The FreeBSD Project

Improve the kinst DTrace provider

Links:
libdtrace: implement inline function tracing URL: https://reviews.freebsd.org/D38825
dtrace(1): add -d flag to dump D script post-dt_sugar URL: https://reviews.freebsd.org/D38732

Contact: Christos Margiolis <christos@FreeBSD.org>
Contact: Mark Johnston <markj@FreeBSD.org>

kinst is a new DTrace provider created by christos@ and markj@ that allows for arbitrary instruction tracing in a kernel function. kinst has been added to the base system in FreeBSD 14.0.

The 2022Q3 status report gives a brief introduction to kinst. We’re now working on inline function tracing (see review D38825 above) — a much-requested DTrace feature — by using kernel DWARF and ELF info to find the call sites of each inline copy and use that information to transform D syntax by turning kinst probes of the form:

   kinst::<inline_func>:<entry/return>
        /<pred>/
        {
                <acts>
        }

To:

   kinst::<caller_func1>:<offset>,
        kinst::<caller_func2>:<offset>,
        kinst::<caller_func3>:<offset>
        /<pred>/
        {
                <acts>
        }

For example:

   # dtrace -dn 'kinst::cam_iosched_has_more_trim:entry { printf("\t%d\t%s", pid, execname); }'
        kinst::cam_iosched_get_trim:13,
        kinst::cam_iosched_next_bio:13,
        kinst::cam_iosched_schedule:40
        {
                printf("\t%d\t%s", pid, execname);
        }

        dtrace: description 'kinst::cam_iosched_has_more_trim:entry ' matched 3 probes
        CPU     ID                    FUNCTION:NAME
          2  79315          cam_iosched_next_bio:13     0       kernel
          2  79316          cam_iosched_schedule:40     0       kernel
          0  79316          cam_iosched_schedule:40     12      intr
          2  79315          cam_iosched_next_bio:13     0       kernel
          2  79316          cam_iosched_schedule:40     0       kernel
          0  79316          cam_iosched_schedule:40     12      intr
        ^C

A new -d flag has also been added to dtrace(1) which dumps the D script after libdtrace has applied syntactic transformations.

Further goals include:

Implement a locals structure in D which stores the local variables of the traced function. For example with kinst::foo:<x>, we could print the local variable bar by doing print(locals→bar) inside a D script.
Port kinst to riscv and/or arm64.

Sponsor: The FreeBSD Foundation

Last modified on: April 18, 2023 by Lorenzo Salvadore