[PATCHSET sched_ext/for-7.1] sched_ext: Implement SCX_ENQ_IMMED

[Tejun Heo]

Hello,

SCX_ENQ_IMMED makes enqueue to local DSQs succeed only if the task can
start running immediately - the current task is done and no other tasks are
waiting. If the condition isn't met, the task is re-enqueued through
ops.enqueue(). This gives the BPF scheduler tighter control over when tasks
actually land on a CPU.

- Patch 1 disallows setting slice to zero via scx_bpf_task_set_slice() as
  zero slice is used by ENQ_IMMED to detect whether the current task is
  done.

- Patch 2 implements SCX_ENQ_IMMED with reenqueue support and loop
  detection.

- Patch 3 adds SCX_OPS_ALWAYS_ENQ_IMMED ops flag to automatically apply
  IMMED to all local DSQ enqueues.

This patchset depends on:

- "sched_ext: Overhaul DSQ reenqueue infrastructure"
  http://lkml.kernel.org/r/20260306190623.1076074-1-tj@xxxxxxxxxx

Based on sched_ext/for-7.1 (4f8b122848db) + scx-reenq (a41719e6ae12).

 0001-sched_ext-Disallow-setting-slice-to-zero-via-scx_bpf.patch
 0002-sched_ext-Implement-SCX_ENQ_IMMED.patch
 0003-sched_ext-Add-SCX_OPS_ALWAYS_ENQ_IMMED-ops-flag.patch

Git tree:

  git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git scx-enq-immed

 include/linux/sched/ext.h            |   3 +
 kernel/sched/ext.c                   | 208 +++++++++++++++++++++++++++++++----
 kernel/sched/ext_internal.h          |  43 ++++++++
 kernel/sched/sched.h                 |   2 +
 tools/sched_ext/include/scx/compat.h |   1 +
 tools/sched_ext/scx_qmap.bpf.c       |   7 +-
 tools/sched_ext/scx_qmap.c           |   9 +-
 7 files changed, 250 insertions(+), 23 deletions(-)

--
tejun