linux-kernel - Re: CBQ broken in 2.6

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Message-Id: <20100127.035635.173712880.davem@davemloft.net>
Date:	Wed, 27 Jan 2010 03:56:35 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	anton.ivanov@...-begemot.co.uk
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: CBQ broken in 2.6

From: Anton Ivanov <anton.ivanov@...-begemot.co.uk>
Date: Wed, 27 Jan 2010 11:05:47 +0000

> I have been looking at CBQ being broken in 2.6 (Debian BUG 534430).

Please direct your question to netdev@...r.kernel.org which is
where the networking experts hang out.

Added to CC:

> The bug is very simple and it makes it unusable for all practical
> purposes - BOUNDED classes always borrow from their parent. As a result
> it impossible to:
> 
> 1. Set and enforce a bandwidth limit to a class with a CBQ tree
> 2. Use CBQ to do QoS at any place but the actual link. For example - put
> a linux box after behind a DSL modem and tell it to QoS at the bandwidth
> at which the modem is configured.
> 
> I have now looked through the code of cbq_sched.c (in 2.6.33-rc5 and
> 2.6.26 as shipped by Debian).

I think you mean net/sched/sch_cbq.c, there is no cbq_sched.c
anywhere in the source tree.

> In both pieces of code the variable in the class structure for CBQ
> cl->borrow is used to signal if borrowing is allowed. If borrowing is
> not allowed (BOUNDED class) it is set to NULL.
> 
> However the routine which parses the class options and defines the class
> cbq_change_class() sets it to the parent class and never resets it to
> NULL for bounded classes.
> 
> Am I missing something here? 
> 
> Please cc me as I am not subscribed to the list.
> 
> Best Regards,
> 
> -- 
>    Understanding is a three-edged sword:
>             your side, their side, and the truth. --Kosh Naranek
> 
> A. R. Ivanov
> E-mail:  anton.ivanov@...-begemot.co.uk
> WWW:     http://www.kot-begemot.co.uk/
> 
> 
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