[PATCH 02/10] sched_ext: Block proxy donors across scheduler transitions

Proxy execution retains mutex-blocked donors on the runqueue so their
scheduling context can execute a lock owner. sched_ext cannot safely
retain such donors unless the BPF scheduler explicitly participates in
their admission and ordering.

Make sched_ext reject retained donors by default. Force blocked EXT
tasks through the regular block path in schedule(), and fully deactivate
a retained donor before sched_setscheduler() moves it into the EXT
class. These hooks establish the safe default that a later opt-in can
relax.

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

Signed-off-by: Andrea Righi <[email protected]>
---
 kernel/sched/core.c     |  3 ++-
 kernel/sched/ext/ext.c  | 21 +++++++++++++++++++++
 kernel/sched/ext/ext.h  |  6 ++++++
 kernel/sched/syscalls.c |  3 +++
 4 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 17f37ad639680..cdeb189f63cdd 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7173,7 +7173,8 @@ static void __sched notrace __schedule(int sched_mode)
 		 * task_is_blocked() will always be false).
 		 */
 		try_to_block_task(rq, prev, &prev_state,
-				  !task_is_blocked(prev));
+				  !task_is_blocked(prev) ||
+				  !scx_allow_proxy_exec(prev));
 		switch_count = &prev->nvcsw;
 	}
 
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 9aeb378fa894b..8c527fa14a15f 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -23,6 +23,27 @@
 
 DEFINE_RAW_SPINLOCK(scx_sched_lock);
 
+bool scx_allow_proxy_exec(const struct task_struct *p)
+{
+	return p->sched_class != &ext_sched_class;
+}
+
+/*
+ * Called after sched_setscheduler() validation and immediately before
+ * sched_change_begin(), with @p's pi and rq locks held.
+ */
+void scx_prepare_setscheduler(struct task_struct *p,
+			      const struct sched_class *next_class)
+{
+	lockdep_assert_held(&p->pi_lock);
+	lockdep_assert_rq_held(task_rq(p));
+
+	if (p->sched_class == next_class || next_class != &ext_sched_class)
+		return;
+
+	sched_proxy_block_task(task_rq(p), p);
+}
+
 /*
  * NOTE: sched_ext is in the process of growing multiple scheduler support and
  * scx_root usage is in a transitional state. Naked dereferences are safe if the
diff --git a/kernel/sched/ext/ext.h b/kernel/sched/ext/ext.h
index 0b7fc46aee08c..d708abf2c3bb8 100644
--- a/kernel/sched/ext/ext.h
+++ b/kernel/sched/ext/ext.h
@@ -18,8 +18,11 @@ bool scx_can_stop_tick(struct rq *rq);
 void scx_rq_activate(struct rq *rq);
 void scx_rq_deactivate(struct rq *rq);
 int scx_check_setscheduler(struct task_struct *p, int policy);
+void scx_prepare_setscheduler(struct task_struct *p,
+			      const struct sched_class *next_class);
 bool task_should_scx(int policy);
 bool scx_allow_ttwu_queue(const struct task_struct *p);
+bool scx_allow_proxy_exec(const struct task_struct *p);
 void init_sched_ext_class(void);
 
 static inline u32 scx_cpuperf_target(s32 cpu)
@@ -52,8 +55,11 @@ static inline bool scx_can_stop_tick(struct rq *rq) { return true; }
 static inline void scx_rq_activate(struct rq *rq) {}
 static inline void scx_rq_deactivate(struct rq *rq) {}
 static inline int scx_check_setscheduler(struct task_struct *p, int policy) { return 0; }
+static inline void scx_prepare_setscheduler(struct task_struct *p,
+					    const struct sched_class *next_class) {}
 static inline bool task_on_scx(const struct task_struct *p) { return false; }
 static inline bool scx_allow_ttwu_queue(const struct task_struct *p) { return true; }
+static inline bool scx_allow_proxy_exec(const struct task_struct *p) { return true; }
 static inline void init_sched_ext_class(void) {}
 
 #endif	/* CONFIG_SCHED_CLASS_EXT */
diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index b215b0ead9a60..2bbba3dc8c890 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -678,6 +678,9 @@ int __sched_setscheduler(struct task_struct *p,
 	if (prev_class != next_class)
 		queue_flags |= DEQUEUE_CLASS;
 
+	if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS))
+		scx_prepare_setscheduler(p, next_class);
+
 	scoped_guard (sched_change, p, queue_flags) {
 
 		if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)) {
-- 
2.55.0