[TPU][V1] MHA Pallas backend (#15288)

Signed-off-by: NickLucche <nlucches@redhat.com>

tests/v1/tpu/test_mha_attn.py

+# SPDX-License-Identifier: Apache-2.0
+"""
+Test:
+
+* Tests for MultiHeadAttention layer
+"""
+
+import pytest
+import torch
+import torch_xla
+import torch_xla.core
+import torch_xla.core.xla_model
+
+from vllm import envs
+from vllm.attention.layer import MultiHeadAttention
+from vllm.attention.selector import _cached_get_attn_backend
+from vllm.platforms import current_platform
+
+if not envs.VLLM_USE_V1:
+    pytest.skip(
+        "Skipping V1 tests. Rerun with `VLLM_USE_V1=1` to test.",
+        allow_module_level=True,
+    )
+
+
+@pytest.fixture(autouse=True)
+def clear_cache():
+    """Clear lru cache to ensure each test case runs without caching.
+    """
+    _cached_get_attn_backend.cache_clear()
+
+
+def ref_attention(
+    query: torch.Tensor,
+    key: torch.Tensor,
+    value: torch.Tensor,
+    scale: float,
+) -> torch.Tensor:
+    """
+    Native implementation of scaled dot product attention without mask:
+    - query, key, value: [batch_size, seq_len, num_heads, head_size]
+    - attn_mask: [batch_size, seq_len, seq_len]
+    """
+    query, key, value = (x.transpose(1, 2) for x in (query, key, value))
+    attn_weights = scale * torch.matmul(query, key.transpose(2, 3))
+    attn_weights = torch.softmax(attn_weights, dim=-1).to(value.dtype)
+    out = torch.matmul(attn_weights, value).transpose(1, 2)
+    return out
+
+
+BATCH_SIZES = [1, 16]
+SEQ_LENS = [1]
+NUM_HEADS = [1, 16]
+NUM_KV_HEADS = [1]
+HEAD_SIZES = [64, 80]
+
+
+@pytest.mark.skipif(not current_platform.is_tpu(),
+                    reason="This test needs a TPU")
+@pytest.mark.parametrize("batch_size", BATCH_SIZES)
+@pytest.mark.parametrize("seq_len", SEQ_LENS)
+@pytest.mark.parametrize("num_heads", NUM_HEADS)
+@pytest.mark.parametrize("num_kv_heads", NUM_KV_HEADS)
+@pytest.mark.parametrize("head_size", HEAD_SIZES)
+@pytest.mark.parametrize("device", [torch_xla.core.xla_model.xla_device()])
+def test_mha_attn_forward(
+    batch_size: int,
+    seq_len: int,
+    num_heads: int,
+    num_kv_heads: int,
+    head_size: int,
+    device: str,
+):
+    current_platform.seed_everything(0)
+    # These are expected to be f32
+    q = torch.randn(batch_size, seq_len, num_heads * head_size, device=device)
+    k = torch.randn(batch_size,
+                    seq_len,
+                    num_kv_heads * head_size,
+                    device=device)
+    v = torch.randn(batch_size,
+                    seq_len,
+                    num_kv_heads * head_size,
+                    device=device)
+    scale = 1.0 / head_size**0.5
+    attn = MultiHeadAttention(num_heads,
+                              head_size,
+                              scale=scale,
+                              num_kv_heads=num_kv_heads)
+    output = attn(q, k, v)
+
+    assert num_heads % num_kv_heads == 0
+    num_queries_per_kv = num_heads // num_kv_heads
+
+    q = q.reshape(batch_size, seq_len, num_heads, head_size)
+    k = k.reshape(batch_size, seq_len, num_kv_heads, head_size)
+    v = v.reshape(batch_size, seq_len, num_kv_heads, head_size)
+    if num_queries_per_kv > 1:
+        k = torch.repeat_interleave(k, num_queries_per_kv, dim=2)
+        v = torch.repeat_interleave(v, num_queries_per_kv, dim=2)
+
+    ref_output = ref_attention(
+        q,
+        k,
+        v,
+        scale=scale,
+    ).reshape(batch_size, seq_len, num_heads * head_size)
+    # torch_xla flash_attn kernel is less accurate but much faster
+    torch.testing.assert_close(output, ref_output, atol=1e-2, rtol=1e-3)

vllm/attention/layer.py

@@ -281,8 +281,7 @@ class MultiHeadAttention(nn.Module):
             backend = _Backend.XFORMERS
 
         self.attn_backend = backend if backend in {
-            _Backend.TORCH_SDPA,
-            _Backend.XFORMERS,
+            _Backend.TORCH_SDPA, _Backend.XFORMERS, _Backend.PALLAS_VLLM_V1
         } else _Backend.TORCH_SDPA
 
     def forward(
@@ -320,6 +319,13 @@ class MultiHeadAttention(nn.Module):
                                                  value,
                                                  scale=self.scale)
             out = out.transpose(1, 2)
+        elif self.attn_backend == _Backend.PALLAS_VLLM_V1:
+            query, key, value = (x.transpose(1, 2)
+                                 for x in (query, key, value))
+            from torch_xla.experimental.custom_kernel import flash_attention
+            out = flash_attention(query, key, value, sm_scale=self.scale)
+            out = out.transpose(1, 2)
+
         return out.reshape(bsz, q_len, -1)