[PATCHSET sched_ext/for-7.0-fixes] sched_ext: Fix SCX_KICK_WAIT deadlock

[Tejun Heo <tj@kernel.org>]

Hello,

SCX_KICK_WAIT busy-waits in kick_cpus_irq_workfn() until the target CPU
reschedules. Because the irq_work runs in hardirq context, the waiting
CPU's kick_sync never advances, and if multiple CPUs form a wait cycle, all
deadlock. This was reported by Christian while testing on arm64.

0001 fixes the deadlock by deferring the wait to a balance callback which
drops the rq lock and enables IRQs, allowing IPIs to be processed and
kick_sync to keep advancing during the wait.

0002 adds a selftest that creates a 3-CPU kick_wait cycle to reproduce the
issue.

Based on sched_ext/for-7.0-fixes (db08b1940f4b).

 0001-sched_ext-Fix-SCX_KICK_WAIT-deadlock-by-deferring-wa.patch
 0002-selftests-sched_ext-Add-cyclic-SCX_KICK_WAIT-stress-.patch

 kernel/sched/ext.c                                 |  95 +++++++---
 kernel/sched/sched.h                               |   3 +
 tools/testing/selftests/sched_ext/Makefile         |   1 +
 .../selftests/sched_ext/cyclic_kick_wait.bpf.c     |  68 ++++++++
 .../testing/selftests/sched_ext/cyclic_kick_wait.c | 194 +++++++++++++++++++++
 5 files changed, 336 insertions(+), 25 deletions(-)

--
tejun