linux-kernel - Re: [PATCHSET] workqueue: concurrency managed workqueue, take#5

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Message-ID: <4C17C938.4040703@kernel.org>
Date:	Tue, 15 Jun 2010 20:40:56 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
CC:	mingo@...e.hu, awalls@...ix.net, linux-kernel@...r.kernel.org,
	jeff@...zik.org, akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com,
	arjan@...ux.intel.com, johannes@...solutions.net, oleg@...hat.com,
	axboe@...nel.dk
Subject: Re: [PATCHSET] workqueue: concurrency managed workqueue, take#5

Hello,

On 06/15/2010 08:29 PM, Stefan Richter wrote:
> Tejun Heo wrote:
>> This is the fifth take of cmwq (concurrency managed workqueue)
>> patchset.  It's on top of v2.6.35-rc3 + sched/core patches.  Git tree
>> is available at
>>
>>   git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git review-cmwq
> 
> A comment and a question:
> 
> As a driver maintainer, I would find it helpful if the WQ_flags in
> include/linux/workqueue.h and/or __create_workqueue_key() in
> kernel/workqueue.c (or its wrappers in include/linux/workqueue.h) were
> better documented.

Sure, it can definitely be improved.

> How about the global workqueue, i.e. schedule_work() and friends?  At
> your current review-cmwq head, they use system_wq, not system_nrt_wq.
> But doesn't have the present global workqueue WQ_NON_REENTRANT
> semantics?  In fact, don't have _all_ workqueues WQ_NON_REENTRANT
> semantics presently?  If so, a good deal of existing users probably
> relies on non-reentrant behaviour.  Or am I thoroughly misunderstanding
> the meaning of WQ_NON_REENTRANT?

Yeah, it's a bit confusing. :-( The current workqueue semantics is
non-reentrant on the same cpu but reentrant on different cpus.
WQ_NON_REENTRANT is non-reentrant regardless of cpu, so it's stronger
guarantee than before.  To summarize,

 current MT == !WQ_NON_REENTRANT < WQ_NON_REENTRANT <
	WQ_SINGLE_CPU < current ST == WQ_SINGLE_CPU + max in_flight of 1.

> (Sorry if this had been discussed before; I followed the discussions of
> some of your previous submissions but not all.  And PS, I am eagerly
> awaiting for this to go into the mainline.)

Ah, yeah, after ten month, I'm pretty eager too.  :-)

Thanks.

-- 
tejun
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