Re: [LARTC] [HTB] htb_dequeue_tree assertion (kernel 2.4.21-ac4)

[Wilfried Weissmann <Wilfried.Weissmann@gmx.at>]

devik wrote:
> If you read comment above htb_dequeue_tree, it should be called
> only when it is sure that there are packets inside of the level/prio.
> It is known by other HTB mechanism (per-level activity lists).
> 
> Thus the bugtrap is to catch case where class was inserted
> into activity list because it had packets in its sub-qdisc
> but when we actually decide to dequeue - it has no packet.
> It is weird - can qdisc lose packets even when dequeue was
> not called ??

If you change the depth of the leave queue then it is possible to drop 
packets or if you completely exchange the queue. Which would also 
explain why the assertion only occurs when the configuration is altered.

Greetings,
Wilfried

> 
> -------------------------------
>     Martin Devera aka devik
> Linux kernel QoS/HTB maintainer
>   http://luxik.cdi.cz/~devik/
> 
> On Fri, 18 Jul 2003, Wilfried Weissmann wrote:
> 
> 
>>Hello,
>>
>>I think the BUG_TRAP() in the htb_dequeue_tree() is wrong. First it
>>checks if the class pointer "cl" is NULL, which is obviously right. But
>>I do not understand why we also check whenever the queue length of the
>>leaf queue is zero "cl->un.leaf.q->q.qlen". I would have put that in the
>>expression of the "if" statements that comes afterwards. A queue length
>>of 0 is not an error condition that should be reported (please, correct
>>me if I misunderstood the code).
>>I can pretty much reliably trigger the assertion with a well utilized
>>gigabit ethernet link when I flush and reactivate the TC configuration
>>every 3 seconds. It looks like the error occurs only when confiuration
>>changes are made.
>>I will some some more tests on monday when I am back at the office to
>>verify that the queue length is (not) the problem.
>>
>>bye,
>>wilfried
>>
>>static struct sk_buff *
>>htb_dequeue_tree(struct htb_sched *q,int prio,int level)
>>{
>>	struct sk_buff *skb = NULL;
>>	//struct htb_sched *q = (struct htb_sched *)sch->data;
>>	struct htb_class *cl,*start;
>>	/* look initial class up in the row */
>>	start = cl = htb_lookup_leaf (q->row[level]+prio,prio,q->ptr[level]+prio);
>>
>>	do {
>>		BUG_TRAP(cl && cl->un.leaf.q->q.qlen); if (!cl) return NULL;
>>		HTB_DBG(4,1,"htb_deq_tr prio=%d lev=%d cl=%X defic=%d\n",
>>				prio,level,cl->classid,cl->un.leaf.deficit[level]);
>>
>>		if (likely((skb = cl->un.leaf.q->dequeue(cl->un.leaf.q)) != NULL))
>>			break;
>>		if (!cl->warned) {


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