Re: [PATCH net v1] net: taprio offload: enforce qdisc to netdev queue mapping

[Vinicius Costa Gomes <vinicius.gomes@intel.com>]

Jakub Kicinski <kuba@kernel.org> writes:

> On Tue, 11 May 2021 19:18:29 +0200 Yannick Vignon wrote:
>> From: Yannick Vignon <yannick.vignon@nxp.com>
>> 
>> Even though the taprio qdisc is designed for multiqueue devices, all the
>> queues still point to the same top-level taprio qdisc. This works and is
>> probably required for software taprio, but at least with offload taprio,
>> it has an undesirable side effect: because the whole qdisc is run when a
>> packet has to be sent, it allows packets in a best-effort class to be
>> processed in the context of a task sending higher priority traffic. If
>> there are packets left in the qdisc after that first run, the NET_TX
>> softirq is raised and gets executed immediately in the same process
>> context. As with any other softirq, it runs up to 10 times and for up to
>> 2ms, during which the calling process is waiting for the sendmsg call (or
>> similar) to return. In my use case, that calling process is a real-time
>> task scheduled to send a packet every 2ms, so the long sendmsg calls are
>> leading to missed timeslots.
>> 
>> By attaching each netdev queue to its own qdisc, as it is done with
>> the "classic" mq qdisc, each traffic class can be processed independently
>> without touching the other classes. A high-priority process can then send
>> packets without getting stuck in the sendmsg call anymore.
>> 
>> Signed-off-by: Yannick Vignon <yannick.vignon@nxp.com>
>> ---
>> 
>> This patch fixes an issue I observed while verifying the behavior of the
>> taprio qdisc in a real-time networking situation.
>> I am wondering if implementing separate taprio qdiscs for the software
>> and accelerated cases wouldn't be a better solution, but that would
>> require changes to the iproute2 package as well, and would break
>> backwards compatibility.
>
> You haven't CCed anyone who worked on this Qdisc in the last 2 years :/
> CCing them now. Comments, anyone?

I guess I should suggest myself as maintainer, to reduce chances of this
happening again.

>
> This looks like a very drastic change. Are you expecting the qdisc will
> always be bypassed?

Only when running in full offload mode it will be bypassed.

And it's kind of by design, in offload mode, the idea was: configure the
netdev traffic class to queue mapping, send the schedule to the hardware
and stay out of the way.

But as per Yannick's report, it seems that taprio doesn't stay enough
out of the yay.

>
> After a 1 minute looks it seems like taprio is using device queues in
> strict priority fashion. Maybe a different model is needed, but a qdisc
> with:
>
> enqueue()
> {
> 	WARN_ONCE(1)
> }
>
> really doesn't look right to me.

This patch takes the "stay out of the way" to the extreme, I kind of
like it/I am not opposed to it, if I had this idea a couple of years
ago, perhaps I would have used this same approach.

I am now thinking if this idea locks us out of anything.

Anyway, a nicer alternative would exist if we had a way to tell the core
"this qdisc should be bypassed" (i.e. don't call enqueue()/dequeue())
after init() runs.

