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sched_ext: Document why built-in DSQs are unsupported sources in scx_bpf_dsq_move_to_local()

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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>
This commit is contained in:
Cheng-Yang Chou
2026-03-27 17:50:39 +08:00
committed by Tejun Heo
parent d6edb15ad9
commit 238aa43f0b
1 changed files with 9 additions and 1 deletions
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10
kernel/sched/ext.c
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@@ -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.