[RFC][PATCH 06/10] sched: Allow sched_class::dequeue_task() to fail

[Peter Zijlstra]

Change the function signature of sched_class::dequeue_task() to return
a boolean, allowing future patches to 'fail' dequeue.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/core.c      |    7 +++++--
 kernel/sched/deadline.c  |    4 +++-
 kernel/sched/fair.c      |    4 +++-
 kernel/sched/idle.c      |    3 ++-
 kernel/sched/rt.c        |    4 +++-
 kernel/sched/sched.h     |    2 +-
 kernel/sched/stop_task.c |    3 ++-
 7 files changed, 19 insertions(+), 8 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2119,7 +2119,10 @@ static inline void enqueue_task(struct r
 		sched_core_enqueue(rq, p);
 }
 
-static inline void dequeue_task(struct rq *rq, struct task_struct *p, int flags)
+/*
+ * Must only return false when DEQUEUE_SLEEP.
+ */
+static inline bool dequeue_task(struct rq *rq, struct task_struct *p, int flags)
 {
 	if (sched_core_enabled(rq))
 		sched_core_dequeue(rq, p, flags);
@@ -2133,7 +2136,7 @@ static inline void dequeue_task(struct r
 	}
 
 	uclamp_rq_dec(rq, p);
-	p->sched_class->dequeue_task(rq, p, flags);
+	return p->sched_class->dequeue_task(rq, p, flags);
 }
 
 void activate_task(struct rq *rq, struct task_struct *p, int flags)
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1842,7 +1842,7 @@ static void enqueue_task_dl(struct rq *r
 		enqueue_pushable_dl_task(rq, p);
 }
 
-static void dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
+static bool dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
 {
 	update_curr_dl(rq);
 
@@ -1852,6 +1852,8 @@ static void dequeue_task_dl(struct rq *r
 	dequeue_dl_entity(&p->dl, flags);
 	if (!p->dl.dl_throttled && !dl_server(&p->dl))
 		dequeue_pushable_dl_task(rq, p);
+
+	return true;
 }
 
 /*
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6828,7 +6828,7 @@ static void set_next_buddy(struct sched_
  * decreased. We remove the task from the rbtree and
  * update the fair scheduling stats:
  */
-static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
+static bool dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 {
 	struct cfs_rq *cfs_rq;
 	struct sched_entity *se = &p->se;
@@ -6896,6 +6896,8 @@ static void dequeue_task_fair(struct rq
 dequeue_throttle:
 	util_est_update(&rq->cfs, p, task_sleep);
 	hrtick_update(rq);
+
+	return true;
 }
 
 #ifdef CONFIG_SMP
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -486,13 +486,14 @@ struct task_struct *pick_next_task_idle(
  * It is not legal to sleep in the idle task - print a warning
  * message if some code attempts to do it:
  */
-static void
+static bool
 dequeue_task_idle(struct rq *rq, struct task_struct *p, int flags)
 {
 	raw_spin_rq_unlock_irq(rq);
 	printk(KERN_ERR "bad: scheduling from the idle thread!\n");
 	dump_stack();
 	raw_spin_rq_lock_irq(rq);
+	return true;
 }
 
 /*
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1493,7 +1493,7 @@ enqueue_task_rt(struct rq *rq, struct ta
 		enqueue_pushable_task(rq, p);
 }
 
-static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
+static bool dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
 {
 	struct sched_rt_entity *rt_se = &p->rt;
 
@@ -1501,6 +1501,8 @@ static void dequeue_task_rt(struct rq *r
 	dequeue_rt_entity(rt_se, flags);
 
 	dequeue_pushable_task(rq, p);
+
+	return true;
 }
 
 /*
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2278,7 +2278,7 @@ struct sched_class {
 #endif
 
 	void (*enqueue_task) (struct rq *rq, struct task_struct *p, int flags);
-	void (*dequeue_task) (struct rq *rq, struct task_struct *p, int flags);
+	bool (*dequeue_task) (struct rq *rq, struct task_struct *p, int flags);
 	void (*yield_task)   (struct rq *rq);
 	bool (*yield_to_task)(struct rq *rq, struct task_struct *p);
 
--- a/kernel/sched/stop_task.c
+++ b/kernel/sched/stop_task.c
@@ -57,10 +57,11 @@ enqueue_task_stop(struct rq *rq, struct
 	add_nr_running(rq, 1);
 }
 
-static void
+static bool
 dequeue_task_stop(struct rq *rq, struct task_struct *p, int flags)
 {
 	sub_nr_running(rq, 1);
+	return true;
 }
 
 static void yield_task_stop(struct rq *rq)