[PATCHSET v2 sched_ext/for-7.3] sched_ext: Bandwidth-limited rescue execution for stranded tasks

[Tejun Heo <tj@kernel.org>]

Hello,

v2: Changes from the sashiko AI review:

    - Declare scx_sched_all outside the CONFIG_EXT_SUB_SCHED block, the
      definition is unconditional (0002).

    - Track the overload grace and usage-decay timestamps in jiffies_64 so
      that 32-bit arithmetic on them can't wrap (0009).

    - Drop the now-unread always_enq_immed rodata mirror (0011).

v1: https://lore.kernel.org/r/20260801085150.2697653-1-tj@kernel.org

A sub-scheduler holds only the cids its parent granted and nothing
guarantees that they cover its tasks' affinities. A task that can run on
none of them currently has no good ending: the scheduler's insert is
cap-rejected and reenqueued until the repeat limit ejects the scheduler, or
the task stalls into the watchdog.

This series adds kernel-side rescue execution. A scheduler marks an insert
that may be cap-rejected with SCX_ENQ_RESCUE and the kernel, instead of
bouncing it, runs the task on the target cpu at a small configured
bandwidth. A rescue starts out non-disruptive. Schedulers that hold caps on
the cid largely stay in control of the cpu - a preemption cap, for example,
still lets its holder preempt the rescuee. When a rescue goes unserved for
too long, it escalates into protected execution.

When a cpu's rescue queue stays oversaturated past the overload threshold,
the kernel ejects the sub-scheduler with the highest recent rescue
consumption on that cpu instead of misblaming the waiter's owner.

- 0001-0005 are preps: naming, helper visibility, dsq-move flag vetting and
  the IGNORE_CAPS preemption waiver.
- 0006-0007 synchronize slice/dsq_vtime writes at insertion commit and add
  SCX_TASK_PROTECTED, the groundwork that lets a kernel-granted slice
  survive scheduler writes.
- 0008-0009 add the rescue machinery and the overload ejection.
- 0010 syncs the tools autogen enum headers.
- 0011-0012 fix scx_qmap's pinned-task direct dispatch and add rescue
  support as the demonstration consumer.

 0001-sched_ext-Rename-scx_local_or_reject_dsq-to-scx_reso.patch
 0002-sched_ext-Make-several-ext.c-helpers-available-outsi.patch
 0003-sched_ext-Factor-out-__scx_bpf_now.patch
 0004-sched_ext-Reject-internal-enq_flags-in-the-dsq-move-.patch
 0005-sched_ext-Make-SCX_ENQ_IGNORE_CAPS-waive-the-preempt.patch
 0006-sched_ext-Synchronize-slice-and-dsq_vtime-writes.patch
 0007-sched_ext-Add-SCX_TASK_PROTECTED.patch
 0008-sched_ext-Add-bandwidth-limited-rescue-execution-for.patch
 0009-sched_ext-Eject-the-top-rescue-consumer-on-overload.patch
 0010-sched_ext-Sync-tools-autogen-enum-headers.patch
 0011-sched_ext-scx_qmap-Idle-check-pinned-tasks-before-di.patch
 0012-sched_ext-scx_qmap-Add-rescue-support.patch

Based on sched_ext/for-7.3 (c5b9316cf3d5).

Git tree: git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git scx-rescue-v2

 include/linux/sched/ext.h                       |   7 +
 kernel/sched/ext/ext.c                          | 690 ++++++++++++++++--------
 kernel/sched/ext/internal.h                     |  78 ++-
 kernel/sched/ext/sub.c                          | 497 ++++++++++++++++-
 kernel/sched/ext/sub.h                          |  41 +-
 kernel/sched/ext/types.h                        |  12 +
 kernel/sched/sched.h                            |  13 +
 tools/sched_ext/include/scx/enum_defs.autogen.h |  16 +
 tools/sched_ext/include/scx/enums.autogen.bpf.h |   3 +
 tools/sched_ext/include/scx/enums.autogen.h     |   1 +
 tools/sched_ext/scx_qmap.bpf.c                  |  89 +--
 tools/sched_ext/scx_qmap.c                      |  24 +-
 tools/sched_ext/scx_qmap.h                      |   1 +
 13 files changed, 1187 insertions(+), 285 deletions(-)

--
tejun