Re: [PATCH 3/4] arm64: wire SDEI NMI into the hardlockup watchdog

[Kiryl Shutsemau <kirill@shutemov.name>]

On Fri, Jun 05, 2026 at 01:03:05PM -0700, Doug Anderson wrote:
> Hi,
> 
> On Wed, Jun 3, 2026 at 7:36 AM Kiryl Shutsemau <kirill@shutemov.name> wrote:
> >
> > From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
> >
> > Select HAVE_HARDLOCKUP_DETECTOR_ARCH so the framework takes its backend
> > from this driver. A per-CPU hrtimer checks its buddy's heartbeat and
> > signals event 0 at a stalled CPU, which runs watchdog_hardlockup_check()
> > NMI-like.
> >
> > The source is chosen at boot: SDEI if firmware provides it, otherwise a
> > perf-NMI counter (pseudo-NMI) fallback -- one image covers both.
> >
> > Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> > ---
> >  arch/arm64/Kconfig          |   1 +
> >  drivers/firmware/Kconfig    |   3 +
> >  drivers/firmware/sdei_nmi.c | 247 +++++++++++++++++++++++++++++++++++-
> >  3 files changed, 248 insertions(+), 3 deletions(-)
> 
> I'm a little confused about this patch. We already have a buddy
> hardlockup detector using the hrtimer, and it's even been improved
> recently to trigger in a smaller time bound. It looks as if you're
> duplicating bits of the perf and buddy detector here?
> 
> I don't think you need this patch at all. The existing buddy detector
> + patches #1 and #2 in your series should be sufficient.

You're mostly right.

Buddy + #2 covers the console case (the remote branch triggers the
culprit's backtrace, which #2 makes deliverable), and #4 gets the wedged
CPU's registers into the vmcore.

The one thing this patch adds that a config can't is boot-time source
selection: PERF-compiled kernels have no detector on a pseudo_nmi=0
boot, and PREFER_BUDDY costs the pseudo-NMI machines perf
self-detection. But that's arguably out of scope for the patchset.

I'll drop this patch in v2 and run PREFER_BUDDY here. If a runtime
perf->buddy fallback ever materializes, the gap closes entirely.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov