ADR-0024 Phase B: update xfail reason with architectural blocker details

Phase B Option A (freeze + defer to ADR-0027): the root cause of
ring_default_ws strict-xfail is that bench workers call torch.zeros /
copy_ which drive env.run in the WORKER-greenlet context. Any pending
KernelLaunchMsg gets stepped inside that worker, spawning kernel_runner
with parent = worker (not main). When the worker yields/finishes, the
kernel greenlet is orphaned and its next switch_to_simpy raises
GreenletExit mid-add — producing rank 0 mean=1 (expected 3).

This is a larger architectural redesign (lazy-deploy tensor API,
coroutine worker, or setup/verify split) and is parked until ADR-0027
(Megatron TP) starts, where the proper solution ships with TP use cases.

No production changes; xfail reason + inline comment only.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_ccl_allreduce_matrix.py

@@ -70,16 +70,27 @@ CASES = [
     # Default fallback — no world_size override → ADR-0024 D1 derives
     # from topology (SIP count = 2). Exercises the new SIP-level TP
     # launcher + cross-SIP ring.
-    # XFAIL: Phase A fix (scheduler-level wait) resolves the greenlet-
+    # XFAIL — architectural blocker (ADR-0024 Phase B, future redesign):
-    # re-entry hang, but Phase 2 DataExecutor still reports only 1 math
+    # Bench workers call torch.zeros / copy_ which internally drive
-    # op for a 2-rank ring (expected 2) — cross-SIP op_log replay
+    # env.run in the WORKER-greenlet context. Any KernelLaunchMsg already
-    # integration pending ADR-0024 Phase B.
+    # pending in the SimPy queue gets stepped inside that worker context,
+    # which in turn spawns kernel_runner + kernel greenlet with parent =
+    # worker (not main). When the worker later yields / finishes, the
+    # kernel greenlet is orphaned; its next switch_to_simpy raises
+    # GreenletExit mid-add, producing rank 0 mean=1 (expected 3).
+    # Fix requires redesigning worker semantics so env.run only ever
+    # drives from main (options: lazy-deploy tensor API, coroutine
+    # worker, or setup/verify split). Not a single-PR change — parked
+    # until ADR-0027 (Megatron TP) starts, at which point a proper
+    # architectural solution lands together with TP use cases.
     pytest.param(
         "ring_allreduce_tcm", "kernbench.ccl.algorithms.ring_allreduce",
         "ring_1d", "tcm", None, 8, 2,
         id="ring_default_ws",
         marks=pytest.mark.xfail(
-            reason="ADR-0024 Phase B: cross-SIP op_log replay integration",
+            reason="ADR-0024 Phase B: worker-greenlet env.run captures "
+                   "kernel greenlet as child → orphaned on worker yield. "
+                   "Needs architectural redesign (see test comment).",
             strict=True,
         ),
     ),