netdev - Re: [PATCH net v3 1/2] net: sched: tbf: fix calculation of max

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Date:	Sat, 23 Nov 2013 11:06:19 -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 v3 1/2] net: sched: tbf: fix calculation of max_size

On Tue, 2013-11-19 at 15:25 +0800, Yang Yingliang wrote:
> commit b757c9336d63f94c6b57532(tbf: improved accuracy at high rates)
> introduce a regression.
> 
> With the follow command:
> tc qdisc add dev eth1 root handle 1: tbf latency 50ms burst 10KB rate 30gbit mtu 64k
> 
> Without this patch, the max_size value is 10751(bytes).
> But, in fact, the real max_size value should be smaller than 7440(bytes).
> Or a packet whose length is bigger than 7440 will cause network congestion.
> Because the packet is so big that can't get enough tokens. Even all the tokens
> in the buffer is given to the packet.
> 
> With this patch, the max_size value is 7440(bytes).
> The packets whose length is bigger than 7440(bytes) will be dropped or reshape
> in tbf_enqueue().

This changelog makes no sense.

If userland asks 'burst 10KB', then burst is 10KB, not 7440.




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