[PATCH 05/10] sched_ext: Handle blocked donor migration with proxy execution

[Andrea Righi <arighi@nvidia.com>]

From: John Stultz <jstultz@google.com>

With proxy execution enabled, mutex-blocked donors stay runnable so
their scheduling context can execute the lock owner.

sched_ext may normally relocate a queued blocked donor. set_task_cpu()
updates both task_cpu() and wake_cpu, making the destination rq the
donor's new callback home. This remains valid if the donor was
previously proxy-migrated: the BPF scheduler is intentionally selecting
a new return destination, which the next proxy migration will preserve.
The usual migration-disabled, affinity, and rq-online checks apply.

An active donor cannot be moved normally because the source rq still
references it through rq->donor for scheduling-class operations and
runtime accounting. Moving it would associate the task with a different
rq while leaving those source-rq references in place. The core proxy
migration path avoids this by switching the rq donor to idle before
moving the scheduling context.

Allow normal sched_ext migration of blocked donors unless the donor is
active on its rq. In that case, defer migration until it is switched out
or leave it to the proxy machinery.

Keep blocked donors on the local DSQ when they are put so they remain
visible to the proxy pick path.

Signed-off-by: John Stultz <jstultz@google.com>
Co-developed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/ext/ext.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index de1eb6f193d9a..c755cbefe2f2f 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -2170,6 +2170,19 @@ static bool task_can_run_on_remote_rq(struct scx_sched *sch,
 	if (task_on_cpu(task_rq(p), p))
 		return false;
 
+	/*
+	 * A blocked donor may be moved normally to select a new callback rq.
+	 * set_task_cpu() updates wake_cpu and makes the destination rq its new
+	 * callback home, even if the donor was previously proxy-migrated.
+	 *
+	 * Don't move an active donor while the source rq still references it for
+	 * scheduling and accounting. The migration can be retried after the donor
+	 * is switched out.
+	 */
+	if (task_is_blocked(p) &&
+	    rcu_access_pointer(task_rq(p)->donor) == p)
+		return false;
+
 	/*
 	 * If @p has migration disabled, @p->cpus_ptr is updated to contain only
 	 * the pinned CPU in migrate_disable_switch() while @p is being switched
@@ -2852,6 +2865,22 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
 	if (p->scx.flags & SCX_TASK_QUEUED) {
 		set_task_runnable(rq, p);
 
+		/*
+		 * Mutex-blocked donors stay queued on the runqueue under proxy
+		 * execution, but the donor never runs as itself, proxy-exec
+		 * walks the blocked_on chain on the next __schedule() and runs
+		 * the lock owner in its place.
+		 *
+		 * Put the donor on the local DSQ directly so pick_next_task()
+		 * can still see it. find_proxy_task() will either run the chain
+		 * owner or deactivate the donor so the wakeup path can return it
+		 * and let BPF make a new dispatch decision once it is unblocked.
+		 */
+		if (task_is_blocked(p)) {
+			scx_dispatch_enqueue(sch, rq, &rq->scx.local_dsq, p, 0);
+			goto switch_class;
+		}
+
 		/*
 		 * If @p has slice left and is being put, @p is getting
 		 * preempted by a higher priority scheduler class or core-sched
-- 
2.55.0