Merge pull request tock#4120 from tock/hudson.ayers/core-wg-notes

Add Core Notes July 26, 2024

doc/wg/core/notes/core-notes-2024-07-26.md

@@ -0,0 +1,101 @@
+# Tock Meeting Notes 2024-07-26
+
+## Attendees
+- Hudson Ayers
+- Pat Pannuto
+- Brad Campbell
+- Leon Schuermann
+- Benjamin Prevor
+- Tyler Potyondy
+- Alyssa Haroldsen
+- Andrew Imwalle
+- Alexandru Radovici
+
+## Updates
+
+* Ben: We have a new employee of the Tock foundation – Benjamin Prevor, he is
+  working on the treadmill testing infrastructure. Before this he was a full
+  time Haskell engineer.
+* Pat: We have a libtock-c PR pulling in a Lora example that brings in GPL
+  code. It is constrained to an example so hopefully contained and does not
+  cause issues, but wanted to flag it so people could check it out.
+  https://github.com/tock/libtock-c/pull/456
+
+## Outstanding PR triage
+
+* Hudson: https://github.com/tock/tock/pull/4109
+* Brad: Lets mark that as waiting on author.
+* Hudson: https://github.com/tock/tock/pull/4110
+* Pat: I am still waiting on the high-level descriptive document detailing goals, philosophy etc. before diving into it.
+* Leon: That was opened by a labmate of ours, it stems from a project on timing isolation. I will be managing this for the most part. Implementation is driving forward regardless because this is a research project, and also outlines how invasive changes would be. In practice we won't move forward with this before a descriptive document is ready.
+* Hudson: https://github.com/tock/tock/pull/4075
+* Leon: Should we merge it on the call? It is last call
+* Everyone: Yes!
+* Brad: as an aside, it would be nice to have a Tock community bot. For example I opened this PR, but then could not approve it even though other people contributed many of the commits.
+* Alyssa: Could we allow people with write access to the Tock repo directly merge without approval? For community PR cases like this?
+* Brad: I think that would be fine, but the issue I am talking about is sort of the opposite – for example I could not put a "changes requested" review on my own PR if Amit adds commits and then approves. Probably not a thing we need to discuss, just a workflow mismatch.
+
+## Dynamic allocation in libtock-rs
+
+[meta: Hudson was notetaker for this meeting so this initial part is copied from Hudson's
+email]
+
+* Hudson: I would like to discuss options for dynamic allocation in libtock-rs.
+  There are two basic approaches
+* Hudson: First is the approach used in the old libtock-rs: Allow apps to
+  specify a heap size at compile time, reserve that much space for the heap
+  using brk() while initializing the runtime, and then use an allocator that
+  assumes a backing store of a fixed size (such as linked\_list\_allocator).
+* Hudson: The second approach is similar to that originally suggested by
+  Johnathan when the old libtock-rs allocator was first implemented: a heap
+  that only takes memory from the kernel as needed, and return memory to the
+  kernel when no longer needed. The idea is generally an allocator that works
+  on a backing store it was given, but will grow the size of this contiguous
+  backing store as needed. One advantage of this approach is that if an app
+  wants to allocate a bunch of memory, then return it before calling any
+  syscalls that lead to grant allocation, that app can use more memory than
+  would otherwise be possible given the size of a grant allocation. This
+  approach can also just be useful toward maximizing the memory available to
+  the app -- rather than having to specify a constant heap size that is known
+  to be smaller than (app memory - allocated grant size), the app can just use
+  everything that is not taken by grants, and then allocations will start to
+  fail once that point is reached.
+* Hudson: The second approach is more challenging to integrate with existing
+  open source embedded Rust allocators, because in practice I have not found
+  any that provide APIs for returning memory given to the allocator back to the
+  kernel, without having to add an OS port to the allocator itself. IMO this
+  approach seems more likely to require us to write our own allocator, or to
+  modify an existing open source allocator with a port to Tock specifically.
+  Interested to hear what others think.
+* Brad: It makes sense that most existing embedded allocators do this, what if
+  the memory freed is not at the end of the heap?
+* Hudson: Yeah, then it would not be returned to the kernel, but you can have
+  heuristics that return memory when there does end up being enough freed near
+  the end of the heap.
+* Brad: If we used an existing allocator, would your design preclude using one
+  that does enable returning memory to the heap later?
+* Hudson: No I don't think so
+* Hudson: I did kinda get the feeling that Johnathan was expecting us to go
+  forward with the latter approach. He had some other comments in 2022 when
+  dczself was looking at implementing an allocator that seemed to kind of
+  assume, oh, we're not gonna want to use an existing allocator we're gonna
+  want to write our own. And he mentioned we're gonna want to use two crates
+  for unit testability, which makes sense if we are writing our own allocator
+  but less if we are using another crate that already has its own unit tests.
+* Hudson: One other observation is that libtock-c uses this kind of Max heap
+  approach as well where apps specify the maximum amount of heap memory that
+  they'll use. Does the libtock-c allocator reserve all of that memory from the
+  kernel as soon as the app is initialized or does it only actually take memory
+  from the kernel gradually as needed?
+* Brad: Gradually as needed
+* Hudson: Does the libtock-c allocator have a mechanism to return memory to the
+  kernel via a negative sbrk?
+* Brad: I was wondering if you would ask that, I am not sure
+* Hudson: OK, I think I will start with an approach that gradually allocates
+  memory from the kernel as it goes, but does not return memory.
+* Brad: Makes sense to me, I don't really see what the downside would be, the
+  memory is there.
+
+## OT WG 
+
+* Hudson: lets defer this discussion to next week with Amit on the call