netdev - RE: [PATCH 3/3] NET: [SCHED] Qdisc changes and sch

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Date:	Thu, 28 Jun 2007 12:04:17 -0700
From:	"Waskiewicz Jr, Peter P" <peter.p.waskiewicz.jr@...el.com>
To:	"Patrick McHardy" <kaber@...sh.net>
Cc:	<davem@...emloft.net>, <netdev@...r.kernel.org>, <jeff@...zik.org>,
	"Kok, Auke-jan H" <auke-jan.h.kok@...el.com>, <hadi@...erus.ca>
Subject: RE: [PATCH 3/3] NET: [SCHED] Qdisc changes and sch_rr added for multiqueue

> PJ Waskiewicz wrote:
> > +#ifdef CONFIG_NET_SCH_MULTIQUEUE
> > +			if (q->mq)
> > +				skb->queue_mapping = 
> > +						
> q->prio2band[band&TC_PRIO_MAX];
> > +			else
> > +				skb->queue_mapping = 0;
> > +#endif
> 
> 
> Setting it to zero here is wrong, consider:
> 
> root qdisc: prio multiqueue
> child qdisc: prio non-multiqueue
> 
> The top-level qdisc will set it, the child qdisc will unset it again.
> When multiqueue is inactive it should not touch it.
> 
> I'll fix that as well.

But the child can't assume the device is multiqueue if the child is
non-multiqueue.  This is the same issue with IP forwarding, where if you
forward through a multiqueue device to a non-mq device, you don't know
if the destination device is multiqueue.  So the last qdisc to actually
dequeue into a device should have control what the queue mapping is.  If
a user had a multiqueue qdisc as root, and configured a child qdisc as
non-mq, that is a configuration error if the underlying device is indeed
multiqueue IMO.

-PJ
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