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

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Message-ID: <1386601926.30495.323.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 09 Dec 2013 07:12:06 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Yang Yingliang <yangyingliang@...wei.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	David Laight <David.Laight@...LAB.COM>, brouer@...hat.com,
	jpirko@...hat.com, jbrouer@...hat.com
Subject: Re: [PATCH RFC ] net: sched: tbf: fix the calculation of max_size

On Mon, 2013-12-09 at 21:10 +0800, Yang Yingliang 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. 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().
> 
> To make consistent with the calculation of tokens, this patch add a helper
> psched_ns_t2l() to calculate burst(max_size) directly to fix this problem.
> 
> After this fix, we can support to using 64bit rates to calculate burst as well.
> 
> Signed-off-by: Yang Yingliang <yangyingliang@...wei.com>
> ---
>  net/sched/sch_tbf.c | 203 +++++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 155 insertions(+), 48 deletions(-)


There is no way I am going to study this patch.

There is no way 32bit * 32bit multiply can overflow 64bit.

If the user gave a stupid input, like a buffer bigger than 4 sec, just
say no. Nobody ever did such stupid things in the past, and nobody will
do in the future because it is so wrong and useless.

q->mtu = max_t(u64, ~0U, PSCHED_TICKS2NS(qopt->mtu));

q->buffer = max_t(u64, ~0U, PSCHED_TICKS2NS(qopt->buffer));

Could we keep this code understandable, please ?



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