[PATCHSET v2 sched_ext/for-7.3-fixes] sched_ext: Idle claim recovery and online cid mask

[Tejun Heo <tj@kernel.org>]

Hello,

v2: - Dropped the idle-to-idle notification patch. Restoring an unused idle
      claim from ops.dispatch() is the established pattern (Andrea Righi).
      Replaced with a patch fixing scx_qmap to do that and documenting the
      pattern in ops.update_idle().
    - Reworded the online cmask getter kerneldoc (Andrea Righi).

v1: https://lore.kernel.org/r/20260914234259.3585373-1-tj@kernel.org

Two issues surfaced while converting a scheduler that maintains its own idle
and online tracking.

An idle CPU can be claimed and kicked without receiving a task. When it
picks idle again there is no idle transition, so ops.update_idle() stays
silent and a scheduler tracking idle CPUs itself loses the CPU until an
unrelated task runs there. scx_qmap has this hole. The first patch restores
the claim from ops.dispatch(), where a kick guarantees a call, and documents
the pattern in ops.update_idle().

A cid-form scheduler that installs its own mapping has no way to learn which
cids are online: the online count identifies the set only under the default
mapping and the CPU-form cpumask is unusable from cid programs. The second
patch adds scx_bpf_online_cmask(), a kernel-maintained cmask in the
scheduler's arena that follows the SCX hotplug notifications.

Verified by building the kernel and tools/sched_ext and running scx_qmap
under CPU and fork load in a VM. The online cmask patch only changed
kerneldoc since v1, where a local selftest offlined and onlined a CPU in a
VM and checked the mask and the callbacks at each step.

Based on sched_ext/for-7.3-fixes (c7a1c6e8004a).

This patchset contains the following 2 patches.

 0001 sched_ext: scx_qmap: Restore unused idle claims from ops.dispatch()
 0002 sched_ext: Maintain an online cid mask in the scheduler arena

The patchset is also available in the following git branch:

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git cid-online-cmask-v2

diffstat follows. Thanks.

 kernel/sched/ext/ext.c                   | 75 +++++++++++++++++++++++++++++---
 kernel/sched/ext/internal.h              |  9 +++-
 kernel/sched/ext/sub.c                   | 10 +++--
 tools/sched_ext/include/scx/common.bpf.h |  1 +
 tools/sched_ext/scx_qmap.bpf.c           | 14 ++++--
 5 files changed, 94 insertions(+), 15 deletions(-)

--
tejun