[PATCH 04/15] sched: Add helper to block retained proxy donors

[Andrea Righi <arighi@nvidia.com>]

Scheduler ownership changes may need to turn a mutex-blocked task
retained on the runqueue for proxy execution back into a normally
blocked task.

Add sched_proxy_block_task() to perform that transition while holding
p->pi_lock and the task's runqueue lock. Reset an active donor to the
physical current task before blocking it. If DELAY_DEQUEUE keeps a FAIR
donor queued, the immediately following sched_change_begin() completes
the dequeue through switching_from_fair() before recording its queued
state.

This is a preparatory change to support proxy execution with sched_ext.

Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/core.c  | 28 ++++++++++++++++++++++++++++
 kernel/sched/sched.h |  6 ++++++
 2 files changed, 34 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e71fd07254d6e..fe142e3bb78c8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6764,6 +6764,34 @@ static void proxy_deactivate(struct rq *rq, struct task_struct *donor)
 	block_task(rq, donor, state);
 }
 
+/*
+ * Remove a retained proxy donor before changing its scheduler ownership.
+ * The caller holds p->pi_lock, so p cannot wake and migrate if block_task()
+ * drops it from the runqueue. If DELAY_DEQUEUE keeps a blocked fair task
+ * queued, switching_from_fair() completes the dequeue in the immediately
+ * following sched_change_begin().
+ */
+void sched_proxy_block_task(struct rq *rq, struct task_struct *p)
+{
+	unsigned long state = READ_ONCE(p->__state);
+
+	lockdep_assert_held(&p->pi_lock);
+	lockdep_assert_rq_held(rq);
+
+	if (!p->is_blocked || !task_on_rq_queued(p))
+		return;
+	if (WARN_ON_ONCE(state == TASK_RUNNING))
+		return;
+
+	if (task_current_donor(rq, p))
+		proxy_reset_donor(rq);
+
+	if (!p->se.sched_delayed)
+		block_task(rq, p, state);
+
+	WARN_ON_ONCE(task_on_rq_queued(p) && !p->se.sched_delayed);
+}
+
 static inline void proxy_release_rq_lock(struct rq *rq, struct rq_flags *rf)
 	__releases(__rq_lockp(rq))
 {
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 7779bcf7c6a12..ee3ba7b5b11f5 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2489,6 +2489,12 @@ static inline bool task_is_blocked(struct task_struct *p)
 	return !!p->blocked_on;
 }
 
+#ifdef CONFIG_SCHED_PROXY_EXEC
+void sched_proxy_block_task(struct rq *rq, struct task_struct *p);
+#else
+static inline void sched_proxy_block_task(struct rq *rq, struct task_struct *p) {}
+#endif
+
 static inline int task_on_cpu(struct rq *rq, struct task_struct *p)
 {
 	return p->on_cpu;
-- 
2.55.0