Re: [PATCH] sched/deadline: Fix server stopping with runnable tasks

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jan 13, 2026 at 10:42:02AM +0100, Gabriele Monaco wrote:
> On Tue, 2026-01-13 at 10:37 +0100, Peter Zijlstra wrote:
> > >  static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se,
> > > s64 delta_exec)
> > >  {
> > > -	bool idle = rq->curr == rq->idle;
> > > +	bool idle = rq->curr == rq->idle && !rq->nr_running && !rq-
> > > >ttwu_pending;
> > 
> > This is idle_cpu(), perhaps we can lift that thing into sched.h or so.
> > 
> 
> Yeah, that's what I meant by quick and dirty.. I have idle_cpu() on the model
> side (and have them matching simplifies a lot of things).
> 
> I just wasn't sure if we wanted a function call in there. But we can probably do
> something nicer without it too.

I'm test building this...

---
diff --git a/include/linux/sched.h b/include/linux/sched.h
index bf96a7d595e2..524bdc0ca53f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1869,7 +1869,6 @@ static inline int task_nice(const struct task_struct *p)
 extern int can_nice(const struct task_struct *p, const int nice);
 extern int task_curr(const struct task_struct *p);
 extern int idle_cpu(int cpu);
-extern int available_idle_cpu(int cpu);
 extern int sched_setscheduler(struct task_struct *, int, const struct sched_param *);
 extern int sched_setscheduler_nocheck(struct task_struct *, int, const struct sched_param *);
 extern void sched_set_fifo(struct task_struct *p);
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 80c9559a3e30..d06b29ae961e 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1420,7 +1420,7 @@ update_stats_dequeue_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se, int
 
 static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64 delta_exec)
 {
-	bool idle = rq->curr == rq->idle;
+	bool idle = idle_rq(rq);
 	s64 scaled_delta_exec;
 
 	if (unlikely(delta_exec <= 0)) {
@@ -1603,8 +1603,8 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
  * | 8 |       B:zero_laxity-wait       |     |    |
  * |   |                                | <---+    |
  * |   +--------------------------------+          |
- * |     |              ^     ^           2        |
- * |     | 7            | 2   +--------------------+
+ * |     |              ^         ^       2        |
+ * |     | 7            | 2, 1    +----------------+
  * |     v              |
  * |   +-------------+  |
  * +-- | C:idle-wait | -+
@@ -1649,8 +1649,11 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
  *   dl_defer_idle = 0
  *
  *
- * [1] A->B, A->D
+ * [1] A->B, A->D, C->B
  * dl_server_start()
+ *   dl_defer_idle = 0;
+ *   if (dl_server_active)
+ *     return; // [B]
  *   dl_server_active = 1;
  *   enqueue_dl_entity()
  *     update_dl_entity(WAKEUP)
@@ -1784,6 +1787,7 @@ void dl_server_start(struct sched_dl_entity *dl_se)
 {
 	struct rq *rq = dl_se->rq;
 
+	dl_se->dl_defer_idle = 0;
 	if (!dl_server(dl_se) || dl_se->dl_server_active)
 		return;
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 58c9d244f12b..ea791550af0e 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1380,6 +1380,28 @@ static inline u32 sched_rng(void)
 #define cpu_curr(cpu)		(cpu_rq(cpu)->curr)
 #define raw_rq()		raw_cpu_ptr(&runqueues)
 
+static inline bool idle_rq(struct rq *rq)
+{
+	return rq->curr == rq->idle && !rq->nr_running && !rq->ttwu_pending;
+}
+
+/**
+ * available_idle_cpu - is a given CPU idle for enqueuing work.
+ * @cpu: the CPU in question.
+ *
+ * Return: 1 if the CPU is currently idle. 0 otherwise.
+ */
+static inline bool available_idle_cpu(int cpu)
+{
+	if (!idle_rq(cpu_rq(cpu)))
+		return 0;
+
+	if (vcpu_is_preempted(cpu))
+		return 0;
+
+	return 1;
+}
+
 #ifdef CONFIG_SCHED_PROXY_EXEC
 static inline void rq_set_donor(struct rq *rq, struct task_struct *t)
 {
diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 0496dc29ed0f..cb337de679b8 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -180,35 +180,7 @@ int task_prio(const struct task_struct *p)
  */
 int idle_cpu(int cpu)
 {
-	struct rq *rq = cpu_rq(cpu);
-
-	if (rq->curr != rq->idle)
-		return 0;
-
-	if (rq->nr_running)
-		return 0;
-
-	if (rq->ttwu_pending)
-		return 0;
-
-	return 1;
-}
-
-/**
- * available_idle_cpu - is a given CPU idle for enqueuing work.
- * @cpu: the CPU in question.
- *
- * Return: 1 if the CPU is currently idle. 0 otherwise.
- */
-int available_idle_cpu(int cpu)
-{
-	if (!idle_cpu(cpu))
-		return 0;
-
-	if (vcpu_is_preempted(cpu))
-		return 0;
-
-	return 1;
+	return idle_rq(cpu_rq(cpu));
 }
 
 /**