On Tue, 2015-09-01 at 18:34 +0200, Daniel Borkmann wrote:
> From: John Fastabend <john.r.fastab...@intel.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,
So all distros will select it, basically.
...
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 877c848..b768bca 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3052,6 +3052,23 @@ static int __dev_queue_xmit(struct sk_buff *skb, void
> *accel_priv)
> rcu_read_lock_bh();
>
> skb_update_prio(skb);
> +#ifdef CONFIG_NET_CLS_PRECLASSIFY
> + q = rcu_dereference_bh(dev->qdisc);
> + if (q && q->preclassify) {
> + switch (q->preclassify(skb, q)) {
> + default:
> + break;
> +#ifdef CONFIG_NET_CLS_ACT
> + case TC_ACT_SHOT:
> + case TC_ACT_STOLEN:
> + case TC_ACT_QUEUED:
> + kfree_skb(skb);
> + rc = NET_XMIT_SUCCESS;
> + goto out;
> +#endif
> + }
> + }
> +#endif
>
Since its a device attribute after all, why are you storing it in
dev->qdisc->preclassify, adding a cache line miss for moderate load ?
(mqprio/mq root qdisc is normally not fetched in fast path ?)
dev->preclassify would be better IMO, close to dev->_tx
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