USDT probes¶
Nextest defines USDT probes which can be used for dynamic tracing. Probes are supported on:
- macOS, illumos and other Solaris derivatives, and FreeBSD, via DTrace
- x86_64 Linux, via bpftrace (aarch64 Linux might work as well)
List of probes¶
The probes available in the current version of cargo-nextest, as well as the arguments available to them, are listed in the nextest-runner documentation.
For all probes, the first argument (arg0) contains JSON-encoded data. With DTrace, this data can be extracted via the json function. (bpftrace does not appear to have a facility for JSON extraction.)
Most probes also have commonly-used arguments available as arg1, arg2, and so on. For example, the test-attempt-start probe's arg1 is the binary ID (a string), and its arg2 is the test name (a string).
The list of probes and their arguments are not part of nextest's stability guarantees.
Note
USDT probes and arguments are added as needed. If you'd like to add more probes to nextest, pull requests are welcome!
Examples¶
To trace all test attempts as they finish, on Linux with bpftrace:
# Run this in another terminal before starting nextest.
# Replace /opt/cargo/bin/cargo-nextest with the output of `which cargo-nextest`.
sudo bpftrace -e 'usdt:/opt/cargo/bin/cargo-nextest:nextest:test-attempt-done { printf("%s\n", str(arg0)); }'
For more about bpftrace, see its One-Liner Tutorial and other documentation.
On macOS, illumos, or FreeBSD, with DTrace:
# Run this in another terminal before starting nextest.
# (On illumos, use pfexec instead of sudo.)
sudo dtrace -x strsize=512 -Z -n '*:cargo-nextest::test-attempt-done { printf("%s\n", copyinstr(arg0)); }'
For more about DTrace, check out the Dynamic Tracing Guide.
Consider using an LLM
If you're new to bpftrace and/or DTrace, your favorite LLM may be able to help you get started. We've had pretty good results with Claude Sonnet 4.5. (It does make mistakes, though, so consult the respective guides if something goes wrong.)