[PATCHSET v3 sched_ext/for-6.20] sched_ext: Fix ops.dequeue() semantics

[Andrea Righi <arighi@nvidia.com>]

The callback ops.dequeue() is provided to let BPF schedulers observe when a
task leaves the scheduler, either because it is dispatched or due to a task
property change. However, this callback is currently unreliable and not
invoked systematically, which can result in missed ops.dequeue() events.

In particular, once a task is removed from the scheduler (whether for
dispatch or due to a property change) the BPF scheduler loses visibility of
the task and the sched_ext core may not always trigger ops.dequeue().

This breaks accurate accounting (i.e., per-DSQ queued runtime sums) and
prevents reliable tracking of task lifecycle transitions.

This patch set fixes the semantics of ops.dequeue(), ensuring that every
ops.enqueue() is balanced by a corresponding ops.dequeue() invocation. In
addition, ops.dequeue() is now properly invoked when tasks are removed from
the sched_ext class, such as on task property changes.

To distinguish between a "regular" dequeue and a property change dequeue a
new dequeue flag is introduced: %SCX_DEQ_SCHED_CHANGE. BPF schedulers can
use this flag to distinguish between regular dispatch dequeues
(%SCX_DEQ_SCHED_CHANGE unset) and property change dequeues
(%SCX_DEQ_SCHED_CHANGE set).

Together, these changes allow BPF schedulers to reliably track task
ownership and maintain accurate accounting.

Changes in v3:
 - Rename SCX_DEQ_ASYNC to SCX_DEQ_SCHED_CHANGE
 - Handle core-sched dequeues (Kuba)
 - Link to v2: https://lore.kernel.org/all/20260121123118.964704-1-arighi@nvidia.com/

Changes in v2:
 - Distinguish between "dispatch" dequeues and "property change" dequeues
   (flag SCX_DEQ_ASYNC)
 - Link to v1: https://lore.kernel.org/all/20251219224450.2537941-1-arighi@nvidia.com

Andrea Righi (2):
      sched_ext: Fix ops.dequeue() semantics
      selftests/sched_ext: Add test to validate ops.dequeue() semantics

 Documentation/scheduler/sched-ext.rst           |  33 ++++
 include/linux/sched/ext.h                       |  11 ++
 kernel/sched/ext.c                              |  89 +++++++++-
 kernel/sched/ext_internal.h                     |   7 +
 tools/sched_ext/include/scx/enum_defs.autogen.h |   2 +
 tools/sched_ext/include/scx/enums.autogen.bpf.h |   2 +
 tools/sched_ext/include/scx/enums.autogen.h     |   1 +
 tools/testing/selftests/sched_ext/Makefile      |   1 +
 tools/testing/selftests/sched_ext/dequeue.bpf.c | 209 ++++++++++++++++++++++++
 tools/testing/selftests/sched_ext/dequeue.c     | 182 +++++++++++++++++++++
 10 files changed, 534 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/sched_ext/dequeue.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/dequeue.c