[PATCHSET sched_ext/for-7.3] sched_ext: NMI-safe exit handling

Hello,

The exit paths weren't NMI-safe: exit claiming walked the sub-scheduler
hierarchy under scx_sched_lock and the bstr exit kfuncs formatted their
messages into a shared buffer under a raw spinlock. Kfuncs in the "any"
category are callable from tracing progs that can attach to functions
running in NMI, and many of them raise scx_error() on invalid inputs, so
an unlucky bad argument from such a prog could deadlock the machine. The
hardlockup handler had the same problem and deferred its abort to an
irq_work, which can't even run on the CPU that detected the lockup.

This series makes exit handling NMI-safe end to end:

 0001 makes exit claiming lock-free: ->aborting is asserted with a
      synchronous lockless sweep and the locked SCX_EXIT_PARENT
      propagation is deferred to an irq_work.

 0002 reverses the bstr exit sequence to claim-first so the message is
      formatted directly into the winner-owned exit_info buffer. With
      these two, scx_bpf_error() and scx_bpf_exit() are safe from any
      context including NMI.

 0003-0005 apply the newly-possible direct error reporting: NMI kicks
      abort the scheduler instead of being silently dropped, the
      hardlockup handler aborts directly from NMI making self-detected
      lockups recoverable, and scx_link_sched() reports failures inline.

 0001-sched_ext-Make-exit-claiming-lock-free.patch
 0002-sched_ext-Format-bstr-exit-messages-after-claiming-t.patch
 0003-sched_ext-Report-NMI-kicks-with-scx_error.patch
 0004-sched_ext-Abort-directly-from-the-hardlockup-handler.patch
 0005-sched_ext-Report-scx_link_sched-failures-inline.patch

Based on sched_ext/for-7.3 (3a21e34eb258).

The patches are also available in the following git branch:

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git scx-exit-nmi

diffstat follows. Thanks.

 kernel/sched/ext/ext.c      | 259 ++++++++++++++++++++++++--------------------
 kernel/sched/ext/internal.h |  10 +-
 kernel/sched/ext/sub.c      |  19 ++--
 3 files changed, 156 insertions(+), 132 deletions(-)

--
tejun