linux-kernel - Re: Overview of concurrency managed workqueue

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Date:	Wed, 16 Jun 2010 06:27:05 -0700
From:	Daniel Walker <dwalker@...eaurora.org>
To:	Tejun Heo <tj@...nel.org>
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: Overview of concurrency managed workqueue

On Wed, 2010-06-16 at 14:10 +0200, Tejun Heo wrote:
> Hello,
> 
> On 06/15/2010 09:43 PM, Daniel Walker wrote:
> > I noticed that you removed the RT workqueue since it's no longer used,
> > but it's possible that a user can raise the priority of a given work
> > queue thread into real time priorities. So with single threaded, and
> > multithreaded workqueues specific to certain areas of the kernel the
> > user would have a greater ability to control priorities of those areas.
> > 
> > It looks like with your patches it would remove that level of
> > flexability effectively making all the work item the same priority with
> > no ability to raise or lower .. Is that accurate ?
> 
> Yes, that is.  With new cmwq, a wq can't assume association with
> specific kthread and thus can't use wq as simple frontend to kthreads,
> but if somebody wants dedicated kthreads instead of shared ones in
> units of work, [s]he should be using kthread.

I'm not talking about coders using workqueues when they should be using
kthreads .. We're talking about currently existing workqueues. Aren't
you converting all _current_ workqueues to your system?

> wq does provide nicer tools for synchronization but in general I don't
> think using kthread is too hard and there aren't too many cases
> anyway.  If there are many users && kthread is difficult to use
> directly, we can definitely write up a wrapping layer tho.  But I
> really think using wq as wrapper around kthreads and manipulating
> worker thread directly is an abusement.

It would be a hack the user would have to patch onto there kernel in
order to get back functionality your taking away.

I think from your perspective workqueue threads are all used for
"concurrency management" only, but I don't think that's true. Some will
be user for prioritization (I'm talking about _current_ workqueues).

Could you address or ponder how the work items could be prioritized
under your system?

Daniel

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