[PATCHSET v4 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.

This 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.

SCX_OPS_LAZY_SLICE_EXPIRY selects lazy slice expiry by default. Schedulers can
override that default in either direction for individual tasks through
scx_bpf_task_set_slice_expiry(). The helper preserves sub-scheduler task
ownership boundaries. Bypass continues to force immediate rescheduling.

Lazy enqueue and kick requests also restore the scheduler tick when targeting an
infinite-slice task on a NO_HZ_FULL CPU, ensuring forward progress even when the
target's tick has stopped.

The selftests cover immediate and lazy preemption, coalescing and precedence
between kick modes, enqueue preemption, per-task slice-expiry overrides, invalid
flags and lazy preemption of infinite-slice tasks on NO_HZ_FULL CPUs.

Tested in separate virtme-ng runs, covering preempt=lazy and nohz_full:

 $ vng -a "preempt=lazy" -- tools/testing/selftests/sched_ext/runner -t kick

  kick:
    PASSED:  6
    SKIPPED: 0
    FAILED:  0

 $ vng -a "preempt=lazy nohz_full=8-15" -- tools/testing/selftests/sched_ext/runner -t nohz_tick

  nohz_tick:
    CPU 8 received 7 finite-slice ticks
    PASSED:  1
    SKIPPED: 0
    FAILED:  0

Changes in v4:

 - Preserve SCX_KICK_WAIT synchronization when it is combined with
   SCX_KICK_PREEMPT_LAZY (Sashiko).
 - Restore the scheduler tick dependency when ops.tick() lazily expires an
   infinite slice, ensuring the request is promoted by a later tick (Sashiko).
 - kselftest: verify that the deferred SCX_KICK_WAIT callback runs (Sashiko).
 - kselftest: keep the kick test controller off the victim CPU and skip cleanly
   when a second allowed CPU is unavailable (Sashiko).
 - kselftest: select the nohz_tick controller from allowed housekeeping CPUs
   outside the NOHZ_FULL mask and skip when the required topology is
   unavailable.
 - Link to v3: https://lore.kernel.org/r/20260914144620.2032614-1-arighi@nvidia.com

Changes in v3:
 - Avoid redundant rq clock updates when restoring the tick dependency and use
   the pinned rq lock helpers for lazy kick delivery (Sashiko).
 - Set PR_SET_PDEATHSIG for the indefinitely spinning kick test workers so they
   don't survive unexpected test-runner exit (Sashiko).
 - Wait for a delta in nr_lazy_victim_running instead of an absolute count that
   may include reschedules from the previous phase (Sashiko).
 - Increase the nohz_tick scheduler watchdog and phase timeouts to tolerate
   scheduling delays in oversubscribed test VMs.
 - Link to v2: https://lore.kernel.org/r/20260914084955.1798562-1-arighi@nvidia.com

Changes in v2:
 - Make lazy slice expiry a per-task property (Tejun Heo).
 - Add scx_bpf_task_set_slice_expiry() for per-task overrides from any sched_ext
   callback (preserving sub-scheduler task ownership).
 - Restore the scheduler tick dependency for lazy enqueue and kick preemption of
   infinite-slice tasks on NO_HZ_FULL CPUs (Tejun Heo).
 - Accumulate immediate and lazy kick requests independently and resolve their
   precedence in kick_one_cpu() (Tejun Heo).
 - Allow PREEMPT|PREEMPT_LAZY and WAIT|PREEMPT_LAZY, immediate preemption, WAIT
   and plain kicks take precedence over lazy preemption (Tejun Heo).
 - Clear both preemption masks together when processing or skipping a preemption
   request (Tejun Heo).
 - Extend the kselftests with combined and ordered kick requests, per-task
   expiry overrides and NO_HZ_FULL coverage.
 - Link to v1: https://lore.kernel.org/r/20260911195800.974364-1-arighi@nvidia.com

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

 include/linux/sched/ext.h                         |   8 +
 kernel/sched/ext/ext.c                            | 153 +++++-
 kernel/sched/ext/internal.h                       |  39 +-
 kernel/sched/ext/sub.c                            |  15 +-
 tools/sched_ext/include/scx/compat.bpf.h          |  13 +
 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      | 182 +++++++
 tools/testing/selftests/sched_ext/kick.c          | 608 ++++++++++++++++++++++
 tools/testing/selftests/sched_ext/nohz_tick.bpf.c |  56 +-
 tools/testing/selftests/sched_ext/nohz_tick.c     | 249 ++++++++-
 14 files changed, 1273 insertions(+), 67 deletions(-)
 create mode 100644 tools/testing/selftests/sched_ext/kick.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/kick.c