Add intel_amx backend for Radix Attention for CPU (#6408)

Co-authored-by: Chunyuan WU <chunyuan.wu@intel.com>
Co-authored-by: Thien Tran <gau.nernst@yahoo.com.sg>

python/sglang/srt/layers/attention/base_attn_backend.py

@@ -109,3 +109,7 @@ class AttentionBackend(ABC):
     ):
         """Run a forward for extend."""
         raise NotImplementedError()
+
+    def support_triton(self):
+        """Check if the current backend supports triton."""
+        return True

python/sglang/srt/layers/attention/intel_amx_backend.py

+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import torch
+
+from sglang.srt.layers.attention.base_attn_backend import AttentionBackend
+from sglang.srt.model_executor.forward_batch_info import ForwardBatch
+
+if TYPE_CHECKING:
+    from sglang.srt.layers.radix_attention import RadixAttention
+    from sglang.srt.model_executor.model_runner import ModelRunner
+
+
+class IntelAMXAttnBackend(AttentionBackend):
+    def __init__(self, model_runner: ModelRunner):
+        import sgl_kernel
+
+        super().__init__()
+        self.forward_metadata = None
+        self.device = model_runner.device
+
+        self.num_head = (
+            model_runner.model_config.num_attention_heads // model_runner.tp_size
+        )
+
+        self.v_head_dim = model_runner.token_to_kv_pool.get_value_buffer(0).shape[-1]
+
+        self.decode_attention_fwd = torch.ops.sgl_kernel.decode_attention_cpu
+        self.extend_attention_fwd = torch.ops.sgl_kernel.extend_attention_cpu
+
+    def init_forward_metadata(self, forward_batch: ForwardBatch):
+        """Init the metadata for a forward pass."""
+
+        bs = forward_batch.batch_size
+        attn_logits = torch.zeros(
+            (
+                bs,
+                self.num_head,
+                8,  # self.num_kv_splits,
+                self.v_head_dim + 1,
+            ),
+            dtype=torch.float32,
+            device=self.device,
+        )
+        if forward_batch.forward_mode.is_decode_or_idle():
+            max_extend_len = None
+        else:
+            max_extend_len = torch.max(forward_batch.extend_seq_lens).item()
+        self.forward_metadata = (attn_logits, max_extend_len)
+
+    def forward_extend(
+        self,
+        q,
+        k,
+        v,
+        layer: RadixAttention,
+        forward_batch: ForwardBatch,
+        save_kv_cache=True,
+    ):
+        if layer.qk_head_dim != layer.v_head_dim:
+            o = q.new_empty((q.shape[0], layer.tp_q_head_num * layer.v_head_dim))
+        else:
+            o = torch.empty_like(q)
+
+        if save_kv_cache:
+            forward_batch.token_to_kv_pool.set_kv_buffer(
+                layer, forward_batch.out_cache_loc, k, v
+            )
+
+        _, max_extend_len = self.forward_metadata
+
+        self.extend_attention_fwd(
+            q.view(-1, layer.tp_q_head_num, layer.qk_head_dim),
+            k,
+            v,
+            o.view(-1, layer.tp_q_head_num, layer.v_head_dim),
+            forward_batch.token_to_kv_pool.get_key_buffer(layer.layer_id),
+            forward_batch.token_to_kv_pool.get_value_buffer(layer.layer_id),
+            forward_batch.req_to_token_pool.req_to_token,
+            forward_batch.req_pool_indices,
+            forward_batch.seq_lens,
+            forward_batch.extend_seq_lens,
+            forward_batch.extend_start_loc,
+            max_extend_len,
+            layer.scaling,
+            layer.logit_cap,
+        )
+        return o
+
+    def forward_decode(
+        self,
+        q: torch.Tensor,
+        k: torch.Tensor,
+        v: torch.Tensor,
+        layer: RadixAttention,
+        forward_batch: ForwardBatch,
+        save_kv_cache=True,
+    ):
+        attn_logits, _ = self.forward_metadata
+
+        q = q.reshape(-1, layer.tp_q_head_num * layer.qk_head_dim)
+
+        if layer.qk_head_dim != layer.v_head_dim:
+            o = q.new_empty((q.shape[0], layer.tp_q_head_num * layer.v_head_dim))
+        else:
+            o = torch.empty_like(q)
+
+        self.decode_attention_fwd(
+            q.view(-1, layer.tp_q_head_num, layer.qk_head_dim),
+            forward_batch.token_to_kv_pool.get_key_buffer(layer.layer_id),
+            forward_batch.token_to_kv_pool.get_value_buffer(layer.layer_id),
+            o.view(-1, layer.tp_q_head_num, layer.v_head_dim),
+            k,
+            v,
+            forward_batch.out_cache_loc,
+            attn_logits,
+            forward_batch.req_to_token_pool.req_to_token,
+            forward_batch.req_pool_indices,
+            forward_batch.seq_lens,
+            layer.scaling,
+            layer.logit_cap,
+        )
+
+        return o
+
+    def support_triton(self):
+        return False

