Messages in this thread | | | Subject | Re: Warning from swake_up_all in 4.14.15-rt13 non-RT | From | Corey Minyard <> | Date | Fri, 9 Mar 2018 07:29:31 -0600 |
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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
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