linux-kernel - Re: [RFC v1] sched/completion: convert completions to use simple wait queues

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Date:	Wed, 30 Mar 2016 17:17:29 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>,
	Daniel Wagner <wagi@...om.org>
Cc:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Wagner <daniel.wagner@...-carit.de>
Subject: Re: [RFC v1] sched/completion: convert completions to use simple wait
 queues

On 03/30/2016 05:07 PM, Peter Zijlstra wrote:
> On Wed, Mar 30, 2016 at 04:53:05PM +0200, Daniel Wagner wrote:
>> From: Daniel Wagner <daniel.wagner@...-carit.de>
>>
>> Completions have no long lasting callbacks and therefore do not need
>> the complex waitqueue variant.  Use simple waitqueues which reduces
>> the contention on the waitqueue lock.
> 
> Changelog really should have talk about the determinism thing. The last
> time you posted this the point was raised that we should wake the
> highest prio waiter in the defer case, you did not address this.

So we really want to go this road? I didn't find any numbers what the
highest count of queued sleepers was in Daniel's complete_all() testing.

As for the latest -RT I received only one report from Clark Williams
with something like 3 to 9 sleepers waked up during one complete_all()
and this happens in the resume code.
Based on this, deferring wake-ups from IRQ-context and a RB-tree (or
something like that for priority sorting) looks like a lot of complexity
and it does not look like we gain much.

> Also, you make no mention of the reduction of UINT_MAX to USHORT_MAX and
> the implications of that.

Wasn't this
|To avoid a size increase of struct completion, I spitted the done
|field into two half.

later he mentions that we can't have 2M sleepers anymore.

Sebastian