Re: Printing backtrace of postgres processes

On Mon, Feb 12, 2024 at 6:52 AM Ashutosh Bapat
<ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> wrote:
>
> >
> > > We defer actual action triggered by a signal till CHECK_FOR_INTERRUPTS
> > > is called. I understand that we can't do that here since we want to
> > > capture the backtrace at that moment and can't wait till next CFI. But
> > > printing the backend can surely wait till next CFI right?
> >
> > Delaying the call of backtrace() to happen during a CFI() would be
> > safe, yes, and writing data to stderr would not really be an issue as
> > at least the data would be sent somewhere. That's less useful, but
> > we do that for memory contexts.
>
> Memory contexts do not change more or less till next CFI, but stack
> traces do. So I am not sure whether it is desirable to wait to capture
> backtrace till next CFI. Given that the user can not time a call to
> pg_log_backend() exactly, so whether it captures the backtrace exactly
> at when interrupt happens or at the next CFI may not matter much in
> practice.
>

Thinking more about this I have following thoughts/questions:

1. Whether getting a backtrace at CFI is good enough?
Backtrace is required to know what a process is doing when it's stuck
or is behaviour unexpected etc. PostgreSQL code has CFIs sprinkled in
almost all the tight loops. Whether those places are enough to cover
most of the cases that the user of this feature would care about?

2. tools like gdb, strace can be used to get the stack trace of any
process, so do we really need this tool?
Most of the OSes provide such tools but may be there are useful in
kubernetes like environment, I am not sure.

3. tools like gdb and strace are able to capture stack trace at any
point during execution. Can we use the same mechanism instead of
relying on CFI?

4. tools like gdb and strace can capture more than just stack trace
e.g. variable values, values of registers etc. Are we planning to add
those facilities as well? OR whether this feature will be useful
without those facilities?

May the feature be more useful if it can provide PostgreSQL specific
details which an external tool can not.

--
Best Wishes,
Ashutosh Bapat