[PATCH] sched_ext: Fix local_dsq_post_enq() to use task's scheduler in sub-sched

[PATCH] sched_ext: Fix local_dsq_post_enq() to use task's scheduler in sub-sched

[zhidao su]

local_dsq_post_enq() calls call_task_dequeue() with scx_root instead of
the scheduler instance actually managing the task. When
CONFIG_EXT_SUB_SCHED is enabled, tasks may be managed by a sub-scheduler
whose ops.dequeue() callback differs from root's. Using scx_root causes
the wrong scheduler's ops.dequeue() to be consulted: sub-sched tasks
dispatched to a local DSQ via scx_bpf_dsq_move_to_local() will have
SCX_TASK_IN_CUSTODY cleared but the sub-scheduler's ops.dequeue() is
never invoked, violating the custody exit semantics.

Fix by adding a 'struct scx_sched *sch' parameter to local_dsq_post_enq()
and move_local_task_to_local_dsq(), and propagating the correct scheduler
from their callers dispatch_enqueue(), move_task_between_dsqs(), and
consume_dispatch_q().

This is consistent with dispatch_enqueue()'s non-local path which already
passes 'sch' directly to call_task_dequeue() for global/bypass DSQs.

Fixes: ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics")
Signed-off-by: zhidao su <suzhidao@xiaomi.com>
---
 kernel/sched/ext.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 9628c64e5592..6ab743777a54 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1467,13 +1467,13 @@ static void call_task_dequeue(struct scx_sched *sch, struct rq *rq,
 	p->scx.flags &= ~SCX_TASK_IN_CUSTODY;
 }
 
-static void local_dsq_post_enq(struct scx_dispatch_q *dsq, struct task_struct *p,
-			       u64 enq_flags)
+static void local_dsq_post_enq(struct scx_sched *sch, struct scx_dispatch_q *dsq,
+			       struct task_struct *p, u64 enq_flags)
 {
 	struct rq *rq = container_of(dsq, struct rq, scx.local_dsq);
 	bool preempt = false;
 
-	call_task_dequeue(scx_root, rq, p, 0);
+	call_task_dequeue(sch, rq, p, 0);
 
 	/*
 	 * If @rq is in balance, the CPU is already vacant and looking for the
@@ -1606,7 +1606,7 @@ static void dispatch_enqueue(struct scx_sched *sch, struct rq *rq,
 	 * concurrently in a non-atomic way.
 	 */
 	if (is_local) {
-		local_dsq_post_enq(dsq, p, enq_flags);
+		local_dsq_post_enq(sch, dsq, p, enq_flags);
 	} else {
 		/*
 		 * Task on global/bypass DSQ: leave custody, task on
@@ -2192,7 +2192,8 @@ static void wakeup_preempt_scx(struct rq *rq, struct task_struct *p, int wake_fl
 		schedule_reenq_local(rq, 0);
 }
 
-static void move_local_task_to_local_dsq(struct task_struct *p, u64 enq_flags,
+static void move_local_task_to_local_dsq(struct scx_sched *sch,
+					 struct task_struct *p, u64 enq_flags,
 					 struct scx_dispatch_q *src_dsq,
 					 struct rq *dst_rq)
 {
@@ -2212,7 +2213,7 @@ static void move_local_task_to_local_dsq(struct task_struct *p, u64 enq_flags,
 	dsq_inc_nr(dst_dsq, p, enq_flags);
 	p->scx.dsq = dst_dsq;
 
-	local_dsq_post_enq(dst_dsq, p, enq_flags);
+	local_dsq_post_enq(sch, dst_dsq, p, enq_flags);
 }
 
 /**
@@ -2433,7 +2434,7 @@ static struct rq *move_task_between_dsqs(struct scx_sched *sch,
 		/* @p is going from a non-local DSQ to a local DSQ */
 		if (src_rq == dst_rq) {
 			task_unlink_from_dsq(p, src_dsq);
-			move_local_task_to_local_dsq(p, enq_flags,
+			move_local_task_to_local_dsq(sch, p, enq_flags,
 						     src_dsq, dst_rq);
 			raw_spin_unlock(&src_dsq->lock);
 		} else {
@@ -2486,7 +2487,7 @@ static bool consume_dispatch_q(struct scx_sched *sch, struct rq *rq,
 
 		if (rq == task_rq) {
 			task_unlink_from_dsq(p, dsq);
-			move_local_task_to_local_dsq(p, enq_flags, dsq, rq);
+			move_local_task_to_local_dsq(sch, p, enq_flags, dsq, rq);
 			raw_spin_unlock(&dsq->lock);
 			return true;
 		}
-- 
2.43.0

Re: [PATCH] sched_ext: Fix local_dsq_post_enq() to use task's scheduler in sub-sched

[Cheng-Yang Chou]

Hi zhidao,

On Thu, Apr 23, 2026 at 10:58:32AM +0800, zhidao su wrote:
> local_dsq_post_enq() calls call_task_dequeue() with scx_root instead of
> the scheduler instance actually managing the task. When
> CONFIG_EXT_SUB_SCHED is enabled, tasks may be managed by a sub-scheduler
> whose ops.dequeue() callback differs from root's. Using scx_root causes
> the wrong scheduler's ops.dequeue() to be consulted: sub-sched tasks
> dispatched to a local DSQ via scx_bpf_dsq_move_to_local() will have
> SCX_TASK_IN_CUSTODY cleared but the sub-scheduler's ops.dequeue() is
> never invoked, violating the custody exit semantics.
> 
> Fix by adding a 'struct scx_sched *sch' parameter to local_dsq_post_enq()
> and move_local_task_to_local_dsq(), and propagating the correct scheduler
> from their callers dispatch_enqueue(), move_task_between_dsqs(), and
> consume_dispatch_q().
> 
> This is consistent with dispatch_enqueue()'s non-local path which already
> passes 'sch' directly to call_task_dequeue() for global/bypass DSQs.
> 
> Fixes: ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics")
> Signed-off-by: zhidao su <suzhidao@xiaomi.com>

This fix looks correct for the parameter mismatch.

Reviewed-by: Cheng-Yang Chou <yphbchou0911@gmail.com>

Regarding the Fixes tag: I'm not entirely sure if it's strictly required
since the sub-scheduler groundwork is still a new feature. However, since
it fixes a specific logic error, keeping the tag seems appropriate.

Thanks.

-- 
Cheers,
Cheng-Yang

Re: [PATCH] sched_ext: Fix local_dsq_post_enq() to use task's scheduler in sub-sched

[Tejun Heo]

Hello,

On Thu, Apr 23, 2026 at 10:58:32AM +0800, zhidao su wrote:
> local_dsq_post_enq() calls call_task_dequeue() with scx_root instead of
> the scheduler instance actually managing the task. When
> CONFIG_EXT_SUB_SCHED is enabled, tasks may be managed by a sub-scheduler
> whose ops.dequeue() callback differs from root's.
...
> Fixes: ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics")
> Signed-off-by: zhidao su <suzhidao@xiaomi.com>

Applied to sched_ext/for-7.1-fixes.

Thanks.

--
tejun