[PATCH 09/18] sched_ext: Block proxy donors across scheduler transitions

[Andrea Righi <arighi@nvidia.com>]

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. Implement
scx_allow_proxy_exec() to force blocked EXT tasks through the regular block
path in schedule().

Also fully deactivate any retained proxy donor in sched_change_begin()
before changing its scheduling class. This prevents sched_setscheduler(),
PI transitions and sched_ext activation from carrying an existing proxy
session into the new class.

Some RT/DL PI transitions could usefully preserve the proxy session. For
example, an RT waiter can boost a FAIR donor which is itself blocked on a
non-PI mutex; retaining the donor would allow the boosted context to keep
proxy-executing that mutex owner. Doing so safely requires defining which
scheduling classes can carry retained proxy state across a transition. Keep
the conservative reset for now and leave compatible RT/DL PI chains for
future work.

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

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/core.c    | 9 +++++++++
 kernel/sched/ext/ext.c | 2 +-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f142ab455b797..4bd956182af8d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -11302,6 +11302,15 @@ sched_change_begin(struct task_struct *p, const struct sched_class *next_class,
 		flags |= DEQUEUE_NOCLOCK;
 	}
 
+	/*
+	 * Don't carry retained proxy state across scheduling class changes.
+	 * Compatible RT/DL PI transitions could preserve the session so that a
+	 * boosted donor continues proxy-executing its lock owner. Defining which
+	 * class transitions can safely retain that state is left for future work.
+	 */
+	if ((flags & DEQUEUE_CLASS) && next_class != p->sched_class)
+		sched_proxy_block_task(rq, p);
+
 	if ((flags & DEQUEUE_CLASS) && p->sched_class->switching_from)
 		p->sched_class->switching_from(rq, p);
 
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index f3e59f4fad4ac..d5ed996ebfb4c 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -26,7 +26,7 @@ DEFINE_RAW_SPINLOCK(scx_sched_lock);
 
 bool scx_allow_proxy_exec(const struct task_struct *p)
 {
-	return true;
+	return p->sched_class != &ext_sched_class;
 }
 
 /*
-- 
2.55.0