>
> Quoting the rest of the patch below for the benefit of those on CC.
>
>> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
>> index 5c91df52b8c2..0bfb03052429 100644
>> --- a/net/sched/sch_taprio.c
>> +++ b/net/sched/sch_taprio.c
>> @@ -438,6 +438,11 @@ static int taprio_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>>  	struct Qdisc *child;
>>  	int queue;
>>  
>> +	if (unlikely(FULL_OFFLOAD_IS_ENABLED(q->flags))) {
>> +		WARN_ONCE(1, "Trying to enqueue skb into the root of a taprio qdisc configured with full offload\n");
>> +		return qdisc_drop(skb, sch, to_free);
>> +	}
>> +
>>  	queue = skb_get_queue_mapping(skb);
>>  
>>  	child = q->qdiscs[queue];
>> @@ -529,23 +534,7 @@ static struct sk_buff *taprio_peek_soft(struct Qdisc *sch)
>>  
>>  static struct sk_buff *taprio_peek_offload(struct Qdisc *sch)
>>  {
>> -	struct taprio_sched *q = qdisc_priv(sch);
>> -	struct net_device *dev = qdisc_dev(sch);
>> -	struct sk_buff *skb;
>> -	int i;
>> -
>> -	for (i = 0; i < dev->num_tx_queues; i++) {
>> -		struct Qdisc *child = q->qdiscs[i];
>> -
>> -		if (unlikely(!child))
>> -			continue;
>> -
>> -		skb = child->ops->peek(child);
>> -		if (!skb)
>> -			continue;
>> -
>> -		return skb;
>> -	}
>> +	WARN_ONCE(1, "Trying to peek into the root of a taprio qdisc configured with full offload\n");
>>  
>>  	return NULL;
>>  }
>> @@ -654,27 +643,7 @@ static struct sk_buff *taprio_dequeue_soft(struct Qdisc *sch)
>>  
>>  static struct sk_buff *taprio_dequeue_offload(struct Qdisc *sch)
>>  {
>> -	struct taprio_sched *q = qdisc_priv(sch);
>> -	struct net_device *dev = qdisc_dev(sch);
>> -	struct sk_buff *skb;
>> -	int i;
>> -
>> -	for (i = 0; i < dev->num_tx_queues; i++) {
>> -		struct Qdisc *child = q->qdiscs[i];
>> -
>> -		if (unlikely(!child))
>> -			continue;
>> -
>> -		skb = child->ops->dequeue(child);
>> -		if (unlikely(!skb))
>> -			continue;
>> -
>> -		qdisc_bstats_update(sch, skb);
>> -		qdisc_qstats_backlog_dec(sch, skb);
>> -		sch->q.qlen--;
>> -
>> -		return skb;
>> -	}
>> +	WARN_ONCE(1, "Trying to dequeue from the root of a taprio qdisc configured with full offload\n");
>>  
>>  	return NULL;
>>  }
>> @@ -1759,6 +1728,37 @@ static int taprio_init(struct Qdisc *sch, struct nlattr *opt,
>>  	return taprio_change(sch, opt, extack);
>>  }
>>  
>> +static void taprio_attach(struct Qdisc *sch)
>> +{
>> +	struct taprio_sched *q = qdisc_priv(sch);
>> +	struct net_device *dev = qdisc_dev(sch);
>> +	unsigned int ntx;
>> +
>> +	/* Attach underlying qdisc */
>> +	for (ntx = 0; ntx < dev->num_tx_queues; ntx++) {
>> +		struct Qdisc *qdisc = q->qdiscs[ntx];
>> +		struct Qdisc *old;
>> +
>> +		if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
>> +			qdisc->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
>> +			old = dev_graft_qdisc(qdisc->dev_queue, qdisc);
>> +			if (ntx < dev->real_num_tx_queues)
>> +				qdisc_hash_add(qdisc, false);
>> +		} else {
>> +			old = dev_graft_qdisc(qdisc->dev_queue, sch);
>> +			qdisc_refcount_inc(sch);
>> +		}
>> +		if (old)
>> +			qdisc_put(old);
>> +	}
>> +
>> +	/* access to the child qdiscs is not needed in offload mode */
>> +	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
>> +		kfree(q->qdiscs);
>> +		q->qdiscs = NULL;
>> +	}
>> +}
>> +
>>  static struct netdev_queue *taprio_queue_get(struct Qdisc *sch,
>>  					     unsigned long cl)
>>  {
>> @@ -1785,8 +1785,12 @@ static int taprio_graft(struct Qdisc *sch, unsigned long cl,
>>  	if (dev->flags & IFF_UP)
>>  		dev_deactivate(dev);
>>  
>> -	*old = q->qdiscs[cl - 1];
>> -	q->qdiscs[cl - 1] = new;
>> +	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
>> +		*old = dev_graft_qdisc(dev_queue, new);
>> +	} else {
>> +		*old = q->qdiscs[cl - 1];
>> +		q->qdiscs[cl - 1] = new;
>> +	}
>>  
>>  	if (new)
>>  		new->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
>> @@ -2020,6 +2024,7 @@ static struct Qdisc_ops taprio_qdisc_ops __read_mostly = {
>>  	.change		= taprio_change,
>>  	.destroy	= taprio_destroy,
>>  	.reset		= taprio_reset,
>> +	.attach		= taprio_attach,
>>  	.peek		= taprio_peek,
>>  	.dequeue	= taprio_dequeue,
>>  	.enqueue	= taprio_enqueue,
>


Cheers,
-- 
Vinicius