> > + atomic_long_t idt_calls;
> > + unsigned long idt_seq_snap;
> > + unsigned long idt_nmi_seq_snap;
> > + unsigned long idt_ignored_snap;
> > + long idt_calls_snap;
> > };
>
> Urgh, this is more than a whole cacheline of extra data :/ Can't we make
> this #ifdef CONFIG_NMI_CHECK_CPU ?

We can. However, the new data is at the end of the structure. Plus doing
that will also require converting the IS_ENABLED() checks to #ifdef or
to lots of tiny functions, neither of which will be at all pretty.

Another approach would be to have a #else that created a union of all
of these fields, thus reducing this unused space so that of a single
unsigned long, while still permitting IS_ENABLED() checks. Thoughts?

> > @@ -509,8 +526,15 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
> >
> > inc_irq_stat(__nmi_count);
> >
> > - if (!ignore_nmis)
> > + if (IS_ENABLED(CONFIG_NMI_CHECK_CPU) && ignore_nmis) {
> > + WRITE_ONCE(nsp->idt_ignored, nsp->idt_ignored + 1);
> > + } else if (!ignore_nmis) {
> > + WRITE_ONCE(nsp->idt_nmi_seq, nsp->idt_nmi_seq + 1);
> > + WARN_ON_ONCE(!(nsp->idt_nmi_seq & 0x1));
> > default_do_nmi(regs);
> > + WRITE_ONCE(nsp->idt_nmi_seq, nsp->idt_nmi_seq + 1);
> > + WARN_ON_ONCE(nsp->idt_nmi_seq & 0x1);
> > + }
> >
> > irqentry_nmi_exit(regs, irq_state);
>
> That is not a NO-OP when !CONFIG_NMI_CHECK_CPU :/

Good catch, will fix!

Thanx, Paul