Re: [PATCH] net: sched: tbf: fix calculation of max_size

[David Miller <davem@davemloft.net>]

From: "David Laight" <David.Laight@ACULAB.COM>
Date: Mon, 25 Nov 2013 12:22:30 -0000

>> From: Yang Yingliang <yangyingliang@huawei.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.
> 
> I think this would change the semantics of the configured 'burst' value
> very slightly (to 'at least' from 'at most') but the average would still
> be correct.
> 
> FWIW I've done similar rate limiters that run directly in units of 'time'.
> The fact that system time advances automatically generates credit.

Yang has responded to your concerns, are they addressed?