Re: [RFC v0] Use swait in completion

From: Sebastian Andrzej Siewior
Date: Tue Mar 08 2016 - 12:52:22 EST

* Daniel Wagner | 2016-03-08 16:59:13 [+0100]:


>As Peter correctly pointed out in [1] a simple conversion from
>wait to swait in completion.c wont work. I played a bit around and
>came up with this rather ugly idea.

besides all the things I mentioned privatly, here is what I have
currently in -RT:

+void swake_up_all_locked(struct swait_queue_head *q)
+ struct swait_queue *curr;
+ int wakes = 0;
+ while (!list_empty(&q->task_list)) {
+ curr = list_first_entry(&q->task_list, typeof(*curr),
+ task_list);
+ wake_up_process(curr->task);
+ list_del_init(&curr->task_list);
+ wakes++;
+ }
+ WARN_ON(wakes > 2);

the remaining part is what you have. The only user so far is complete()
and currently I see ony complete_all() with zero or one waiter.
If none of my boxes die over the night, I intend to release this
tomorrow in -RT and see if someone else triggers the limit.

However I don't think if your DEFER flag solution is all that bad. I
have also the block-mq in -RT using swait and they perform wakes with
irqs-off. Not in -RT but mainline. So me might need something to make it
work properly. But if we defer the wakeup they might come at us and
complain about the latencyâ