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

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Date:   Sun, 11 Jul 2021 20:23:26 -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 8:02 PM Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
>
> On Mon, Jul 12, 2021 at 9:47 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > On Sun, Jul 11, 2021 at 5:50 PM Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
> > >
> > > On Mon, Jul 12, 2021 at 3:03 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
> > > >
> > > > From: Qitao Xu <qitao.xu@...edance.com>
> > > >
> > > > Tracepoint trace_qdisc_enqueue() is introduced to trace skb at
> > > > the entrance of TC layer on TX side. This is kinda symmetric to
> > > > trace_qdisc_dequeue(), and together they can be used to calculate
> > > > the packet queueing latency. It is more accurate than
> > > > trace_net_dev_queue(), because we already successfully enqueue
> > > > the packet at that point.
> > > >
> > > > Note, trace ring buffer is only accessible to privileged users,
> > > > it is safe to use %px to print a real kernel address here.
> > > >
> > > > Reviewed-by: Cong Wang <cong.wang@...edance.com>
> > > > Cc: Jamal Hadi Salim <jhs@...atatu.com>
> > > > Cc: Jiri Pirko <jiri@...nulli.us>
> > > > Signed-off-by: Qitao Xu <qitao.xu@...edance.com>
> > > > ---
> > > >  include/trace/events/qdisc.h | 26 ++++++++++++++++++++++++++
> > > >  net/core/dev.c               |  9 +++++++++
> > > >  2 files changed, 35 insertions(+)
> > > >
> > > > diff --git a/include/trace/events/qdisc.h b/include/trace/events/qdisc.h
> > > > index 58209557cb3a..c3006c6b4a87 100644
> > > > --- a/include/trace/events/qdisc.h
> > > > +++ b/include/trace/events/qdisc.h
> > > > @@ -46,6 +46,32 @@ TRACE_EVENT(qdisc_dequeue,
> > > >                   __entry->txq_state, __entry->packets, __entry->skbaddr )
> > > >  );
> > > >
> > > > +TRACE_EVENT(qdisc_enqueue,
> > > > +
> > > > +       TP_PROTO(struct Qdisc *qdisc, const struct netdev_queue *txq, struct sk_buff *skb),
> > > > +
> > > > +       TP_ARGS(qdisc, txq, skb),
> > > > +
> > > > +       TP_STRUCT__entry(
> > > > +               __field(struct Qdisc *, qdisc)
> > > > +               __field(void *, skbaddr)
> > > > +               __field(int, ifindex)
> > > > +               __field(u32, handle)
> > > > +               __field(u32, parent)
> > > > +       ),
> > > > +
> > > > +       TP_fast_assign(
> > > > +               __entry->qdisc = qdisc;
> > > > +               __entry->skbaddr = skb;
> > > > +               __entry->ifindex = txq->dev ? txq->dev->ifindex : 0;
> > > > +               __entry->handle  = qdisc->handle;
> > > > +               __entry->parent  = qdisc->parent;
> > > > +       ),
> > > Hi qitao, cong
> > > Why not support the txq, we get more info from txq.
> >
> > Because we only want to calculate queueing latency, not anything
> > else. If you need it, you are welcome to add anything reasonable
> > in the future, it won't break ABI (see 3dd344ea84e122f791ab).
> Thanks!
> > > and we should take care of the return value of q->enqueue, because we
> > > can know what happens in the qdisc queue(not necessary to work with
> > > qdisc:dequeue).
> > > and we can use a tracepoint filter for the return value too.
> >
> > Disagree. Because we really have no interest in dropped packets.
> > Even if we really do, we could trace kfree_skb(), not really here.
> The qdisc returns not only the NET_XMIT_DROP, right ?
> skbprio_enqueue, sfq_enqueue and red_enqueue may return the NET_XMIT_CN.
>

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?

>
> > > we should introduce a new function to instead of now codes, that may
> > > make the codes clean.  Please review my patch for more info.
> >
> > Just 3 lines of code, it is totally personal taste.
> >
> > > https://patchwork.kernel.org/project/netdevbpf/patch/20210711050007.1200-1-xiangxia.m.yue@gmail.com/
> >
> > I did review it. Like I said, %p does not work. Have you tested your
> > patches? ;)
> Yes, I tested them, I didn't find the error.  my patch is based on
> commit id 89212e160b81e778f829b89743570665810e3b13

I seriously doubt it, because we actually used %p in the beginning
too and got the same address for two different packets, this is why
we have to move to %px. It is 100% reproducible, so it probably
means you didn't test it at all.

Thanks.