gqa: single-KV-group LLaMA-3.1-70B prefill milestone (Increments 1-5)

End-to-end wires C=8 P=8 d_head=128 prefill with snake-ring inter-CUBE
SFR, intra-CUBE PE-SP (all 64 ranks active), and the milestone bench
panel. Decode kernel gains lrab-adapted center-root reduce for the
2×4 sub-mesh per ADR-0060 §4.2.

Increment 1 — SFR multi-row snake
  src/kernbench/ccl/sfr_config.py: configure_sfr_intercube_ring gains
  submesh_shape / submesh_origin kwargs; installs a Hamiltonian snake
  ring through a rectangular sub-mesh (every hop is 1-hop physical
  neighbour). Backward-compat: 1D-row behaviour preserved when
  submesh_shape is None.
  tests/test_intercube_snake_ring.py (12 tests)

Increment 2 — Decode lrab-adapted center-root reduce
  src/kernbench/benches/_gqa_attention_decode_long.py: new sub_w param
  (default 0 = existing 1D-chain). sub_w >= 2 selects the ADR-0060
  §4.2 prescribed lrab-adapted Phase 1+2 reduce (bidirectional row +
  bidirectional col converge to the center cube), with log-sum-exp
  _merge_running replacing the plain + of lrab.
  tests/attention/test_gqa_decode_long_2d_reduce.py (4 tests)

Increment 3 — Prefill kernel at C=8 (no production change)
  Verified by inspection that the existing prefill_long kernel +
  Increment 1's snake SFR already work at C=8 without any kernel
  edit. The kernel speaks logical W/E; the snake routes it.
  tests/attention/test_gqa_prefill_long_c8_snake.py (3 tests)

Increment 4 — Intra-CUBE PE-SP in prefill (all 64 ranks)
  src/kernbench/benches/_gqa_attention_prefill_long.py: new P param
  (default 1 = existing PE-0-only). P > 1 splits T_q query-axis-wise
  across the P PEs of each CUBE; output rows are disjoint per PE so
  no intra-CUBE reduce is needed; each PE drives its own same-lane
  ring (P parallel rings).
  tests/attention/test_gqa_prefill_long_pe_sp.py (5 tests)

Increment 5 — LLaMA-scale milestone bench panel
  src/kernbench/benches/milestone_gqa_headline.py: new panel
  single_kv_group_prefill_gqa_c8_p8 (C=8, P=8, T_q=S_kv=32K,
  d_head=128). _run_prefill_panel extended with P/T_q/d_head
  defaults; routes snake SFR when C > mesh_w.
  tests/attention/test_milestone_gqa_single_kv_group_prefill_panel.py (3 tests)

Total: 4 production files modified, 5 new test files, 27 new tests.
Followed the Phase 1/2 protocol per CLAUDE.md throughout.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/attention/test_gqa_decode_long_2d_reduce.py

