[DTrace-devel] sdt provider and access to the trace_event_raw_* struct

[Alan Maguire]

hi folks

I've come across a case where I need to trace a kernel tracepoint with a
lot of associated trace info.  It seems that the current approach for
sdt probes looks at the "struct trace_event_raw_<tracepoint_name>"
structure and maps its fields into args[] values, translating each
member into a separate argument.  That works great for tracepoints with
a limited number of fields. However in the case of a tracepoint with a
lot of such fields (i.e. more than the number of args[] supported), it
would be useful to also have a convenient way to access the raw "struct
trace_event_raw_*" data, especially since we have access to it directly
via CTF. It's possible to do this via a hack, e.g. the following works:

#!/usr/sbin/dtrace -s

sdt:sched::sched_switch
{
        s = (struct trace_event_raw_sched_switch *)(arg0-8);
        print(s);
}


...but presumably that only works because the first arg value isn't
scalar. It would be good to have a helper or builtin variable to access
this pointer directly. Maybe there's a better way to do this, or maybe
we could add a helper/builtin to make this pointer accessible? What do
folks think?

Thanks!

Alan