sched_ext: Document why built-in DSQs are unsupported sources in scx_bpf_dsq_move_to_...

Add a comment explaining the design intent behind rejecting built-in DSQs
(%SCX_DSQ_GLOBAL and %SCX_DSQ_LOCAL*) as sources. Local DSQs support
reenqueueing but the BPF scheduler cannot directly iterate or move tasks
from them. %SCX_DSQ_GLOBAL is similar but also doesn't support
reenqueueing because it maps to multiple per-node DSQs, making the scope
difficult to define.

Also annotate @dsq_id to make clear it must be a user-created DSQ.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 7043fb94113078bfad2bfb95823fdafc368c6382..bfe923b7ffe09c4bbf8f3728b71b564ce6d90f41 100644 (file)
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -8212,7 +8212,7 @@ __bpf_kfunc void scx_bpf_dispatch_cancel(const struct bpf_prog_aux *aux)
 
 /**
  * scx_bpf_dsq_move_to_local - move a task from a DSQ to the current CPU's local DSQ
- * @dsq_id: DSQ to move task from
+ * @dsq_id: DSQ to move task from. Must be a user-created DSQ
  * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
  * @enq_flags: %SCX_ENQ_*
  *
@@ -8220,6 +8220,14 @@ __bpf_kfunc void scx_bpf_dispatch_cancel(const struct bpf_prog_aux *aux)
  * local DSQ for execution with @enq_flags applied. Can only be called from
  * ops.dispatch().
  *
+ * Built-in DSQs (%SCX_DSQ_GLOBAL and %SCX_DSQ_LOCAL*) are not supported as
+ * sources. Local DSQs support reenqueueing (a task can be picked up for
+ * execution, dequeued for property changes, or reenqueued), but the BPF
+ * scheduler cannot directly iterate or move tasks from them. %SCX_DSQ_GLOBAL
+ * is similar but also doesn't support reenqueueing, as it maps to multiple
+ * per-node DSQs making the scope difficult to define; this may change in the
+ * future.
+ *
  * This function flushes the in-flight dispatches from scx_bpf_dsq_insert()
  * before trying to move from the specified DSQ. It may also grab rq locks and
  * thus can't be called under any BPF locks.