netdev - Re: [PATCH 0/6] Add qdisc->ops->peek() support.

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Message-ID: <48F88613.1060404@trash.net>
Date:	Fri, 17 Oct 2008 14:33:23 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Jarek Poplawski <jarkao2@...il.com>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH 0/6] Add qdisc->ops->peek() support.

Jarek Poplawski wrote:
> Patrick McHardy wrote, On 10/16/2008 02:38 PM:
> 
>> Jarek Poplawski wrote:
> ...
>>> PS: after this patchset only netem_enqueue() needs qdisc->requeue(),
>>> but I hope this won't take too long.
>> Assuming work-conserving qdiscs are used with netem, the currently
>> code will always send out a reorder packet immediately. This behaviour
>> is trivial to implement without ->requeue. The problematic case is
>> non-work-conserving inner qdiscs, but that doesn't seem important
>> at all since you'd usually add it as parent of netem, which still
>> works.
> 
> How about something like this (example only)?
> @@ -233,7 +233,9 @@ 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);
> +		q->qdisc->flags |= TCQ_F_REQUEUE;
> +		ret = qdisc_equeue(skb, q->qdisc);
> +		q->qdisc->flags &= ~TCQ_F_REQUEUE;

Well, the inner qdisc would still need to logic to order packets
apprioriately. Its probably not that hard, but as I said, I don't
think its necessary at all. It only makes a difference with a
non-work-conserving inner qdisc, but a lot of the functionality of
netem requires the inner tfifo anyways and rate-limiting is usually
done on top of netem. So I would suggest so either hard-wire the
tfifo qdisc or at least make the assumption that inner qdiscs are
work-conserving.
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