netdev - Re: [PATCH net-next 1/4] net: qdisc: add op to run filters/actions before enqueue

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Date:	Tue, 1 Sep 2015 23:22:49 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Daniel Borkmann <daniel@...earbox.net>,
	Jamal Hadi Salim <jhs@...atatu.com>
Cc:	David Miller <davem@...emloft.net>,
	john fastabend <john.fastabend@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	netdev <netdev@...r.kernel.org>,
	John Fastabend <john.r.fastabend@...el.com>
Subject: Re: [PATCH net-next 1/4] net: qdisc: add op to run filters/actions
 before enqueue

(Why not Cc'ing Jamal for net_sched pathes?)

On Tue, Sep 1, 2015 at 9:34 AM, Daniel Borkmann <daniel@...earbox.net> wrote:
> From: John Fastabend <john.r.fastabend@...el.com>
>
> Add a new ->preclassify() op to allow multiqueue queuing disciplines
> to call tc_classify() or perform other work before dev_pick_tx().
>
> This helps, for example, with mqprio queueing discipline that has
> offload support by most popular 10G NICs, where the txq effectively
> picks the qdisc.
>
> Once traffic is being directed to a specific queue then hardware TX
> rings may be tuned to support this traffic type. mqprio already
> gives the ability to do this via skb->priority where the ->preclassify()
> provides more control over packet steering, it can classify the skb
> and set the priority, for example, from an eBPF classifier (or action).
>
> Also this allows traffic classifiers to be run without holding the
> qdisc lock and gives one place to attach filters when mqprio is
> in use. ->preclassify() could also be added to other mq qdiscs later
> on: f.e. most classful qdiscs first check major/minor numbers of
> skb->priority before actually consulting a more complex classifier.
>
> For mqprio case today, a filter has to be attached to each txq qdisc
> to have all traffic hit the filter. Since ->preclassify() is currently
> only used by mqprio, the __dev_queue_xmit() fast path is guarded by
> a generic, hidden Kconfig option (NET_CLS_PRECLASSIFY) that is only
> selected by mqprio, otherwise it defaults to off. Also, the Qdisc
> structure size will stay the same, we move __parent, used by cbq only
> into a write-mostly hole. If actions are enabled, __parent is written
> on every enqueue, and only read, rewritten in reshape_fail() phase.
> Therefore, this place in the read-mostly cacheline could be used by
> preclassify, which is written only once.
>

I don't like this approach. Ideally, qdisc layer should be totally
on top of tx queues, which means tx queue selection should
happen after dequeue. I looked at this before, the change is not
trivial at all given the fact that qdisc ties too much with tx queue
probably due to historical reasons, especially the tx softirq part.
But that is really a long-term solution for me.

I have no big objection for this as a short-term solution, however,
once we add these filters before enqueue, we can't remove them
any more. We really need to think twice about it.

Jamal, do you have any better idea?
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