[PATCH 04/17] sched/core: Avoid false migration warning for proxy donors

[Andrea Righi <arighi@nvidia.com>]

Proxy execution can move a blocked donor's scheduling context to the
lock owner's CPU even when the donor is migration-disabled. The donor
does not execute there, and its original execution CPU remains recorded
in wake_cpu.

set_task_cpu() warns unconditionally for migration-disabled tasks, so a
subsequent proxy migration or the wakeup path returning the donor home
triggers a false positive. Moving a blocked scheduling context does not
violate the migration-disabled execution context.

Exclude blocked proxy donors from the warning. The proxy wakeup path
restores an executable placement before clearing the blocked state.

This is a preparatory fix for enabling proxy execution together with
sched_ext.

Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/core.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f47514ba6c241..f2d34b7f96e67 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3352,6 +3352,8 @@ void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
 void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
 {
 	unsigned int state = READ_ONCE(p->__state);
+	bool proxy_migrated = sched_proxy_exec() && p->is_blocked &&
+			      task_cpu(p) != p->wake_cpu;
 
 	/*
 	 * We should never call set_task_cpu() on a blocked task,
@@ -3387,7 +3389,12 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
 	 */
 	WARN_ON_ONCE(!cpu_online(new_cpu));
 
-	WARN_ON_ONCE(is_migration_disabled(p));
+	/*
+	 * Proxy execution can move a blocked task's scheduling context to any
+	 * CPU without moving its migration-disabled execution context. The
+	 * wakeup path will return the task to a CPU where it can execute.
+	 */
+	WARN_ON_ONCE(is_migration_disabled(p) && !proxy_migrated);
 
 	trace_sched_migrate_task(p, new_cpu);
 
-- 
2.55.0