Commit 238aa43f authored by Cheng-Yang Chou's avatar Cheng-Yang Chou Committed by Tejun Heo
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sched_ext: Document why built-in DSQs are unsupported sources in scx_bpf_dsq_move_to_local() 



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: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarCheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent d6edb15a

kernel/sched/ext.c

+9 −1
Original line number Diff line number Diff line
@@ -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.