[PATCH v3 sched_ext/for-7.2 0/2] sched_ext: Add dispatch transaction API

[Cheng-Yang Chou <yphbchou0911@gmail.com>]

scx_bpf_dsq_insert() captures the task's sequence number at insert time,
so any pre-dispatch validity checks a BPF scheduler performs before the
insert fall outside the race detection window.

Patch 1: Introduce a dispatch transaction API with two new kfuncs.
- scx_bpf_dsq_insert_begin() captures the sequence number before any
  pre-dispatch checks, opening the transaction.
- scx_bpf_dsq_insert_commit() inserts the task using the early-captured
  token, extending the race detection window. If the token has gone stale
  (task was dequeued or claimed by another CPU), finish_dispatch()
  silently discards the entry. This is intended for schedulers that do
  not implement properly synchronized dequeue.

Patch 2: Add a selftest exercising the dispatch transaction API. The BPF
scheduler captures the token in ops.dispatch() and flips CPU affinity on
child tasks to trigger dequeue and re-enqueue, verifying that both
successful and stale-token dispatch paths are exercised.

Test plan:
- Applied this series and ran selftests with vng, all tests pass.
- Confirmed selftest skips on older kernel.

This series is discussed in [1].
[1]: https://lore.kernel.org/r/20260319083518.94673-1-arighi@nvidia.com/

Changes in v3:
- Rename cookie API to dispatch transaction API: rename kfuncs from
  scx_bpf_task_get_cookie/scx_bpf_dsq_insert_with_cookie to
  scx_bpf_dsq_insert_begin/scx_bpf_dsq_insert_commit (Tejun Heo)
- Rename internal scx_dsq_insert_commit() to scx_dsq_insert_buf() to
  avoid collision with the new public kfunc (Tejun Heo)
- Move __ksym __weak declarations in common.bpf.h into the API commit
- Redesign selftest to call insert from ops.dispatch() with CPU affinity
  flips to actually exercise the race detection window. v2 called from
  ops.enqueue() where the cookie was silently ignored (Tejun Heo)
- Skip selftest on older kernels lacking the dispatch transaction API
- Apply the same fix logic as commit e18002d2c791 to the selftest
- Link to v2:
  https://lore.kernel.org/r/20260506160412.522199-1-yphbchou0911@gmail.com/

Changes in v2:
- Reword kdoc for both kfuncs to avoid exposing qseq and ops_state as
  implementation details (Kuba Piecuch)
- Add __ksym __weak declarations for new kfuncs in common.bpf.h
- Add dispatch_cookie selftest (Kuba Piecuch)
- Link to v1:
  https://lore.kernel.org/r/20260506075925.371138-1-yphbchou0911@gmail.com/

Thanks,
Cheng-Yang

---

Cheng-Yang Chou (2):
  sched_ext: Add dispatch transaction API
  selftests/sched_ext: Add dispatch_cookie test

 kernel/sched/ext.c                            |  72 ++++++++++++-
 tools/sched_ext/include/scx/common.bpf.h      |   2 +
 tools/testing/selftests/sched_ext/Makefile    |   1 +
 .../selftests/sched_ext/dispatch_cookie.bpf.c | 102 ++++++++++++++++++
 .../selftests/sched_ext/dispatch_cookie.c     | 100 +++++++++++++++++
 5 files changed, 272 insertions(+), 5 deletions(-)
 create mode 100644 tools/testing/selftests/sched_ext/dispatch_cookie.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/dispatch_cookie.c

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2.48.1