netdev - Re: [UPDATED] [NET-NEXT PATCH 1/2] pkt_sched: Add multiqueue scheduler support

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Date:	Tue, 2 Sep 2008 05:54:11 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>, jeff@...zik.org,
	netdev@...r.kernel.org, davem@...emloft.net,
	Alexander Duyck <alexander.h.duyck@...el.com>
Subject: Re: [UPDATED] [NET-NEXT PATCH 1/2] pkt_sched: Add multiqueue
	scheduler support

On Mon, Sep 01, 2008 at 03:49:14PM -0700, Alexander Duyck wrote:
> On Mon, 2008-09-01 at 23:05 +0200, Jarek Poplawski wrote:
> 
> > Mostly looks OK to me, but a few (late) doubts below:
> 
> Most of your suggestions I agree with, with the following exceptions.
> > ...
> > > +static int multiq_tune(struct Qdisc *sch, struct nlattr *opt)
> > > +{
> > > +	struct multiq_sched_data *q = qdisc_priv(sch);
> > > +	struct tc_multiq_qopt *qopt;
> > > +	struct Qdisc **queues;
> > > +	int i;
> > > +
> > > +	if (sch->parent != TC_H_ROOT)
> > > +		return -EINVAL;
> > 
> > Is it necessary?
> > 
> I think so.  Basically I want to have this qdisc as the root for all
> other qdiscs because the hardware queue decision needs to be made as
> soon as possible in order to avoid any head of line blocking issues.
> This way you don't end up with multiple qdiscs fighting over hardware
> queues.

OK, but I wonder if it's not enough to treat this as a recommendation?
Actually, since dequeuing is under the common lock here, the main
difference seems to be this checking for subqueue_stopped could happen
a bit earlier, but this should be safe (a subqueue can't get another
packets in the meantime). So maybe I miss something but this looks
like blocking safe even when used as prio's leaf.

Thanks,
Jarek P.
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