[PATCHSET sched_ext/for-7.2-fixes] sched_ext: Fix finite-slice ticks on nohz_full

[Andrea Righi <arighi@nvidia.com>]

When an EXT task with a finite slice is scheduled on a nohz_full CPU, the
scheduler may incorrectly keep the tick disabled because it still observes the
outgoing task (e.g., the idle task) during the context switch. As a result, the
incoming task can run past its assigned slice.

Fix this by always enabling the scheduler tick for finite-slice EXT tasks and
provide a kselftest that reproduces the scenario and verifies that scheduler
ticks are delivered correctly. Test is skipped in kernels without available
nohz_full CPUS.

Test case (using virtme-ng):

  $ vng -vb --config tools/testing/selftests/sched_ext/config \
        --configitem CONFIG_NO_HZ_FULL=y
  $ vng --append "nohz_full=all" -- \
        tools/testing/selftests/sched_ext/runner -t nohz_tick

Before:

  $ vng --append "nohz_full=all" -- tools/testing/selftests/sched_ext/runner -t nohz_tick
  ===== START =====
  TEST: nohz_tick
  DESCRIPTION: Verify finite EXT slices restart the NOHZ_FULL tick
  OUTPUT:
  ERR: nohz_tick.c:270
  Finite-slice worker received only 1 scheduler ticks
  not ok 1 nohz_tick #
  =====  END  =====


  =============================

  RESULTS:

  PASSED:  0
  SKIPPED: 0
  FAILED:  1

  Failed tests:
    - nohz_tick

After:

  $ vng --append "nohz_full=all" -- tools/testing/selftests/sched_ext/runner -t nohz_tick
  ===== START =====
  TEST: nohz_tick
  DESCRIPTION: Verify finite EXT slices restart the NOHZ_FULL tick
  OUTPUT:
  CPU 1 received 4 finite-slice ticks
  ok 1 nohz_tick #
  =====  END  =====


  =============================

  RESULTS:

  PASSED:  1
  SKIPPED: 0
  FAILED:  0

Andrea Righi (2):
  sched_ext: Enable tick for finite slices on nohz_full
  selftests/sched_ext: Verify nohz_full tick behavior

 kernel/sched/ext/ext.c                            |  14 +-
 tools/testing/selftests/sched_ext/Makefile        |   1 +
 tools/testing/selftests/sched_ext/nohz_tick.bpf.c |  65 +++++
 tools/testing/selftests/sched_ext/nohz_tick.c     | 309 ++++++++++++++++++++++
 4 files changed, 386 insertions(+), 3 deletions(-)