Re: [PATCH] Identify LWLocks in tracepoints

On Sat, 20 Mar 2021, 04:21 Peter Eisentraut, <
peter(dot)eisentraut(at)enterprisedb(dot)com> wrote:

>
> On 18.03.21 07:34, Craig Ringer wrote:
> > The main reason I didn't want to add more tracepoints than were strictly
> > necessary is that Pg doesn't enable the systemtap semaphores feature, so
> > right now we do a T_NAME(lock) evaluation each time we pass a tracepoint
> > if --enable-dtrace is compiled in, whether or not anything is tracing.
> > This was fine on pg11 where it was just:
> >
> > #define T_NAME(lock) \
> > (LWLockTrancheArray[(lock)->tranche])
> >
> > but since pg13 it instead expands to
> >
> > GetLWTrancheName((lock)->tranche)
> >
> > where GetLWTrancheName isn't especially trivial. We'll run that function
> > every single time we pass any of these tracepoints and then discard the
> > result, which is ... not ideal. That applies so long as Pg is compiled
> > with --enable-dtrace. I've been meaning to look at enabling the
> > systemtap semaphores feature in our build so these can be wrapped in
> > unlikely(TRACE_POSTGRESQL_LWLOCK_RELEASE_ENABLED()) guards, but I wanted
> > to wrap this patch set up first as there are some complexities around
> > enabling the semaphores feature.
>
> There is already support for that. See the documentation at the end of
> this page:
>
> https://www.postgresql.org/docs/devel/dynamic-trace.html#DEFINING-TRACE-POINTS

Pretty sure it won't work right now.

To use systemtap semaphores (the _ENABLED macros) you need to run dtrace -g
to generate a probes.o then link that into postgres.

I don't think we do that. I'll double check soon.