[PATCH 16/16] sched_ext: Require cid-form struct_ops for sub-sched support

[Tejun Heo <tj@kernel.org>]

Sub-scheduler support is tied to the cid-form struct_ops: sub_attach /
sub_detach will communicate allocation via cmask, and the hierarchy assumes
all participants share a single topological cid space. A cpu-form root that
accepts sub-scheds would need cpu <-> cid translation on every cross-sched
interaction, defeating the purpose.

Enforce this at validate_ops():
- A sub-scheduler (scx_parent(sch) non-NULL) must be cid-form.
- A root that exposes sub_attach / sub_detach must be cid-form.

scx_qmap, which is currently the only scheduler demoing sub-sched support,
was converted to cid-form in the preceding patch, so this doesn't cause
breakage.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 02bdd393bbe4..71d6a2b39e64 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -6753,6 +6753,23 @@ static int validate_ops(struct scx_sched *sch, const struct sched_ext_ops *ops)
 	if (!sch->is_cid_type && (ops->cpu_acquire || ops->cpu_release))
 		pr_warn("ops->cpu_acquire/release() are deprecated, use sched_switch TP instead\n");
 
+	/*
+	 * Sub-scheduler support is tied to the cid-form struct_ops. A sub-sched
+	 * attaches through a cid-form-only interface (sub_attach/sub_detach),
+	 * and a root that accepts sub-scheds must expose cid-form state to
+	 * them. Reject cpu-form schedulers on either side.
+	 */
+	if (!sch->is_cid_type) {
+		if (scx_parent(sch)) {
+			scx_error(sch, "sub-sched requires cid-form struct_ops");
+			return -EINVAL;
+		}
+		if (ops->sub_attach || ops->sub_detach) {
+			scx_error(sch, "sub_attach/sub_detach requires cid-form struct_ops");
+			return -EINVAL;
+		}
+	}
+
 	return 0;
 }
 
-- 
2.53.0