netdev - Re: netem and hierarchical ingress traffic shaping

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Date:	Fri, 23 Dec 2011 18:54:01 +0100
From:	Dave Taht <dave.taht@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	"John A. Sullivan III" <jsullivan@...nsourcedevel.com>,
	netdev@...r.kernel.org
Subject: Re: netem and hierarchical ingress traffic shaping

On Fri, Dec 23, 2011 at 6:39 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le vendredi 23 décembre 2011 à 18:33 +0100, Eric Dumazet a écrit :
>> Le dimanche 18 décembre 2011 à 11:55 -0800, Stephen Hemminger a écrit :
>> > On Sun, 18 Dec 2011 00:12:12 -0500
>> > "John A. Sullivan III" <jsullivan@...nsourcedevel.com> wrote:
>> >
>> > > Since netem appears to be classless, we realized we would need to
>> > > replace the SFQ on each leaf with netem which we really didn't want to
>> > > do - not only to not lose SFQ but because we didn't want to maintain the
>> > > netem parameters on each leaf.  So, we activated our ifb1 interface,
>> > > placed netem on it and redirected all the egress traffic to ifb1.  Taht
>> > > worked fine.
>> >
>> > Current versions of netem can take one class.
>> > --
>>
>> Hmm, I can see that (commit 10f6dfcfde884441)
>>
>> But it wont work very well, it assumes qdisc uses a single queue if
>> netem reordering is requested :
>>
>> (__skb_queue_head(&q->qdisc->q, skb))
>>
>> We should allow reordering if netem queue is changed from tfifo, only if
>> new qdisc is compatible with __skb_queue_head(&q->qdisc->q, skb)
>>
>> (maybe providing a new ->queue_at_head() new ops)
>
> Also, child qdisc must not scratch skb->cb[], since netem stores
> time_to_send in it.

Are there any place where all 48 bytes of cb are used?

I wouldn't mind if 'time_to_send' became a separate skb field
for a more generic 'time_in_queue'...

> I guess nobody actually tried this netem mis-feature :(


>
> I'll send a fix.
>
>
>
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Dave Täht
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