[DTrace-devel] [PATCH v4] Implement the lockstat provider

[Kris Van Hees]

On Wed, May 24, 2023 at 08:17:45PM -0400, Eugene Loh via DTrace-devel wrote:
> Why does lockstat not report a function (e.g., dtrace -lP lockstat)?  I
> thought that some probes instrumented source code (e.g., lockstat) and
> therefore reported a module and function; others (like profile, cpc, dtrace)
> did not and therefore did not. It appears that lockstat is reporting a
> module but not a function, but clearly lockstat is instrumenting source
> code.

No, lockstat is not instrumenting source code.  It is providing probes that
allow instrumentation of locking primitives in the kernel.  The fact that the
legacy version reported multiple probes (each with a function name listed) for
SDT probes is simply an artifact of the implementation requiring multiple
probes locations for the same (conceptual) probe.  That is no longer needed
because we overlay multiple probe locations with a single probe.  Which is
perfectly valid because we are not probing location but rather logic.

> On 5/24/23 17:56, Kris Van Hees via DTrace-devel wrote:
> 
> > diff --git a/test/unittest/lockstat/tst.lv-adaptive-acquire-error.r.p b/test/unittest/lockstat/tst.lv-adaptive-acquire-error.r.p
> > new file mode 100755
> > +#!/usr/bin/awk -f
> > +NR == 1 { next; }
> > +NR == 2 && NF == 5 { $1 = "PROBE"; $4 = ""; gsub(/  +/, " "); print; next; }
> > +NR == 2 && NF == 4 { $1 = "PROBE"; gsub(/  +/, " "); print; next; }
> > +/^ *[0-9]+/ { exit; }
> > +{ print; }
> 
> Since this is nontrivial (as Nick already mentioned) and awk can be
> commented, how about a few comments to explain what's going on?

Even though you and Nick) mention it is non-trivial, it is clearly easy enough
to understand if you know awk.  And with your suggestion below I think it is
even more clear.  I don't see what comments can make it more obvious than it
already is.

> The two NR==2 lines could perhaps be reduced to:
>         NR==2 { print "PROBEID", $2, $3, $NF; next; }

Sure.

> What's the exit line for?

If this is used e.g. on the legacy version where the .r files were generated,
you do not want multiple occurence (due to multiple probe locations) to be
listed because they all will have the same argument declarations anyway.  So,
we bail if there is a 2nd probe listed.