kern/58377: if_stats events lack dtrace probes

[campbell+netbsd%mumble.net@localhost]

>Number:         58377
>Category:       kern
>Synopsis:       if_stats events lack dtrace probes
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Jun 29 01:45:01 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10, 9, ...
>Organization:
The NetBSDerrs Foundation
>Environment:
>Description:
You can tell how many outgoing packets have been dropped because of output errors on an interface with `netstat -I ifN -n', but that doesn't help to diagnose _why_ the errors happen.  Most drivers have multiple reasons for errors, or other events about which we count statistics.  It should be possible to dtrace these events, e.g. to record stack traces and not just numbers of events.
>How-To-Repeat:
try to diagnose why oerrors is going up without rebuilding the kernel and rebooting, which might lose the state you needed to diagnose
>Fix:
First approximation: New SDT probe sdt:net:if:stat(struct ifnet *ifp, if_stat_t stat, int64_t delta).

We might potentially want to have a separate probe for each stat, so that you can, e.g., cheaply put probes on oerrors without incurring more than a predicted-not-taken branch on on opkts.  But that's a little more work.