linux-kernel - Re: [PATCHSET] workqueue: implement and use WQ

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Date:	Wed, 21 Jul 2010 00:39:13 +0200
From:	Tejun Heo <tj@...nel.org>
To:	David Howells <dhowells@...hat.com>
CC:	Arjan van de Ven <arjan@...ux.intel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	torvalds@...ux-foundation.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, oleg@...hat.com, axboe@...nel.dk,
	dwalker@...eaurora.org, stefanr@...6.in-berlin.de,
	florian@...kler.org, andi@...stfloor.org, mst@...hat.com,
	randy.dunlap@...cle.com, Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCHSET] workqueue: implement and use WQ_UNBOUND


Hello, David.

"David Howells" <dhowells@...hat.com> wrote:
> Does this mean you don't get reentrancy guarantees with unbounded work queues?

It means that unbound wq behaves like a generic worker pool.  Bound wq limits concurrency to minimal level but unbound one executes works as long as resources are available.  I'll continue below.

>I can't work out how you're achieving it with unbounded queues.  I presume with
>CPU-bound workqueues your doing it by binding the work item to the current CPU
>still...

Unbound works are served by a dedicated gcwq whose workers are not affine to any particular CPU.  As all unbound works are served by the same gcwq, non reentrancy is automatically guaranteed.

>Btw, how does this fare in an RT system, where work items bound to a CPU can't
>get executed because their CPU is busy with an RT thread, even though there are
>other, idle CPUs?

Sure, there's nothing special about unbound workers.  They're just normal kthreads.

>> Oh, and Frederic suggested that we would be better off with something based
>> on tracing API and I agree, so the debugfs thing is currently dropped from
>> the tree.  What do you think?
>
>I probably disagree.  I just want to be able to cat a file and see the current
>runqueue state.  I don't want to have to write and distribute a special program
>to do this.  Of course, I don't know that much about the tracing API, so
>cat'ing a file to get the runqueue listed nicely may be possible with that.

I'm relatively sure we can do that.  Frederic?

Thanks.

-- 
tejun
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