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

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Date:	Tue, 20 Jul 2010 23:01:42 +0100
From:	David Howells <dhowells@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	dhowells@...hat.com, 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

Tejun Heo <tj@...nel.org> wrote:

> David, this should work for fscache/slow-work the same way too.  That
> should relieve your concern, right?

Not at the moment.  What does this mean:

	 * Unbound workqueues aren't concurrency managed and should be
	 * dispatched to workers immediately.

Does this mean you don't get reentrancy guarantees with unbounded work queues?

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...

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?

> 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.

David
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