linux-kernel - Re: [RFC v0] Use swait in completion

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Date:	Tue, 8 Mar 2016 18:52:06 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Daniel Wagner <wagi@...om.org>
Cc:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Wagner <daniel.wagner@...-carit.de>
Subject: Re: [RFC v0] Use swait in completion

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

>Hi,
Hi,

>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);
+}
+EXPORT_SYMBOL(swake_up_all_locked);

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 latency…

Sebastian