netdev - Re: [PATCH 1/2] Traffic control cgroups subsystem

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Message-ID: <20080911000726.GM20815@postel.suug.ch>
Date:	Thu, 11 Sep 2008 02:07:26 +0200
From:	Thomas Graf <tgraf@...g.ch>
To:	Paul Menage <menage@...gle.com>
Cc:	Ranjit Manomohan <ranjitm@...gle.com>, davem@...emloft.net,
	akpm@...ux-foundation.org, kaber@...sh.net, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] Traffic control cgroups subsystem

* Paul Menage <menage@...gle.com> 2008-09-10 16:51
> On Wed, Sep 10, 2008 at 4:45 PM, Thomas Graf <tgraf@...g.ch> wrote:
> >
> > That's argueable I guess. Likely or not, it doesn't make sense to
> > add such restrictions if not required, especially not when it is
> > trivial to just look at the cgroup of the task directly.
> 
> Isn't a very large fraction of outgoing network traffic driven from
> net_tx_action(), which doesn't execute in the context of the sending
> process? Or am I missing something?

net_tx_action() dequeues from the qdisc. The classification is done
before the packets is enqueued onto the qdisc.

A special case exists when the packets needs to be requeued, in this
case the enqueue() is done in softirq context but it should not invoke
the classifier again if I remember correctly.

> You claimed "In the most common case, the packet is not queued before
> it hits the qdisc so this takes away a lot of complexity and is very
> fast but should still be sufficient.". But is that really true on a
> system where the network is busy? (Which is after all exactly the case
> where you care about accurate traffic accounting/policing)

To my best knowledge it is, otherwise I wouldn't say it. Feel free to
prove me wrong.

F.e. it doesn't work if an ifb device is added to egress between
the user and the cgroup classifier. That's what I mean with common
cases.
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