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

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Date:	Wed, 30 Mar 2016 17:21:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Daniel Wagner <wagi@...om.org>, 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 Wed, Mar 30, 2016 at 05:17:29PM +0200, Sebastian Andrzej Siewior wrote:
> 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? 

Dunno, but at least mention why it wouldn't matter.

> 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.

Sure, but that equally puts the whole defer thing into question, if we
can put a hard cap on the max number (and WARN when exceeded) we're also
good.

> > 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.

That wasn't in this changelog, therefore it wasn't read ;-)