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

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Date:	Fri, 6 Jun 2014 15:36:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	jjherne@...ux.vnet.ibm.com, Sasha Levin <sasha.levin@...cle.com>,
	Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: workqueue: WARN at at kernel/workqueue.c:2176

On Thu, Jun 05, 2014 at 06:54:35PM +0800, Lai Jiangshan wrote:
> 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);
>  }

OK, so this won't apply to a recent kernel.

> @@ -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);

So I would really rather like to avoid this if possible, its doing full
remote queueing, exactly what we tried to avoid.

> +
>  	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.

Oh man, another variant.. why did you change it again? And without
explanation for why you changed it.

I don't see a reason to call sched_ttwu_pending() with rq->lock held,
seeing as how we append to that list without it held.

I'm still thinking the previous version is good, can you explain why you
changed it?






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