netdev - Re: [PATCH 5/9]: sch_netem: Use requeue list instead of ops->requeue()

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Date:	Tue, 19 Aug 2008 19:36:09 +0200
From:	Bastian Bloessl <basti@...l.de>
To:	David Miller <davem@...emloft.net>
CC:	netdev@...r.kernel.org, jarkao2@...il.com
Subject: Re: [PATCH 5/9]: sch_netem: Use requeue list instead of ops->requeue()

David Miller wrote:
> sch_netem: Use requeue list instead of ops->requeue()
> 
> This code just wants to make this packet the "front" one, and that's
> just as simply done by queueing to the ->requeue list.
> 

Hi,
I think that might make some difference.
Suppose we have an upper qdisc that dequeues from netem and
puts the packet in it's requeue-list. But after that another
packet could arrive that should be send earlier (delay
distribution, reordering).
As far as I understand it the requeued packet would now
be send before the other.

> --- a/net/sched/sch_netem.c
> +++ b/net/sched/sch_netem.c
> @@ -233,7 +233,8 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
>  		 */
>  		cb->time_to_send = psched_get_time();
>  		q->counter = 0;
> -		ret = q->qdisc->ops->requeue(skb, q->qdisc);
> +		__skb_queue_tail(&q->qdisc->requeue, skb);

I think this should be __skb_queue_head() because in it's
requeue-list netem has packets that it doesn't want to send yet. So
their time_to_send is likely to be in the future. But reordered
packets have current time as time_to_send.

Or did I get it wrong?

Regards
Bastian Bloessl
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