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

From: Daniel Borkmann <[email-masked]>
Date: Tue, 01 Sep 2015 20:50:30 +0200

On 09/01/2015 07:21 PM, Eric Dumazet wrote:
> On Tue, 2015-09-01 at 18:34 +0200, Daniel Borkmann wrote:
>> From: John Fastabend <[email-masked]>
>>
>> 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

Yes, makes sense, as this cacheline is accessed anyway few cycles later.
I'll look into how well this approach integrates into tc's configuration
path. I think mqprio/mq/... could just install/remove dev->preclassify()
handler as soon as first filter is attached/detached.

Thanks,
Daniel