Re: [PATCH net-next v2 2/2] net: sched: Lockless Token Bucket (LTB) qdisc

[Eric Dumazet]

On 7/8/20 9:38 AM, YU, Xiangning wrote:
> Lockless Token Bucket (LTB) is a qdisc implementation that controls the
> use of outbound bandwidth on a shared link. With the help of lockless
> qdisc, and by decoupling rate limiting and bandwidth sharing, LTB is
> designed to scale in the cloud data centers.
> 

> +
> +static int ltb_enqueue(struct sk_buff *skb, struct Qdisc *sch,
> +                    spinlock_t *root_lock, struct sk_buff **to_free)
> +{
> +     struct ltb_sched *ltb = qdisc_priv(sch);
> +     struct ltb_pcpu_sched *pcpu_q;
> +     struct ltb_pcpu_data *pcpu;
> +     struct ltb_class *cl;
> +     int cpu;
> +
> +     pcpu = this_cpu_ptr(ltb->pcpu_data);
> +     pcpu_q = qdisc_priv(pcpu->qdisc);
> +     cpu = smp_processor_id();
> +     ltb_skb_cb(skb)->cpu = cpu;
> +
> +     cl = ltb_classify(sch, ltb, skb);
> +     if (unlikely(!cl)) {
> +             kfree_skb(skb);
> +             return NET_XMIT_DROP;
> +     }
>

Silently dropping a packet in a qdisc is forbidden.

Instead we always make sure to increment a counter, so that "tc -s qdisc" can 
give a clue.

qdisc_drop() is how we handle this.

Then, you might hit an issue if 5,000,000 packets per second need to be dropped.
(So you will probably need per cpu counters)