Thursday, 17 October 2013

SDT Provider for DTrace

The SDT provider in dtrace is one of the simplest and many people want
to create their own. This is ideal for instrumenting the kernel source
or your own dynamic driver.

My approach to supporting Solaris-like SDT probes involves some
(too) clever technology, but I missed the point. Nice as it is,
to augment the kernel with probes, that is one problem. Instrumenting
ones own drivers (such as ZFS) is something people have been asking about.

Easy! Isnt it?!

Err...well, no. Since starting the dtrace port, I skimped over this
because we are not touching the kernel source tree. But lets say
we want to touch our own tree. Or, more likely, our own driver, what
does this involve?

Well, again, its easy! Just sprinkle your code with DTRACE_PROBE
macros. Hang on.

What happens next?


Ok, lets examine a solution. Lets say that DTRACE_PROBE is suitably
defined. The requirements for this are we insert a call instruction
at the probe point - just like USDT. But this call instruction has nowhere
to go. With USDT, we link with drti.o and it arranges to scan all probe
calls, and make them no-ops, until you ask for the probe to be tracked.
At which point dtrace will replace the nop with an INT3 instruction
and the kernel code will map that breakpoint to the USDT probe firing.

In the kernel, its different. We dont have drti.o. But we know
a module is loading, so during the module scan, we can look for
these "strange" call instructions, stub them out (actually, leave them
in). We *dynamically* create an SDT probe as if it had been predeclared
and the job is done.

This is what I am currently trialling. I have created a simple
"Hello world" (/proc/helloworld) driver, with DTRACE_PROBE calls, and
the goal is that when the driver is loaded, we can trace these probes,
thus demoing a working facility.

This is strong enough for dynamically loaded modules and in theory is
also the correct solution for a vmlinux which has DTRACE_PROBEs in there.

Stay tuned for some results.

Post created by CRiSP v11.0.20a-b6616

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