netdev - Re: [PATCH] net: sched: tbf: fix calculation of max

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Date:	Tue, 26 Nov 2013 10:35:19 +0800
From:	Yang Yingliang <yangyingliang@...wei.com>
To:	David Laight <David.Laight@...LAB.COM>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	netdev <netdev@...r.kernel.org>, <davem@...emloft.net>,
	<brouer@...hat.com>, <jpirko@...hat.com>, <jbrouer@...hat.com>
Subject: Re: [PATCH] net: sched: tbf: fix calculation of max_size

On 2013/11/26 9:28, Yang Yingliang wrote:
> On 2013/11/25 20:22, David Laight wrote:
>>> From: Yang Yingliang <yangyingliang@...wei.com>
>>>
>>> Current max_size is caluated from rate table. Now, the rate table
>>> has been replaced and it's wrong to caculate max_size based on this
>>> rate table. It can lead wrong calculation of max_size.
>>>
>>> The burst in kernel may be lower than user asked, because burst may gets
>>> some loss when transform it to buffer(E.g. "burst 40kb rate 30mbit/s")
>>> and it seems we cannot avoid this loss. And burst's value(max_size) based
>>> on rate table may be equal user asked. If a packet's length is max_size,
>>> this packet will be stalled in tbf_dequeue() because its length is above
>>> the burst in kernel so that it cannot get enough tokens. The max_size guards
>>> against enqueuing packet sizes above q->buffer "time" in tbf_enqueue().
>>
>> Why not adjust the calculations so that the number of allocated tokens
>> can go negative?
>>
>> So allow the transfer if the number of tokens is +ve and then subtract
>> the number needed for the message itself.
> 
> Thanks for your advice!
> I had considered it before. But, I think that we calculate tokens from ns
> but max_size is calculated based on rate table causes the problem. I think we
> should make them consistent.
> 

Forgot to say, fixing calculation of max_size is also needed by 64bit rates.

Regards,
Yang


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