[PATCH v2 02/10] sched_ext: Add select_cpu kfuncs to scx_kfunc_ids_unlocked

[Tejun Heo]

select_cpu_from_kfunc() has an extra scx_kf_allowed_if_unlocked() branch
that accepts calls from unlocked contexts and takes task_rq_lock() itself
- a "callable from unlocked" property encoded in the kfunc body rather
than in set membership. That's fine while the runtime check is the
authoritative gate, but the upcoming verifier-time filter uses set
membership as the source of truth and needs it to reflect every context
the kfunc may be called from.

Add the three select_cpu kfuncs to scx_kfunc_ids_unlocked so their full
set of callable contexts is captured by set membership. This follows the
existing dual-set convention used by scx_bpf_dsq_move{,_vtime} and
scx_bpf_dsq_move_set_{slice,vtime}, which are members of both
scx_kfunc_ids_dispatch and scx_kfunc_ids_unlocked.

While at it, add brief comments on each duplicate BTF_ID_FLAGS block
(including the pre-existing dsq_move ones) explaining the dual
membership.

No runtime behavior change: the runtime check in select_cpu_from_kfunc()
remains the authoritative gate until it is removed along with the rest
of the scx_kf_mask enforcement in a follow-up.

v2: Clarify dispatch-set comment to name scx_bpf_dsq_move*() explicitly so it
    doesn't appear to cover scx_bpf_sub_dispatch() (Andrea Righi).

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 kernel/sched/ext.c      |    6 ++++++
 kernel/sched/ext_idle.c |    4 ++++
 2 files changed, 10 insertions(+)

--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -8497,6 +8497,7 @@ BTF_ID_FLAGS(func, scx_bpf_dispatch_nr_s
 BTF_ID_FLAGS(func, scx_bpf_dispatch_cancel, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, scx_bpf_dsq_move_to_local, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, scx_bpf_dsq_move_to_local___v2, KF_IMPLICIT_ARGS)
+/* scx_bpf_dsq_move*() also in scx_kfunc_ids_unlocked: callable from unlocked contexts */
 BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_slice, KF_RCU)
 BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_vtime, KF_RCU)
 BTF_ID_FLAGS(func, scx_bpf_dsq_move, KF_RCU)
@@ -8612,10 +8613,15 @@ __bpf_kfunc_end_defs();

 BTF_KFUNCS_START(scx_kfunc_ids_unlocked)
 BTF_ID_FLAGS(func, scx_bpf_create_dsq, KF_IMPLICIT_ARGS | KF_SLEEPABLE)
+/* also in scx_kfunc_ids_dispatch: also callable from ops.dispatch() */
 BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_slice, KF_RCU)
 BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_vtime, KF_RCU)
 BTF_ID_FLAGS(func, scx_bpf_dsq_move, KF_RCU)
 BTF_ID_FLAGS(func, scx_bpf_dsq_move_vtime, KF_RCU)
+/* also in scx_kfunc_ids_select_cpu: also callable from ops.select_cpu()/ops.enqueue() */
+BTF_ID_FLAGS(func, __scx_bpf_select_cpu_and, KF_IMPLICIT_ARGS | KF_RCU)
+BTF_ID_FLAGS(func, scx_bpf_select_cpu_and, KF_RCU)
+BTF_ID_FLAGS(func, scx_bpf_select_cpu_dfl, KF_IMPLICIT_ARGS | KF_RCU)
 BTF_KFUNCS_END(scx_kfunc_ids_unlocked)

 static const struct btf_kfunc_id_set scx_kfunc_set_unlocked = {
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -1482,6 +1482,10 @@ static const struct btf_kfunc_id_set scx
  * contexts where @p's pi_lock state is unknown. Keep them out of
  * BPF_PROG_TYPE_TRACING by registering them in their own set which is exposed
  * only to STRUCT_OPS and SYSCALL programs.
+ *
+ * These kfuncs are also members of scx_kfunc_ids_unlocked (see ext.c) because
+ * they're callable from unlocked contexts in addition to ops.select_cpu() and
+ * ops.enqueue().
  */
 BTF_KFUNCS_START(scx_kfunc_ids_select_cpu)
 BTF_ID_FLAGS(func, __scx_bpf_select_cpu_and, KF_IMPLICIT_ARGS | KF_RCU)