[PATCH 0/2] sched_ext: Reset runnable_at when a task leaves its scheduler

[Michal Blaszczyk <michalblk@google.com>]

A task that is switched out of SCHED_EXT while it's waiting keeps its
p->scx.runnable_at. When it comes back, the watchdog counts the time it
spent in another class as waiting, and can abort the scheduler for a stall
that didn't happen. Patch 1 restarts the wait when a task leaves its
scheduler. Patch 2 adds a selftest that reproduces the spurious stall.

The bug goes back to the watchdog's introduction in v6.12. The series is
based on sched_ext/for-7.4, and patch 1 also applies to for-7.3-fixes.

Tested in a virtme-ng VM (16 CPUs, PROVE_LOCKING): class_switch_stall
fails every time without patch 1 and passes with it. The other sched_ext
selftests give the same results with and without the series (enq_blocked
fails in this VM either way).

I used an AI coding assistant to find the bug and to write and test both
patches.

Michal Blaszczyk (2):
  sched_ext: Reset runnable_at when a task leaves its scheduler
  selftests/sched_ext: Add class_switch_stall test

 kernel/sched/ext/ext.c                        |  14 +-
 tools/testing/selftests/sched_ext/Makefile    |   1 +
 .../sched_ext/class_switch_stall.bpf.c        | 121 +++++++++
 .../selftests/sched_ext/class_switch_stall.c  | 240 ++++++++++++++++++
 4 files changed, 375 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/sched_ext/class_switch_stall.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/class_switch_stall.c


base-commit: 1f5dde349ac2fca1f119ae8de3f0d2f1c2a8d338
-- 
2.56.0.176.g3575e65682-goog