Pull to receive the __arena argument conversion:

 67f1f4a48c24 ("sched_ext: Pass kernel arena pointers to ops_cid callbacks")
 a8dc810968af ("sched_ext: Convert sub-cap kfuncs to __arena cmask arguments")
 a05c5b5cb5cf ("sched_ext: Convert scx_bpf_cid_override() to __arena array arguments")

along with the bpf-next branch carrying the __arena argument support they
depend on.

Conflict in kernel/sched/ext/ext.c between:

 c384ab8a0b13 ("sched_ext: Move the config-off sub-cap kfunc stubs into sub.c")

and:

 a8dc810968af ("sched_ext: Convert sub-cap kfuncs to __arena cmask arguments")

which updated the stubs in their old ext.c location. Resolved by keeping
ext.c without the stubs and applying the prototype conversion to the
relocated stubs in sub.c.

Signed-off-by: Tejun Heo <tj@kernel.org>

dequeue_task_scx() masks SCX_DEQ_CORE_SCHED_EXEC out of the
SCX_DEQ_SCHED_CHANGE decision, but the test can never fire: the incoming
flags are an int of generic DEQUEUE_* bits while the flag is bit 32, and the
core-sched execute path never goes through class dequeue anyway -
set_next_task_scx() calls ops_dequeue() with the flag directly. The test was
live when the SCX_DEQ_SCHED_CHANGE computation sat in ops_dequeue() and
became dead when 03f5304aad0f ("sched_ext: Pass full dequeue flags to
ops.quiescent()") moved the computation here. Drop it.

Signed-off-by: Tejun Heo <tj@kernel.org>

With sub-schedulers, tasks of different schedulers routinely share rqs and
SMT siblings, but scx_prio_less() consults ops.core_sched_before() only when
both tasks belong to the same scheduler. Every pair spanning two schedulers
falls back to the default ordering, so no scheduler can express ordering
across a scheduler boundary, including a root over its sub-schedulers'
tasks.

Order a pair spanning schedulers by the nearest common ancestor that
implements ops.core_sched_before(): both tasks are in its subtree, making
this the one op where a scheduler is called on tasks it delegated to its
sub-schedulers and may not be scheduling anymore. Same-scheduler pairs keep
using the owning scheduler's op so a parent never orders inside a subtree it
delegated. The op is skipped when the deciding scheduler is bypassing on
either task's CPU.

Update scx_qmap to fall back to the kernel's default ordering when handed a
delegated task it has no task_ctx for.

Signed-off-by: Tejun Heo <tj@kernel.org>

The default core-sched ordering runs the longest waiting task first by
comparing p->scx.core_sched_at stamps. The stamp is maintained under two
rules. touch_core_sched() stamps when a task starts waiting for a CPU and
when its slice runs out. If the scheduler implements
ops.core_sched_before(), touch_core_sched_dispatch() re-stamps on every
dispatch.

A comparison can see one stamp taken under each rule, which isn't a
meaningful ordering. The dispatch rule also buys little - it only aligns
bypass-mode comparisons with the local DSQ order. Multiple schedulers make
the mixed comparisons more common.

Wait time is what p->scx.runnable_at already tracks for the stall watchdog.
Delete core_sched_at with both touch functions and compare runnable_at in
the scx_prio_less() fallback.

runnable_at is refreshed only on enqueue and goes stale while a task keeps
occupying its CPU. Instead of re-stamping, order a running task after every
waiting task as it is the most recently serviced.

Signed-off-by: Tejun Heo <tj@kernel.org>

scx_prio_less() implements prio_less() semantics - %true means that @a is
the lower priority and should run after @b. ops.core_sched_before() is
documented to return %true when @a should run before @b. scx_prio_less()
returns the op's value as-is, inverting the documented semantics at runtime.

Call the op with the arguments swapped.

scx_qmap followed the wiring instead of the documentation and returned %true
for the younger task, so the two inversions canceled out and it behaved as
intended. Flip its comparison to match. scx_qmap is likely the only current
user in or out of the kernel tree. Any scheduler written the same way needs
the same flip, while schedulers following the documentation are fixed by
this change.

Fixes: 7b0888b7cc19 ("sched_ext: Implement core-sched support")
Cc: stable@vger.kernel.org # v6.12+
Signed-off-by: Tejun Heo <tj@kernel.org>

The EOPNOTSUPP stubs for the sub-cap kfuncs live in ext.c under #ifndef
CONFIG_EXT_SUB_SCHED while the real definitions live in sub.c. Move the
stubs into sub.c so all sub kfunc definitions live in one file. Pure code
move, no functional change.

Signed-off-by: Tejun Heo <tj@kernel.org>

sched_class->balance() is gone from sched_ext and what balance_one() does is
run dispatch to produce something pickable. Update the balance-era names to
dispatch terms:

- balance_one() -> dispatch_one()
- SCX_RQ_IN_BALANCE -> SCX_RQ_IN_DISPATCH

No BPF scheduler reads the flag. The enum autogen headers gain the new name
with the old entry retained like other removed enumerators, zero-filling at
load time. No functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>

The fixup demoting a keep verdict when @prev is not on ext_sched_class
guarded against the rq-level SCX_RQ_BAL_KEEP flag going stale back when
balancing and picking were separate operations.

The verdict now travels in the return value, created and consumed in one
invocation against the @prev it evaluated, and every keep decision tests
SCX_TASK_QUEUED under the rq lock, which implies ext_sched_class as a class
switch dequeues first. Drop the fixup along with dispatch_core_pick()'s
copy.

Signed-off-by: Tejun Heo <tj@kernel.org>

kick_sync_wait_bal_cb() assumes it runs on the rq's CPU from the
__schedule() tail: the snapshots it compares against live in that CPU's
percpu area and the busy-wait runs with the rq lock dropped and IRQs
enabled.

However, dispatch can now drop the rq lock while the callback sits queued,
and rq lock takers in that window (the sched class change paths, the scx
task iterator) flush pending balance callbacks on release, running the
callback on a foreign CPU. Such a run compares against unrelated snapshots
and can deadlock when the executing CPU is itself a wait target.

Bail on a foreign CPU and leave the wait state alone. The wait only observes
progress that the resched kicks already guarantee and the rq's next wait
picks up the stale cpus_to_sync bits.

Fixes: 4c95380701f5 ("sched/ext: Fold balance_scx() into pick_task_scx()")
Cc: stable@vger.kernel.org # v6.19+
Signed-off-by: Tejun Heo <tj@kernel.org>

scx_bpf_dsq_reenq() queues a deferred reenq (dru) that runs from
run_deferred(), not ops.dispatch(). If the DSQ is destroyed before the dru
runs, process_deferred_reenq_users() sees dsq->id == SCX_DSQ_INVALID and
hits the BUG_ON. destroy_dsq() doesn't flush pending drus, so just skip.

tj: Read dsq->id once with READ_ONCE(). Reading it separately in the INVALID
    check and the BUG_ON would leave a window where destroy_dsq() can
    invalidate the id between the two reads and still trigger the BUG_ON.

Fixes: 84b1a0ea0b7c ("sched_ext: Implement scx_bpf_dsq_reenq() for user DSQs")
Cc: stable@vger.kernel.org # v7.1+
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>