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

["Jason J. Herne" <jjherne@linux.vnet.ibm.com>]

On 06/05/2014 06:54 AM, Lai Jiangshan wrote:
> ------------
>
> 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@cn.fujitsu.com>
> ---
> 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.
>

FYI, this patch appears to fix the problem. I was able to run for 3 days 
without hitting the warning.

I see that you guys are still discussing the details of the fix. When 
you decide on a final solution I'm happy to retest. Just be sure to ask 
:). It is hard to tell what to test with so many patches and code 
snippets flying around all the time.

Happy coding.

-- 
-- Jason J. Herne (jjherne@linux.vnet.ibm.com)