netdev - Re: qdisc tbf dropping many packets

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Date:	Tue, 14 Oct 2008 21:48:49 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	abasit@...t6.net
Cc:	linux-net@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: qdisc tbf dropping many packets 

From: Abdul Basit <abasit@...t6.net>
Date: Tue, 14 Oct 2008 21:37:04 -0400

[ netdev CC:'d, you'll reach more knowledgable people there ]

> I am running linux kernel 2.6.23.1 on PowerPC and trying to set qdisc tbf  to limit the rate:-
> *
> *==> tc qdisc add dev eth0 root tbf rate 2048kbit buffer 10kb latency 50ms
> 
> The problem is no matter what rate i specify, if I download a file it always get down to ~16-18Kbps download speed,
> after investigating a bit more, i found by tc -s qdisc show that many dropped packets show up in statistic s,
> looking further in kernel code, I found that in linux/net/sched/sch_tbf.c:126
> 
> static int tbf_enqueue(struct sk_buff *skb, struct Qdisc* sch)
> {
>        struct tbf_sched_data *q = qdisc_priv(sch);
>        int ret;
>  if (skb->len > q->max_size) {
>                 printk (KERN_INFO "tbf_enqueue, drop skb->len = %d, q->max_size = %d\n", skb->len, q->max_size);
>                 sch->qstats.drops++;
> 
>         ....
> }
> 
> whenever it try to enqueue the packet, the skb->len ( range from 2980 - 2977 ) is usually greator than q->max_size ( 2907 )
> hence it drops the packet.
> 
> On a different machine with same source code, skb->len never become > 1500 and there the tbf works fine.
> 
> Could someone please provide more insight or help where should i look more?
> 
> Thanks,
> ~Basit
> 
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