udp: add tracepoints for queueing skb to rcvbuf

This patch adds a tracepoint to __udp_queue_rcv_skb to get the
return value of ip_queue_rcv_skb. It indicates why kernel drops
a packet at this point.

ip_queue_rcv_skb returns following values in the packet drop case:

rcvbuf is full : -ENOMEM
sk_filter returns error : -EINVAL, -EACCESS, -ENOMEM, etc.
__sk_mem_schedule returns error: -ENOBUF

Signed-off-by: Satoru Moriya <[e-mail]>
Acked-by: Neil Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/trace/events/udp.h

···
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM udp
+
+#if !defined(_TRACE_UDP_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_UDP_H
+
+#include <linux/udp.h>
+#include <linux/tracepoint.h>
+
+TRACE_EVENT(udp_fail_queue_rcv_skb,
+
+	TP_PROTO(int rc, struct sock *sk),
+
+	TP_ARGS(rc, sk),
+
+	TP_STRUCT__entry(
+		__field(int, rc)
+		__field(__u16, lport)
+	),
+
+	TP_fast_assign(
+		__entry->rc = rc;
+		__entry->lport = inet_sk(sk)->inet_num;
+	),
+
+	TP_printk("rc=%d port=%hu", __entry->rc, __entry->lport)
+);
+
+#endif /* _TRACE_UDP_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>

net/core/net-traces.c

···
 #include <trace/events/skb.h>
 #include <trace/events/net.h>
 #include <trace/events/napi.h>
+#include <trace/events/udp.h>
 
 EXPORT_TRACEPOINT_SYMBOL_GPL(kfree_skb);
 

net/ipv4/udp.c

···
 #include <net/route.h>
 #include <net/checksum.h>
 #include <net/xfrm.h>
+#include <trace/events/udp.h>
 #include "udp_impl.h"
 
 struct udp_table udp_table __read_mostly;
···
 					 is_udplite);
 		UDP_INC_STATS_BH(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
 		kfree_skb(skb);
+		trace_udp_fail_queue_rcv_skb(rc, sk);
 		return -1;
 	}