Re: [PATCH -rt 2/5] Thread Migration Preemption - v2

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

How about somethign like this?

---

Avoid busy looping on unmigratable tasks by pushing the migration requests
onto a delayed_migration_queue, which we try on each wakeup.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 kernel/sched.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -288,6 +288,7 @@ struct rq {
 
 	struct task_struct *migration_thread;
 	struct list_head migration_queue;
+	struct list_head delayed_migration_queue;
 #endif
 
 #ifdef CONFIG_SCHEDSTATS
@@ -5623,6 +5624,11 @@ static int migration_thread(void *data)
 		head = &rq->migration_queue;
 
 		if (list_empty(head)) {
+			/*
+			 * we got a wakeup, give the delayed list another shot.
+			 */
+			if (current->state != TASK_INTERRUPTIBLE)
+				list_splice(&rq->delayed_migration_queue, head);
 			spin_unlock_irq(&rq->lock);
 			schedule();
 			set_current_state(TASK_INTERRUPTIBLE);
@@ -5641,8 +5647,7 @@ static int migration_thread(void *data)
 			 * wake us up.
 			 */
 			spin_lock_irq(&rq->lock);
-			head = &rq->migration_queue;
-			list_add(&req->list, head);
+			list_add(&req->list, &rq->delayed_migration_queue);
 			set_tsk_thread_flag(req->task, TIF_NEED_MIGRATE);
 			spin_unlock_irq(&rq->lock);
 			wake_up_process(req->task);
@@ -7006,6 +7011,7 @@ void __init sched_init(void)
 		rq->cpu = i;
 		rq->migration_thread = NULL;
 		INIT_LIST_HEAD(&rq->migration_queue);
+		INIT_LIST_HEAD(&rq->delayed_migration_queue);
 #endif
 		atomic_set(&rq->nr_iowait, 0);