Re: workqueue: WARN at at kernel/workqueue.c:2176

[Lai Jiangshan]

The patch is not tested by Jason, I don't know whether the patch fix the problem.
The changlog including the "Reported-by:" and "Tested-by:" need to be updated
after it is proved.

------------

Subject: [PATCH] sched: migrate the waking tasks

Current code skips to migrate the waking task silently when TTWU_QUEUE is enabled.

When a task is waking, it is pending on the wake_list of the rq, but
it is not on queue (task->on_rq == 0). In this case, set_cpus_allowed_ptr()
and __migrate_task() will not migrate it due to it is not on queue.

This behavior is incorrect, because the task had been already waken-up, it will
be running on the wrong CPU without correct placement until the next wake-up
or update for cpus_allowed.

To fix this problem, we need to make the waking tasks on-queue (transfer
the waking tasks to running state) before migrate them.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 268a45e..d05a5a1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1474,20 +1474,24 @@ static int ttwu_remote(struct task_struct *p, int wake_flags)
 }
 
 #ifdef CONFIG_SMP
-static void sched_ttwu_pending(void)
+static void sched_ttwu_pending_locked(struct rq *rq)
 {
-	struct rq *rq = this_rq();
 	struct llist_node *llist = llist_del_all(&rq->wake_list);
 	struct task_struct *p;
 
-	raw_spin_lock(&rq->lock);
-
 	while (llist) {
 		p = llist_entry(llist, struct task_struct, wake_entry);
 		llist = llist_next(llist);
 		ttwu_do_activate(rq, p, 0);
 	}
+}
 
+static void sched_ttwu_pending(void)
+{
+	struct rq *rq = this_rq();
+
+	raw_spin_lock(&rq->lock);
+	sched_ttwu_pending_locked(rq);
 	raw_spin_unlock(&rq->lock);
 }
 
@@ -4530,6 +4534,11 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
 		goto out;
 
 	dest_cpu = cpumask_any_and(cpu_active_mask, new_mask);
+
+	/* Ensure it is on rq for migration if it is waking */
+	if (p->state == TASK_WAKING)
+		sched_ttwu_pending_locked(rq);
+
 	if (p->on_rq) {
 		struct migration_arg arg = { p, dest_cpu };
 		/* Need help from migration thread: drop lock and wait. */
@@ -4576,6 +4585,10 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
 	if (!cpumask_test_cpu(dest_cpu, tsk_cpus_allowed(p)))
 		goto fail;
 
+	/* Ensure it is on rq for migration if it is waking */
+	if (p->state == TASK_WAKING)
+		sched_ttwu_pending_locked(rq_src);
+
 	/*
 	 * If we're not on a rq, the next wake-up will ensure we're
 	 * placed properly.
--
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