Re: [PATCH v5 0/4] arm64: cross-CPU NMI via SDEI

[Kiryl Shutsemau <kas@kernel.org>]

On Wed, Jul 29, 2026 at 02:37:37PM -0700, Doug Anderson wrote:
> Hi,
> 
> On Tue, Jul 28, 2026 at 5:18 AM Kiryl Shutsemau <kas@kernel.org> wrote:
> >
> > The piece that isn't boot-time yet is the hardlockup detector that drives
> > all this. perf and buddy are mutually exclusive at compile time. We want
> > to use perf watchdog for FEAT_NMI while SDEI needs buddy.
> 
> FWIW: can I ask why? You're worried about the "all CPUs lock up at the
> same time" problem? I am aware of some work towards solving that (even
> with buddy) by letting the hardware watchdog driver's "bark" interrupt
> run at NMI-level (or pseudo-NMI-level). With that change, I'm not
> aware of any benefits of the "perf" detector over the buddy one. 

Honestly, no strong reason. I flagged perf mostly as "the detector you'd
reach for once NMIs are back", not from a concrete need.

The all-CPUs-wedge-at-once case is the only gap I was vaguely worried
about, since there's no live buddy left to notice it. If the NMI-level
bark work you mention closes that for buddy, then I don't see a reason to
want perf at all -- and as you say, buddy is cheaper.

Buddy everywhere would be strictly better for us.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov