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

Christian Loehle
2026-03-30T08:52:05Z

On 3/29/26 00:18, Tejun Heo wrote:
> 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

For both:
Tested-by: Christian Loehle <christian.loehle@arm.com>

Thanks for fixing this!