This RFC is to seek for suggestion to track the reason that the sk_buff is dropped. Sometimes the kernel may not directly call kfree_skb() to drop the sk_buff. Instead, it "goto drop" and call kfree_skb() at 'drop'. This makes it difficult to track the reason that the sk_buff is dropped. The commit c504e5c2f964 ("net: skb: introduce kfree_skb_reason()") has introduced the kfree_skb_reason() to help track the reason. However, we may need to define many reasons for each driver/subsystem. I am going to trace the "goto drop" in TUN and TAP drivers. However, I will need to introduce many new reasons if I re-use kfree_skb_reason(). There are some other options. 1. The 1st option is to introduce a new tracepoint, e.g., trace_drop_skb() as below to track the function and line number. We would call trace_drop_skb() before "goto drop". TP_PROTO(struct sk_buff *skb, struct net_device *dev, const char *function, unsigned int line), 2. The 2nd option is to directly call trace_kfree_skb() before "goto drop". And we may replace kfree_skb() with below kfree_skb_notrace() as suggested by Joao Martins. /** * kfree_skb_notrace - free an sk_buff without tracing * @skb: buffer to free * * Drop a reference to the buffer and free it if the usage count has * hit zero. */ void kfree_skb_notrace(struct sk_buff *skb) { if (!skb_unref(skb)) return; __kfree_skb(skb); } 3. The last option is this RFC. To avoid introducing so many new reasons, we use (__func__, __LINE__) to uniquely identify the location of each "goto drop". The 'reason' introduced by the commit c504e5c2f964 ("net: skb: introduce kfree_skb_reason()") is replaced by the (function, line) pair. The below is the sample output from trace_pipe by this RFC, when the sk_buff is dropped by TUN driver. <idle>-0 [016] ..s1. 432.701987: kfree_skb: skbaddr=00000000a65c0a72 protocol=0 location=000000008a49d80c function=none line=0 <idle>-0 [003] b.s2. 432.704397: kfree_skb: skbaddr=00000000665e5ccd protocol=2048 location=00000000ec3b7129 function=tun_net_xmit line=1116 <idle>-0 [003] ..s1. 432.704400: kfree_skb: skbaddr=00000000e4c806f8 protocol=2048 location=000000002929642d function=none line=0 <idle>-0 [002] b.s2. 432.734617: kfree_skb: skbaddr=00000000079749b3 protocol=2048 location=00000000ec3b7129 function=tun_net_xmit line=1116 <idle>-0 [015] b.s2. 432.880571: kfree_skb: skbaddr=00000000e1542f1e protocol=34525 location=00000000ec3b7129 function=tun_net_xmit line=1116 <idle>-0 [015] ..s1. 432.880577: kfree_skb: skbaddr=000000004f3022b6 protocol=34525 location=00000000547c5c25 function=none line=0 <idle>-0 [002] b.s2. 432.886247: kfree_skb: skbaddr=0000000062990a71 protocol=2054 location=00000000ec3b7129 function=tun_net_xmit line=1116 drivers/net/tap.c | 30 ++++++++++++++++++++++-------- drivers/net/tun.c | 33 +++++++++++++++++++++++++-------- include/linux/skbuff.h | 24 +++++++----------------- include/trace/events/skb.h | 37 ++++++++----------------------------- net/core/dev.c | 3 ++- net/core/skbuff.c | 10 ++++++---- net/ipv4/tcp_ipv4.c | 14 +++++++------- net/ipv4/udp.c | 14 +++++++------- 8 files changed, 84 insertions(+), 81 deletions(-) Would you please share your suggestion and feedback? Thank you very much! Dongli Zhang