linux-kernel - Re: [PATCH] RT: Add priority-queuing and priority-inheritance to workqueue infrastructure

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Date:	Mon, 6 Aug 2007 19:38:16 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Daniel Walker <dwalker@...sta.com>,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RT: Add priority-queuing and priority-inheritance to workqueue infrastructure

On 08/06, Gregory Haskins wrote:
>
> On Mon, 2007-08-06 at 18:45 +0400, Oleg Nesterov wrote:
> > On 08/06, Peter Zijlstra wrote:
> 
> > > 
> > > still this does not change the fundamental issue of a high prio piece of
> > > work waiting on a lower prio task.
> >        ^^^^^^^
> > waiting. This is a "key" word, and this was my (perhaps wrong) point.
> 
> Actually, I think Peter is making a really important point here.

Yes. Please see another email I just sent.

> "Waiting" can be defined in more ways than the REQUEST/RESPONSE pattern
> that I have been rambling about.
> 
> Using Peters NIC vs USB example:  What if a NIC driver is using a
> workqueue as a bottom-half mechanism for its RX packet queuing.  In a
> nice RT environment it would be highly ideal if we allow the deferred
> work to complete with respect to the priority that was assigned to the
> subsystem.
> 
> So while the submitter isn't technically blocking on the work, the
> application that is receiving packets is now subject to the arbitrary
> priority of the keventd as opposed to the NIC irq.  Thus there is still
> "waiting" being subject to inversion, its just not in a REQUEST/RESPONSE
> pattern.

Oleg.

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