gh-128002: use per threads tasks linked list in asyncio

[kumaraditya303]

• Issue: Audit asyncio thread safety #128002

Use per-thread linked list of tasks in asyncio. This design allows for lock free register/unregister of tasks of loops running concurrently in different threads. It uses the stop the world pause to traverse the list of tasks from all threads from the thread where all_tasks is called. This has no performance impact on regular builds as per benchmarks and performs a bit faster on free-threading benchmarks. pyperformance benchmarks aren't good for this because it uses just one thread so there is little lock contention, this however performs much better when multiple threads are running.

On free-threading:

Benchmark	bm-20250111-linux-x86_64-python-3a570c6d58bd5ad7d7c1-3.14.0a3+-3a570c6	bm-20250112-linux-x86_64-kumaraditya303-per_thread_tasks-3.14.0a3+-cca4057
async_tree_cpu_io_mixed_tg	556 ms	536 ms: 1.04x faster
async_tree_none_tg	303 ms	294 ms: 1.03x faster
coroutines	26.2 ms	25.5 ms: 1.03x faster
async_tree_memoization_tg	397 ms	387 ms: 1.03x faster
async_tree_cpu_io_mixed	598 ms	583 ms: 1.03x faster
async_generators	498 ms	486 ms: 1.02x faster
async_tree_io	748 ms	733 ms: 1.02x faster
async_tree_io_tg	696 ms	682 ms: 1.02x faster
async_tree_memoization	442 ms	436 ms: 1.02x faster
async_tree_none	349 ms	344 ms: 1.01x faster
Geometric mean	(ref)	1.02x faster

Benchmark hidden because not significant (1): asyncio_websockets

[1st1]

First, great work on this, this is legitimately a cool PR.

That said, I'm feeling really uneasy about _PyEval_StopTheWorld. Maybe instead of this approach we try spin locks + a custom mini hash map data structure? That would obviously be a lot more work, but a cleaner and ultimately better perf approach.

Also please wait for reviews from @pablogsal and @ambv. I'm curious if this would make external introspection harder.

[graingert]

Does this work where an event loop is used on one thread, stopped then resumed on another thread?

[kumaraditya303]

I have pushed some more changes:

• Added a interpreter list of tasks which gets used when a task is alive but the thread state gets deallocated as suggested by @colesbury
• Added thread id tracking in free-threading builds to avoid locking in the general case of loops running independently in threads without sharing tasks
• Added some tests

TODO: benchmark it before merging

[pablogsal]

If I am not mistaken this solution seems incompatible with the asyncio introspecction workflow we are adding on #124640. Please, ensure that this change is compatible with the changes in that PR to avoid problems in the future.

[bedevere-app]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[kumaraditya303]

If I am not mistaken this solution seems incompatible with the asyncio introspecction workflow we are adding on #124640. Please, ensure that this change is compatible with the changes in that PR to avoid problems in the future.

This PR has nothing to do with asyncio introspection. As I said in other PR, the change which would affect that is moving current task to per-loop which isn't done in this PR.

.

[pablogsal]

Hummm, I must have misread how this affects the task management. Let me dismiss my request for changes meanwhile. Thanks for the patience with this @kumaraditya303!

[kumaraditya303]

@colesbury PTAL

[colesbury]

This LGTM, but would you please also get this reviewed by another asyncio expert?