netdev - Re: [RFC Patch net-next 4/4] net_sched: make ingress qdisc lockless

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Date:	Thu, 9 Jan 2014 17:21:54 -0800
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	John Fastabend <john.fastabend@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [RFC Patch net-next 4/4] net_sched: make ingress qdisc lockless

On Thu, Jan 9, 2014 at 5:11 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> Well, there is one qdisc, and if your NIC is multiqueue, with for
> example 32 queues, you can have 32 cpu happily using this qdisc at once.

I did see this, but still don't see the problem.

>
> Thats why you need the spinlock.
>

It looks like you are saying we queue the packets somewhere
in qdisc_enqueue_root() therefore needs a spinlock, but looking
at the code:

static inline int qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch)
{
        qdisc_calculate_pkt_len(skb, sch);
        return sch->enqueue(skb, sch);
}

static inline int qdisc_enqueue_root(struct sk_buff *skb, struct Qdisc *sch)
{
        qdisc_skb_cb(skb)->pkt_len = skb->len;
        return qdisc_enqueue(skb, sch) & NET_XMIT_MASK;
}

so it almost equals to calling ->enqueue directly, for ingress, which is
ingress_enqueue(). Except updating some stats, the only thing
it does is calling tc_classify().

So where is the problem at qdisc layer? I must miss something too obvious...
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