netdev - Re: [PATCH net v4 1/2] net: sched: tbf: fix calculation of max

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Date:	Tue, 03 Dec 2013 00:12:19 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Yang Yingliang <yangyingliang@...wei.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, brouer@...hat.com,
	jpirko@...hat.com, jbrouer@...hat.com
Subject: Re: [PATCH net v4 1/2] net: sched: tbf: fix calculation of max_size

On Tue, 2013-12-03 at 15:44 +0800, Yang Yingliang wrote:
> On 2013/12/3 12:59, Eric Dumazet wrote:
> > On Tue, 2013-12-03 at 11:26 +0800, Yang Yingliang wrote:
> > 
> >> +	for (max_size = 0; max_size < MAX_PKT_LEN; max_size++)
> >> +		if (psched_l2t_ns(&q->rate, max_size) > q->buffer)
> >> +			break;
> >> +	if (--max_size <= 0)
> >> +		goto unlock_done;
> >> +
> > 
> > This seems dubious. With your new code, max_size < 65536
> > 
> > Prior code had :
> > 
> > for (n = 0; n < 256; n++)
> >     if (rtab->data[n] > qopt->buffer)
> >         break;
> > max_size = (n << qopt->rate.cell_log) - 1;
> > 
> > So we could have much bigger max_size.
> > 
> > The reason I ask is that its possible to have qdisc_pkt_len(skb) being
> > bigger than 65536, for TCP packets with low MSS value.
> > 
> 
> Hmmm, if qdisc_pkt_len(skb) is bigger than 65536, skb_is_gso(skb) is true,
> it will go into tbf_segment(). If I am wrong, please point me out, thanks!
> 
> BTW, 65536 is suggested by Jesper, I'm a little uncertain about it. He is, too.
> Do you or some other developers have stronger opinions on this?

We do not want to go to tbf_segment() if we programmed tbf to allow TSO
packets of 68.000 bytes being sent without being segmented.

TSO packet of 64KB -> about 45 frames if MSS=1448, 45*1514 = 68130 bytes

Have you tried to use TBF on a 10 Gbps link, say with one TCP flow ?


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