[PATCHSET sched_ext/for-7.4] sched_ext: Add lazy preemption support

[Andrea Righi <arighi@nvidia.com>]

The fair scheduling class can defer a scheduling boundary through lazy
rescheduling, but sched_ext currently exposes only immediate preemption to BPF
schedulers.

The series adds lazy variants of the enqueue and kick preemption flags.  They
expire the running sched_ext task's slice while allowing the scheduling boundary
to be deferred until a return to user space or the next scheduler tick.
Non-local lazy enqueue requests retain the head-insertion semantics of immediate
preemption.

SCX_OPS_LAZY_SLICE_EXPIRY also lets a BPF scheduler request lazy rescheduling
when a slice expires in task_tick_scx(), matching the choice available to the
fair scheduling class. Immediate behavior remains the default and is used while
sched_ext is being disabled.

An fexit probe on __resched_curr() records the exact reschedule TIF after the
running task's slice is expired. Together with the stopping callback, the
selftests cover immediate and lazy kicks, coalescing in both orders, tick-driven
slice expiry and invalid flag combinations. Individual modes are skipped when
the running kernel does not expose the corresponding flag.

Andrea Righi (2):
      sched_ext: Add lazy preemption support
      selftests/sched_ext: Add kick selftest

 kernel/sched/ext/ext.c                          | 115 ++++--
 kernel/sched/ext/internal.h                     |  36 +-
 kernel/sched/ext/sub.c                          |  15 +-
 tools/sched_ext/include/scx/enum_defs.autogen.h |   3 +
 tools/sched_ext/include/scx/enums.autogen.bpf.h |   6 +
 tools/sched_ext/include/scx/enums.autogen.h     |   2 +
 tools/sched_ext/include/scx/enums_abi.autogen.h |   5 +-
 tools/testing/selftests/sched_ext/Makefile      |   1 +
 tools/testing/selftests/sched_ext/kick.bpf.c    | 144 +++++++
 tools/testing/selftests/sched_ext/kick.c        | 479 ++++++++++++++++++++++++
 10 files changed, 773 insertions(+), 33 deletions(-)
 create mode 100644 tools/testing/selftests/sched_ext/kick.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/kick.c