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

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Date:   Mon, 6 Jul 2020 13:10:48 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     "YU, Xiangning" <xiangning.yu@...baba-inc.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 2/2] net: sched: Lockless Token Bucket (LTB) Qdisc

On Mon, Jul 6, 2020 at 11:11 AM YU, Xiangning
<xiangning.yu@...baba-inc.com> wrote:
> +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_class *cl;
> +       struct ltb_pcpu_data *pcpu = this_cpu_ptr(ltb->pcpu_data);
> +       int cpu;
> +
> +       cpu = smp_processor_id();
> +       pcpu_q = qdisc_priv(pcpu->qdisc);
> +       ltb_skb_cb(skb)->cpu = cpu;
> +
> +       cl = ltb_classify(sch, ltb, skb);
> +       if (unlikely(!cl)) {
> +               kfree_skb(skb);
> +               return NET_XMIT_DROP;
> +       }
> +
> +       pcpu->active = true;
> +       if (unlikely(kfifo_put(&cl->aggr_queues[cpu], skb) == 0)) {
> +               kfree_skb(skb);
> +               atomic64_inc(&cl->stat_drops);
> +               return NET_XMIT_DROP;
> +       }


How do you prevent out-of-order packets?


> +static int ltb_init(struct Qdisc *sch, struct nlattr *opt,
...
> +       ltb->default_cls = ltb->shadow_cls; /* Default hasn't been created */
> +       tasklet_init(&ltb->fanout_tasklet, ltb_fanout_tasklet,
> +                    (unsigned long)ltb);
> +
> +       /* Bandwidth balancer, this logic can be implemented in user-land. */
> +       init_waitqueue_head(&ltb->bwbalancer_wq);
> +       ltb->bwbalancer_task =
> +           kthread_create(ltb_bw_balancer_kthread, ltb, "ltb-balancer");
> +       wake_up_process(ltb->bwbalancer_task);

Creating a kthread for each qdisc doesn't look good. Why do you
need a per-qdisc kthread or even a kernel thread at all?

Thanks.