From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <shemminger@vyatta.com>
Subject: Re: netem and hierarchical ingress traffic shaping
Date: Fri, 23 Dec 2011 11:07:49 -0800
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Cc: Dave Taht <dave.taht@gmail.com>,
	"John A. Sullivan III" <jsullivan@opensourcedevel.com>,
	netdev@vger.kernel.org
To: Eric Dumazet <eric.dumazet@gmail.com>
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On Fri, 23 Dec 2011 19:28:27 +0100
Eric Dumazet <eric.dumazet@gmail.com> wrote:

> Le vendredi 23 d=E9cembre 2011 =E0 18:54 +0100, Dave Taht a =E9crit :
>=20
> > Are there any place where all 48 bytes of cb are used?
> >=20
>=20
> Yes, but on in qdisc layer.
>=20
> struct tcp_skb_cb is known to be 44 bytes (when IPv6 is enabled)
>=20
> In qdisc layer, we use a small part of it, for the moment.
>=20
> > I wouldn't mind if 'time_to_send' became a separate skb field
> > for a more generic 'time_in_queue'...
> >=20
>=20
> This wont happen.
>=20
> As I posted in an earlier patch, this can be added in "struct
> qdisc_skb_cb"
>=20
>=20
>=20

skb_cb is the dumping ground of the networking layer.
The assumption was that the qdisc could use the skb_cb
for it's own scratchpad. Netem is using it for tagging
packets in the queue.=20

So basically, netem, choke, and sfb are incompatible with
each other. This is not that bad, why not add a flag to qdisc
ops to indicate which qdisc are using cb and block user from
trying to do something bogus.