netdev - Re: [RFC PATCH v2 08/10] net: sched: pfifo_fast use alf

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Date:	Fri, 15 Jul 2016 10:29:52 -0700
From:	John Fastabend <john.fastabend@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	fw@...len.de, jhs@...atatu.com, alexei.starovoitov@...il.com,
	eric.dumazet@...il.com, netdev@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [RFC PATCH v2 08/10] net: sched: pfifo_fast use alf_queue

On 16-07-15 03:09 AM, Jesper Dangaard Brouer wrote:
> On Thu, 14 Jul 2016 17:09:43 -0700
> John Fastabend <john.fastabend@...il.com> wrote:
> 
>>>>  static int pfifo_fast_enqueue(struct sk_buff *skb, struct Qdisc *qdisc,
>>>>  			      struct sk_buff **to_free)
>>>>  {
>>>> -	if (skb_queue_len(&qdisc->q) < qdisc_dev(qdisc)->tx_queue_len) {
>>>> -		int band = prio2band[skb->priority & TC_PRIO_MAX];
>>>> -		struct pfifo_fast_priv *priv = qdisc_priv(qdisc);
>>>> -		struct sk_buff_head *list = band2list(priv, band);
>>>> -
>>>> -		priv->bitmap |= (1 << band);
>>>> -		qdisc->q.qlen++;
>>>> -		return __qdisc_enqueue_tail(skb, qdisc, list);
>>>> -	}
>>>> +	int band = prio2band[skb->priority & TC_PRIO_MAX];
>>>> +	struct pfifo_fast_priv *priv = qdisc_priv(qdisc);
>>>> +	struct skb_array *q = band2list(priv, band);
>>>> +	int err;
>>>>  
>>>> -	return qdisc_drop(skb, qdisc, to_free);
>>>> +	err = skb_array_produce_bh(q, skb);  
>>>
>>> Do you need the _bh variant here?  (Doesn't the qdisc run with BH disabled?)
>>>
>>>   
>>
>> Yep its inside rcu_read_lock_bh().
> 
> The call rcu_read_lock_bh() already disabled BH (local_bh_disable()).
> Thus, you can use the normal variants of skb_array_produce(), it is
> (approx 20 cycles) faster than the _bh variant...
> 

hah I was agreeing with you as in yep no need for the _bh variant :)
I must have been low on coffee or something when I wrote that response
because when I read it now it sounds like I really think the _bh is
needed.

At any rate _bh removed thanks!