@@ -0,0 +1,191 @@
+"""Tests for lrab-adapted center-root inter-CUBE reduce in decode_long.
+
+Verifies the ADR-0060 §4.2 prescribed Level-1 collective for the
+single-KV-group LLaMA-3.1-70B target (C=8 over a 2×4 sub-mesh, root at
+the geometric center CUBE).
+
+Activation contract:
+  - ``sub_w == 0`` (default; omitted from launch args) → existing 1D
+    inter-CUBE chain that converges at CUBE 0. Byte-for-byte unchanged.
+  - ``sub_w >= 2`` with ``sub_h = C // sub_w >= 2`` → lrab-adapted
+    center-root mesh reduce. Root cube is
+    ``(sub_h//2)*sub_w + (sub_w//2)``.
+
+Degenerate (``sub_h < 2`` or ``sub_w < 2``) combinations are rejected at
+launch time per ADR-0060 §4.2 + CLAUDE.md "Simplicity First": those
+configurations belong to the 1D-chain code path, not lrab.
+
+Phase 1 (this commit): tests only — production code lands in Phase 2.
+T1, T2, T4 fail today (kernel signature does not accept ``sub_w``).
+T3 passes today as the backward-compat anchor for the existing
+1D-chain path.
+"""
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+import pytest
+
+from kernbench.benches._gqa_attention_decode_long import gqa_attention_decode_long_kernel  # noqa: F401
+from kernbench.ccl.install import load_ccl_config, resolve_algorithm_config
+from kernbench.ccl.sfr_config import configure_sfr_intercube_multisip
+from kernbench.policy.placement.dp import DPPolicy
+from kernbench.runtime_api.bench_runner import run_bench
+from kernbench.runtime_api.types import resolve_device
+from kernbench.sim_engine.engine import GraphEngine
+from kernbench.topology.builder import resolve_topology
+
+TOPOLOGY_DEFAULT = Path(__file__).resolve().parents[2] / "topology.yaml"
+
+D_HEAD = 64
+DTYPE = "f16"
+_CUBE_RE = re.compile(r"\bcube(\d+)\b")
+
+
+def _ccl_cfg():
+    return resolve_algorithm_config(
+        load_ccl_config(), name="lrab_hierarchical_allreduce",
+    )
+
+
+def _engine_factory(t, d):
+    return GraphEngine(getattr(t, "topology_obj", t), enable_data=True)
+
+
+def _dma_write_cubes(op_log) -> list[int]:
+    """Return the list of CUBE ids that emitted a ``dma_write``.
+
+    Decode's final-store path uses one ``tl.store`` from the root rank.
+    Multiple HBM-channel-level write records may correspond to the same
+    logical store; this just reports the source CUBE for each.
+    """
+    cubes: list[int] = []
+    for r in op_log:
+        if r.op_name != "dma_write":
+            continue
+        m = _CUBE_RE.search(r.component_id)
+        if m is not None:
+            cubes.append(int(m.group(1)))
+    return cubes
+
+
+def _run_decode_long(
+    *, C: int, P: int, S_kv: int, sub_w: int | None,
+):
+    """Drive a decode_long launch. ``sub_w=None`` means omit the arg
+    entirely (exercises the kernel's current default behaviour)."""
+    topo = resolve_topology(str(TOPOLOGY_DEFAULT))
+
+    def _bench_fn(ctx):
+        configure_sfr_intercube_multisip(ctx.engine, ctx.spec, _ccl_cfg())
+        dp_full = DPPolicy(cube="replicate", pe="replicate",
+                           num_cubes=C, num_pes=P)
+        dp_kv = DPPolicy(cube="row_wise" if C > 1 else "replicate",
+                         pe="row_wise", num_cubes=C, num_pes=P)
+        T_q = 1
+        h_q = 8
+        h_kv = 1
+        ctx.zeros((T_q, h_q * D_HEAD),
+                  dtype=DTYPE, dp=dp_full, name=f"q_dec_2d_c{C}")
+        k = ctx.zeros((S_kv, h_kv * D_HEAD),
+                      dtype=DTYPE, dp=dp_kv, name=f"k_dec_2d_c{C}")
+        v = ctx.zeros((S_kv, h_kv * D_HEAD),
+                      dtype=DTYPE, dp=dp_kv, name=f"v_dec_2d_c{C}")
+        o = ctx.empty((T_q, h_q * D_HEAD),
+                      dtype=DTYPE, dp=dp_full, name=f"o_dec_2d_c{C}")
