On Fri, Oct 23, 2020 at 10:52:14PM +0200, Davide Caratti wrote:

kfree_skb_list() calls kfree_skb(), thus triggering as many dropwatch
events as the number of skbs in the list. This can disturb the analysis
of packet drops, e.g. with fragmented echo requests generated by ICMP
sockets, or with regular SCTP packets: when consume_skb() frees them,
the kernel's drop monitor may wrongly account for several packet drops:

 consume skb()
   skb_release_data()
     kfree_skb_list()
       kfree_skb() <-- false dropwatch event

Seems the problem lies with skb_release_data() calling
kfree_skb_list() while it should have been a, say, consume_skb_list(),
and not generate further kfree_skb calls.

Maybe a bool parameter on skb_release_data to signal that it should
call consume_skb_list (which doesn't exist) instead?

don't call kfree_skb() when freeing a skb list, use a dedicated
tracepoint instead. By printing "cur" and "next", it also becomes
possible to reconstruct the skb list from its members.

I like the new probe alone. It helps to have more visibility on drops
such as those from __dev_xmit_skb() and how they happen.

But as a solution to the problem stated, seems it can be confusing.
Say one is debugging a tx drop issue. AFAICT one would have to watch
both probe points anyway, as the drop could be on a layer below than
SCTP. So I'm not seeing how it helps much, other than possibly causing
drop_watch to miss drops (by not listening to the new trace point).

  Marcelo