Re: [Patch net-next v3] net_sched: introduce tracepoint trace_qdisc_enqueue()

On Thu, Jul 15, 2021 at 4:24 AM Yunsheng Lin [off-list ref] wrote:

On 2021/7/15 14:03, Cong Wang wrote:

quoted

From: Qitao Xu <redacted>

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 <redacted>
Signed-off-by: Qitao Xu <redacted>
---
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.