+        q = ctx.zeros((T_q, h_q * D_HEAD),
+                      dtype=DTYPE, dp=dp_full, name=f"q_dec_2d_c{C}_2")
+        launch_args = [q, k, v, o, T_q, S_kv, h_q, h_kv, D_HEAD, C, P]
+        if sub_w is not None:
+            launch_args.append(sub_w)
+        ctx.launch(
+            f"gqa_decode_long_2d_c{C}_sw{sub_w}",
+            gqa_attention_decode_long_kernel,
+            *launch_args,
+            _auto_dim_remap=False,
+        )
+
+    return run_bench(
+        topology=topo, bench_fn=_bench_fn,
+        device=resolve_device(None), engine_factory=_engine_factory,
+    )
+
+
+# ── T1: C=8 lrab-adapted center-root completes ───────────────────────
+
+
+def test_decode_2d_sub_w4_sub_h2_completes():
+    """ADR-0060 §4.2 prescribed center-root mesh reduce at the
+    milestone target: C=8 over a 2×4 sub-mesh (sub_w=4, sub_h=2),
+    P=8 PEs per CUBE. With zero inputs, output is trivially zero;
+    the test guards that the kernel reaches completion without
+    deadlock or scratch overflow.
+    """
+    result = _run_decode_long(C=8, P=8, S_kv=2048, sub_w=4)
+    assert result.completion.ok, (
+        f"decode at C=8, sub_w=4 (lrab-adapted) must complete; "
+        f"got {result.completion}"
+    )
+
+
+# ── T2: root lives at the geometric center CUBE (cube 6) ─────────────
+
+
+def test_decode_2d_sub_w4_sub_h2_root_at_center_cube_6():
+    """For sub_w=4, sub_h=2: root_col=2, root_row=1, root_cube=6.
+
+    Only the root CUBE issues the final ``tl.store`` — every other
+    rank short-circuits. The dma_write records must come exclusively
+    from CUBE 6.
+    """
+    result = _run_decode_long(C=8, P=8, S_kv=2048, sub_w=4)
+    assert result.completion.ok
+    cubes = _dma_write_cubes(result.engine.op_log)
+    assert cubes, "expected at least one dma_write for the final O store"
+    distinct = set(cubes)
+    assert distinct == {6}, (
+        f"final dma_write must come exclusively from CUBE 6 (sub_w=4, "
+        f"sub_h=2 root); got cubes={sorted(distinct)}"
+    )
+
+
+# ── T3: backward-compat — sub_w omitted → existing 1D chain at C=4 ───
+
+
+def test_decode_2d_backward_compat_sub_w0_default():
+    """When ``sub_w`` is omitted from the launch, the kernel must
+    behave identically to today: 1D inter-CUBE chain along W,
+    converging at CUBE 0. This serves as a regression anchor — Phase 2
+    must not change the existing C=4 panel behaviour.
+
+    Passes today as well as after Phase 2.
+    """
+    result = _run_decode_long(C=4, P=8, S_kv=2048, sub_w=None)
+    assert result.completion.ok, (
+        f"decode at C=4 with default sub_w (1D chain) must complete; "
+        f"got {result.completion}"
+    )
+    cubes = _dma_write_cubes(result.engine.op_log)
+    assert cubes, "expected at least one dma_write for the final O store"
+    distinct = set(cubes)
+    assert distinct == {0}, (
+        f"1D-chain root must be CUBE 0; got cubes={sorted(distinct)}"
+    )
+
+
+# ── T4: degenerate sub_w configurations are rejected ─────────────────
+
+
+def test_decode_2d_invalid_sub_w_rejected_when_sub_h_lt_2():
+    """ADR-0060 §4.2 + CLAUDE.md "Simplicity First": only sub-meshes
+    with both sub_w >= 2 and sub_h >= 2 use lrab. C=4 with sub_w=4
+    would give sub_h=1 (degenerate; Phase 2 of lrab becomes a no-op).
+    Callers must use the 1D-chain path (sub_w=0/omitted) for that case.
+
+    The kernel must reject the degenerate combination with a clear
+    error rather than silently producing a 1D-chain result at the
+    wrong root location.
+    """
+    with pytest.raises((ValueError, AssertionError), match=r"sub_[wh]"):
+        _run_decode_long(C=4, P=8, S_kv=2048, sub_w=4)