linux-kernel - Re: Warning from swake_up_all in 4.14.15-rt13 non-RT

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Date:   Fri, 9 Mar 2018 07:29:31 -0600
From:   Corey Minyard <cminyard@...sta.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-rt-users <linux-rt-users@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: Warning from swake_up_all in 4.14.15-rt13 non-RT

On 03/09/2018 05:04 AM, Sebastian Andrzej Siewior wrote:
> On 2018-03-08 13:54:17 [-0600], Corey Minyard wrote:
>>> It will work but I don't think pushing this into workqueue/tasklet is a
>>> good idea. You want to wakeup all waiters on waitqueue X (probably one
>>> waiter) and instead there is one one wakeup + ctx-switch which does the
>>> final wakeup.
>> True, but this is an uncommon and already fairly expensive operation being
>> done.  Adding a context switch doesn't seem that bad.
> still no need to make it more expensive if it can be avoided.
>
>>> But now I had an idea: swake_up_all() could iterate over list and
>>> instead performing wakes it would just wake_q_add() the tasks. Drop the
>>> lock and then wake_up_q(). So in case there is wakeup pending and the
>>> task removed itself from the list then the task may observe a spurious
>>> wakeup.
>> That sounds promising, but where does wake_up_q() get called?  No matter
>> what
>> it's an expensive operation and I'm not sure where you would put it in this
>> case.
> Look at this:
>
...
>   
>   void swake_up(struct swait_queue_head *q)
>   {
> @@ -66,25 +62,14 @@ EXPORT_SYMBOL(swake_up);
>    */
>   void swake_up_all(struct swait_queue_head *q)
>   {
> -	struct swait_queue *curr;
> -	LIST_HEAD(tmp);
> +	unsigned long flags;
> +	DEFINE_WAKE_Q(wq);
>   
> -	WARN_ON(irqs_disabled());
> -	raw_spin_lock_irq(&q->lock);
> -	list_splice_init(&q->task_list, &tmp);
> -	while (!list_empty(&tmp)) {
> -		curr = list_first_entry(&tmp, typeof(*curr), task_list);
> +	raw_spin_lock_irqsave(&q->lock, flags);
> +	swake_add_all_wq(q, &wq);
> +	raw_spin_unlock_irqrestore(&q->lock, flags);
>   
> -		wake_up_state(curr->task, TASK_NORMAL);
> -		list_del_init(&curr->task_list);
> -
> -		if (list_empty(&tmp))
> -			break;
> -
> -		raw_spin_unlock_irq(&q->lock);
> -		raw_spin_lock_irq(&q->lock);
> -	}
> -	raw_spin_unlock_irq(&q->lock);
> +	wake_up_q(&wq);

 From what I can tell, wake_up_q() is unbounded, and you have undone what
the previous code had tried to accomplish.  In the scenario I'm talking 
about,
interrupts are still disabled here.  That's why I was asking about where 
to put
wake_up_q(), I knew you could put it here, but it didn't seem to me to help
at all.

>   }
>   EXPORT_SYMBOL(swake_up_all);
>   
>
>> I had another idea.  This is only occurring if RT is not enabled, because
>> with
>> RT all the irq disable things go away and you are generally running in task
>> context.  So why not have a different version of swake_up_all() for non-RT
>> that does work from irqs-off context?
> With the patch above I have puzzle part which would allow to use swait
> based completions upstream. That ifdef would probably not help.

I agree that having a bounded time way to wake up a bunch of threads while
interrupts are disabled would solve a bunch of issues.  I just don't see 
how it
can be done without pushing it off to a softirq or workqueue.

-corey

>> -corey
> Sebastian