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

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Date:   Tue, 7 Jul 2020 13:06:15 -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 1:34 PM YU, Xiangning
<xiangning.yu@...baba-inc.com> wrote:
>
>
>
> On 7/6/20 1:10 PM, Cong Wang wrote:
> > 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?
> >
>
> Hi Cong,
>
> That's a good question. In theory there will be out of order packets during aggregation. While keep in mind this is per-class aggregation, and it runs at a high frequency, that the chance to have any left over skbs from the same TCP flow on many CPUs is low.
>
> Also, based on real deployment experience, we haven't observed an increased out of order events even under heavy work load.
>

Yeah, but unless you always classify packets into proper flows, there
is always a chance to generate out-of-order packets here, which
means the default configuration is flawed.


> >
> >> +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?
> >
>
> We moved the bandwidth sharing out of the critical data path, that's why we use a kernel thread to balance the current maximum bandwidth used by each class periodically.
>
> This part could be implemented at as timer. What's your suggestion?

I doubt you can use a timer, as you call rtnl_trylock() there.
Why not use a delayed work?

Thanks.