linux-kernel - Re: Overview of concurrency managed workqueue

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Date:	Thu, 17 Jun 2010 08:21:05 +0200
From:	Florian Mickler <florian@...kler.org>
To:	Florian Mickler <florian@...kler.org>
Cc:	Daniel Walker <dwalker@...eaurora.org>, Tejun Heo <tj@...nel.org>,
	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 Thu, 17 Jun 2010 07:29:20 +0200
Florian Mickler <florian@...kler.org> wrote:

> On Wed, 16 Jun 2010 21:20:36 +0200
> Tejun Heo <tj@...nel.org> wrote:
> 
> > On 06/16/2010 08:46 PM, Tejun Heo wrote:
> > > * I'm very sorry I'm breaking your hacky workaround but seriously
> > >   that's another problem to solve.  Let's talk about the problem
> > >   itself instead of your hacky workaround.  (I think for most cases
> > >   not using workqueue in RT path would be the right thing to do.)
> > 
> > For example, for the actual case of amba-pl022.c you mentioned, where
> > interrupt handler sometimes offloads to workqueue, convert
> > amba-pl022.c to use threaded interrupt handler.  That's why it's
> > there.
> > 
> > If you actually _solve_ the problem like this, other users wouldn't
> > experience the problem at all once the update reaches them and you
> > won't have to worry about your workaround breaking with the next
> > kernel update or unexpected suspend/resume and we won't be having this
> > discussion about adjusting workqueue priorities from userland.
> > 
> > There are many wrong things about working around RT latency problems
> > by setting workqueue priorities from userland.  Please think about why
> > the driver would have a separate workqueue for itself in the first
> > place.  It was to work around the limitation of workqueue facility and
> > you're arguing that, because that work around allows yet another very
> > fragile workaround, the property which made the original work around
> > necessary in the first place needs to stay.  That sounds really
> > perverse to me.
> > 
> 
> For what its worth, IMO the right thing to do would probably be to
> propagate the priority through the subsystem into the driver. 

I was thinking about input devices here... anyway, my point is, that
the user of the workqueue-interface should pass the priority-context to
the workqueue subsystem... 

If a userspace needs a driver to have a specific priority... can it
inform the system about it (thinking sysfs here..)? that would prevent
userspace having "to kick" the kernel if smth get's stuck....

Cheers,
Flo
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