make jl_active_task_stack more accurate

[JeffBezanson]

In most cases this used to return the whole stack buffer; instead look at the context or thread state to get the actual SP.

[fingolfin]

Ohh, very nice :-)

[kpamnany]

@JeffBezanson: is this done/working? Merge?

@qinsoon, you probably want to backport this once it is merged.

[JeffBezanson]

We concluded that this probably can't be called from the GC, so maybe we shouldn't do this?

[fingolfin]

I am confused as to how to parse thus... "can't be called" as in "wouldn't work" or as in "doesn't actually happen" or ... ?

[kpamnany]

@fingolfin: for the moving GC (MMTk), we need to pin pointers that should not be moved in the next collection. We're using a conservative stack scan to figure out what needs to be pinned. To make the scan quicker, we wanted to get rsp rather than scanning the entire stack. The approach in this PR leverages the logic in jl_rec_backtrace(), but that code does other things that probably can't be safely done from a GC thread.

We concluded that this probably can't be called from the GC, so maybe we shouldn't do this?

@qinsoon: have you been able to implement a get_sp()? If so, we can close this PR.

[fingolfin]

@kpamnany OK so the problem is that the function being modified here is not safe to be called during GC, got it (note that we've been doing conservative stack scanning in Julia since... 2020 or so)

[fingolfin]

Are there still plans by anyone to work on this in some form? I'd be really interested in this. I am also willing to help out with both development and testing. But it seems you guys already had some discussions "on the side" regarding pitfalls and implementation approaches...?

[qinsoon]

I don't know if any one is actively working on this. Probably no.

The issue with the PR so far is that it requires the GC threads that call the function to have a Julia TLS. However, having a Julia TLS does not seem to be a requirement for GC threads in other cases. We can make it a requirement, then there would be no issue for this PR.

One alternative @kpamnany mentioned is that to get the stack pointer value from the saved registers in #55003, which also works around the TLS requirement. I am not working on that either.

[vtjnash]

We concluded this would probably need to be a new field in the Task struct that stores the active extent whenever the GC is running, as often there is no other way to retrieve that information