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sched_ext: Gate kicks on SCX_CAP_BASE and preemption on SCX_CAP_PREEMPT

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A kick forces a scheduling event on the target cpu, and a preemption also
evicts the running task. Gate both on caps. Any kick requires baseline
access on the cid, and preempting a task the sub-sched does not own -
whether by a SCX_ENQ_PREEMPT insert or a SCX_KICK_PREEMPT kick - requires
the new SCX_CAP_PREEMPT. Gating either alone would leave a hole - the
weakest cap authorizing preempting kicks, or plain kicks disturbing cpus the
kicker has no access to.

Preempting the sched's own subtree is always allowed, and the cap extends
the right to any task on the cid. PREEMPT implies ENQ, and so ENQ_IMMED.

A preempting insert tests the running task under the target rq lock and is
rejected and reenqueued unless the victim is in the inserter's subtree or it
holds PREEMPT. A migration-disabled task is admitted regardless, but with
SCX_ENQ_PREEMPT stripped.

Kicks are enforced on the delivery path, where the effective caps can be
read coherently under the target rq's lock. A kick from a sub-sched lacking
SCX_CAP_BASE on the cid is dropped, and a SCX_KICK_PREEMPT kick without
PREEMPT for a task outside the kicker's subtree degrades to a plain
reschedule.

Unlike the enqueue caps, PREEMPT is checked only at the instant of the
insert or kick, never as a standing property of a queued task.

v2: Clear SCX_ENQ_PREEMPT on the offline and migration_pending force-admits.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
This commit is contained in:
Tejun Heo 2026-07-13 22:18:44 -10:00
commit 6ea3be3680
4 changed files with 77 additions and 15 deletions
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