netdev - Re: [Patch net-next v2] net_sched: introduce tracepoint trace_qdisc

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Date:   Sun, 11 Jul 2021 21:02:44 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Qitao Xu <qitao.xu@...edance.com>,
        Cong Wang <cong.wang@...edance.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: [Patch net-next v2] net_sched: introduce tracepoint trace_qdisc_enqueue()

On Sun, Jul 11, 2021 at 8:49 PM Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
>
> On Mon, Jul 12, 2021 at 11:39 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > On Sun, Jul 11, 2021 at 8:36 PM Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
> > >
> > > On Mon, Jul 12, 2021 at 11:23 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
> > > > Sure, in that case a different packet is dropped, once again you
> > > > can trace it with kfree_skb() if you want. What's the problem?
> > > It's ok, but we can make it better. Yunsheng Lin may have explained why?
> >
> > Why it is better to trace dropped packets both in enqueue and in kfree_skb()?
> I mean we can use one tracepoint to know what happened in the queue,
> not necessary to trace enqueue and  kfree_skb()

This is wrong, packets can be dropped for other reasons too, tracing
enqueue is clearly not sufficient even if you trace it unconditionally.
For a quick example, codel drops packets at dequeue rather than
enqueue. ;)

> If so, we must match when the packet is dropped and what packets. if
> we use the return value in trace_qdisc_requeue. It
> is easy to know what happened(when, where, what packets were dropped ).

I am afraid you have to watch the dropped packets.

Thanks.