netdev - Re: [RFC PATCH] sched: only dequeue if packet can be queued to hardware queue.

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Message-ID: <5f2db9d90809180751h2b62f9f2m1462ca65b2a612bf@mail.gmail.com>
Date:	Thu, 18 Sep 2008 07:51:13 -0700
From:	"Alexander Duyck" <alexander.duyck@...il.com>
To:	"David Miller" <davem@...emloft.net>
Cc:	alexander.h.duyck@...el.com, netdev@...r.kernel.org,
	jarkao2@...il.com, herbert@...dor.apana.org.au, kaber@...sh.net
Subject: Re: [RFC PATCH] sched: only dequeue if packet can be queued to hardware queue.

On Thu, Sep 18, 2008 at 2:46 AM, David Miller <davem@...emloft.net> wrote:
> So what is the difference between qdisc->dequeue and qdisc->ops->dequeue?
> The same applies to ->enqueue.
>
> qdisc->{dequeue,enqueue} are given the value of ops->{dequeue,enqueue}
> at the time of qdisc creation.  I can only see two reasons for their
> existence:
>
> 1) We used to allow overriding ->enqueue and ->dequeue by certain
>   modules.  I see no such use like this in the current tree.
>
> 2) For performance it's kept as a copy in the qdisc.
>
> Either way, changing ->ops->dequeue into ->dequeue doesn't seem to be
> correct, unless you have some explanation.
>
> This is done in a few other places in your patch.

I redefined qdisc->dequeue to be set to smart_dequeue in sch_generic.c:
@@ -475,7 +491,7 @@ struct Qdisc *qdisc_alloc(struct netdev_queue *dev_queue,
       skb_queue_head_init(&sch->q);
       sch->ops = ops;
       sch->enqueue = ops->enqueue;
-       sch->dequeue = ops->dequeue;
+       sch->dequeue = ops->smart_dequeue;
       sch->dev_queue = dev_queue;
       dev_hold(qdisc_dev(sch));
       atomic_set(&sch->refcnt, 1);

Most of the changes from qdisc->dequeue to qdisc->ops->dequeue are to have the
standard dequeue call use nothing but standard dequeue calls in it's
path.  I needed
to maintain qdisc->ops->dequeue because there are several functions throughout
the qdisc code that require the ability to dequeue a packet regardless
of hw queue
state.

Thanks,

Alex
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