On Fri, 11 Feb 2022, Mark Johnston wrote:
On Fri, Feb 11, 2022 at 09:23:47AM -0500, Mark Johnston wrote:
On Fri, Feb 11, 2022 at 02:10:30PM +0000, Bjoern A. Zeeb wrote:
Hi,
one of the drawbacks of Dtrace (and other tracing frameworks out there
on various OSes) is that they do need a list of probes upfront before
they can enable.
The probes don't have to exist, add -Z to the dtrace(1) parameters.
Say I want to trace a kernel module from the moment it is loaded, that
is currently not possible.
So something like:
# dtrace -n 'fbt::coretemp_identify:entry {stack();}' -Z
# kldload coretemp
ought to work, I think, but it doesn't. It's been a while since I
looked at this code but I think it might be related to the unimplemented
(on FreeBSD) portion of dtrace_probe_provide(). IIRC that's due to a
lock order reversal...
I see now: the problem is that FBT registers probes after KLD SYSINITs
and module hooks are invoked. See dtrace_module_loaded(), which
(asynchronously) calls dtrace_enabling_matchall() to see if any newly
registered probes match pending "retained enablings", in this case,
enablings created by dtrace -Z. We could perhaps add a new eventhandler
that gets called before anything in the KLD is executed, and register
probes at that point. Or, if the code you're interested runs after
SYSINITs are finished, then -Z might be sufficient today.
-Z didn't do it last I tried. I wasn't expecting it to actually
enable the probes at a later time based on the man page.
Having something which will do the job before SYSINTs are run, that
would be awesome.
What I am wondering is how hard it would be to "ignore" unknown probes
but enable them the moment on loading matching ones. I assume that
will probably need a callback through user space to complicate things
but it should not be impossible.
In case anyone has some spare time to draft this up, I'll be happily a
first tester; otherwise the idea is out at least for someone to find
it one day ...
Lots of health,
/bz
--
Bjoern A. Zeeb r15:7
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Bjoern A. Zeeb r15:7
