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

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Message-Id: <20080918.024655.22661052.davem@davemloft.net>
Date:	Thu, 18 Sep 2008 02:46:55 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	alexander.h.duyck@...el.com
Cc:	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.

From: Alexander Duyck <alexander.h.duyck@...el.com>
Date: Wed, 17 Sep 2008 23:43:02 -0700

> diff --git a/net/sched/sch_atm.c b/net/sched/sch_atm.c
> index 43d3725..91a40b2 100644
> --- a/net/sched/sch_atm.c
> +++ b/net/sched/sch_atm.c
> @@ -516,12 +516,31 @@ static struct sk_buff *atm_tc_dequeue(struct Qdisc *sch)
>  
>  	pr_debug("atm_tc_dequeue(sch %p,[qdisc %p])\n", sch, p);
>  	tasklet_schedule(&p->task);
> -	skb = p->link.q->dequeue(p->link.q);
> +	skb = p->link.q->ops->dequeue(p->link.q);
>  	if (skb)
>  		sch->q.qlen--;
>  	return skb;
>  }
>  

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.

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