netdev - Re: [Patch net-next v3] net_sched: introduce tracepoint trace_qdisc

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Date:   Thu, 15 Jul 2021 08:25:15 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Qitao Xu <qitao.xu@...edance.com>,
        Cong Wang <cong.wang@...edance.com>
Subject: Re: [Patch net-next v3] net_sched: introduce tracepoint trace_qdisc_enqueue()

On Thu, Jul 15, 2021 at 4:24 AM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> On 2021/7/15 14:03, Cong Wang 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 similar to
> > trace_qdisc_dequeue():
> >
> > 1. For both we only trace successful cases. The failure cases
> >    can be traced via trace_kfree_skb().
> >
> > 2. They are called at entrance or exit of TC layer, not for each
> >    ->enqueue() or ->dequeue(). This is intentional, because
> >    we want to make trace_qdisc_enqueue() symmetric to
> >    trace_qdisc_dequeue(), which is easier to use.
> >
> > The return value of qdisc_enqueue() is not interesting here,
> > we have Qdisc's drop packets in ->dequeue(), it is impossible to
> > trace them even if we have the return value, the only way to trace
> > them is tracing kfree_skb().
> >
> > We only add information we need to trace ring buffer. If any other
> > information is needed, it is easy to extend it without breaking ABI,
> > see commit 3dd344ea84e1 ("net: tracepoint: exposing sk_family in all
> > tcp:tracepoints").
> >
> > Reviewed-by: Cong Wang <cong.wang@...edance.com>
> > Signed-off-by: Qitao Xu <qitao.xu@...edance.com>
> > ---
> > v3: expand changelog
> >     add helper dev_qdisc_enqueue()
> >
> > v2: improve changelog
> >
> >  include/trace/events/qdisc.h | 26 ++++++++++++++++++++++++++
> >  net/core/dev.c               | 20 ++++++++++++++++----
> >  2 files changed, 42 insertions(+), 4 deletions(-)
> >
> > 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;
> > +     ),
> > +
> > +     TP_printk("enqueue ifindex=%d qdisc handle=0x%X parent=0x%X skbaddr=%px",
> > +               __entry->ifindex, __entry->handle, __entry->parent, __entry->skbaddr)
> > +);
> > +
> >  TRACE_EVENT(qdisc_reset,
> >
> >       TP_PROTO(struct Qdisc *q),
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index c253c2aafe97..0dcddd077d60 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -131,6 +131,7 @@
> >  #include <trace/events/napi.h>
> >  #include <trace/events/net.h>
> >  #include <trace/events/skb.h>
> > +#include <trace/events/qdisc.h>
> >  #include <linux/inetdevice.h>
> >  #include <linux/cpu_rmap.h>
> >  #include <linux/static_key.h>
> > @@ -3844,6 +3845,18 @@ static void qdisc_pkt_len_init(struct sk_buff *skb)
> >       }
> >  }
> >
> > +static int dev_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *q,
> > +                          struct sk_buff **to_free,
> > +                          struct netdev_queue *txq)
> > +{
> > +     int rc;
> > +
> > +     rc = q->enqueue(skb, q, to_free) & NET_XMIT_MASK;
> > +     if (rc == NET_XMIT_SUCCESS)
>
> I do not really understand the usecase you metioned previously,
> but tracepointing the rc in trace_qdisc_enqueue() will avoid the
> above checking, which is in the fast path.

Good point, the solution is clearly not just tracing return value, instead
I use trace_qdisc_enqueue_enabled() which is based on jump label:

        static inline bool                                              \
        trace_##name##_enabled(void)                                    \
        {                                                               \
                return static_key_false(&__tracepoint_##name.key);      \
        }


>
> If there is no extra checking overhead when this tracepoint is
> disabled, then ignore my comment.

Yes, trace_qdisc_enqueue_enabled() should give 0 overhead.

Thanks.