python/sglang/srt/layers/attention/torch_native_backend.py

@@ -265,3 +265,6 @@ class TorchNativeAttnBackend(AttentionBackend):
         )
 
         return o
+
+    def support_triton(self):
+        return False

python/sglang/srt/managers/schedule_batch.py

@@ -60,7 +60,7 @@ from sglang.srt.model_executor.forward_batch_info import CaptureHiddenMode, Forw
 from sglang.srt.sampling.sampling_batch_info import SamplingBatchInfo
 from sglang.srt.sampling.sampling_params import SamplingParams
 from sglang.srt.server_args import ServerArgs
-from sglang.srt.utils import flatten_nested_list, get_compiler_backend
+from sglang.srt.utils import flatten_nested_list, get_compiler_backend, support_triton
 
 if TYPE_CHECKING:
     from sglang.srt.speculative.eagle_utils import EagleDraftInput, EagleVerifyInput
@@ -1257,7 +1257,7 @@ class ScheduleBatch(ScheduleBatchDisaggregationDecodeMixin):
         self.extend_input_logprob_token_ids = extend_input_logprob_token_ids
 
         # Write to req_to_token_pool
-        if global_server_args_dict["attention_backend"] != "torch_native":
+        if support_triton(global_server_args_dict.get("attention_backend")):
             # TODO: some tensors can be reused for ForwardBatchInfo (e.g., extend_lens, cumsum_start)
 
             write_req_to_token_pool_triton[(bs,)](

python/sglang/srt/model_executor/forward_batch_info.py

@@ -39,7 +39,7 @@ import triton
 import triton.language as tl
 
 from sglang.srt.layers.rotary_embedding import MRotaryEmbedding
-from sglang.srt.utils import flatten_nested_list, get_compiler_backend
+from sglang.srt.utils import flatten_nested_list, get_compiler_backend, support_triton
 
 if TYPE_CHECKING:
     from sglang.srt.layers.attention.base_attn_backend import AttentionBackend
@@ -351,7 +351,7 @@ class ForwardBatch:
             ret.extend_prefix_lens = torch.tensor(
                 batch.extend_prefix_lens, dtype=torch.int32
             ).to(device, non_blocking=True)
-            if model_runner.server_args.attention_backend != "torch_native":
+            if support_triton(model_runner.server_args.attention_backend):
                 ret.extend_num_tokens = batch.extend_num_tokens
                 positions, ret.extend_start_loc = compute_position_triton(
                     ret.extend_prefix_lens,

python/sglang/srt/model_executor/model_runner.py

@@ -91,6 +91,7 @@ from sglang.srt.speculative.spec_info import SpeculativeAlgorithm
 from sglang.srt.torch_memory_saver_adapter import TorchMemorySaverAdapter
 from sglang.srt.utils import (
     MultiprocessingSerializer,
+    cpu_has_amx_support,
     enable_show_time_cost,
     get_available_gpu_memory,
     get_bool_env_var,
@@ -317,6 +318,16 @@ class ModelRunner:
     def model_specific_adjustment(self):
         server_args = self.server_args
 
+        if (
+            server_args.attention_backend == "intel_amx"
+            and server_args.device == "cpu"
+            and not cpu_has_amx_support()
+        ):
+            logger.info(
+                "The current platform does not support Intel AMX, will fallback to torch_native backend."
+            )
+            server_args.attention_backend = "torch_native"
+
         if server_args.attention_backend is None:
             """
             Auto select the fastest attention backend.
@@ -369,7 +380,10 @@ class ModelRunner:
                         f"Invalid attention backend for MLA: {server_args.attention_backend}"
                     )
             else:
-                raise ValueError("MLA optimization not supported on CPU.")
+                if server_args.attention_backend != "intel_amx":
+                    raise ValueError(
+                        "MLA optimization not supported on CPU except for intel_amx backend."
+                    )
 
         if (
             server_args.attention_backend == "fa3"
@@ -1067,6 +1081,13 @@ class ModelRunner:
             )
 
             return CutlassMLABackend(self)
+        elif self.server_args.attention_backend == "intel_amx":
+            from sglang.srt.layers.attention.intel_amx_backend import (
+                IntelAMXAttnBackend,
+            )
+
+            logger.info(f"Intel AMX attention backend is enabled.")
+            return IntelAMXAttnBackend(self)
         else:
             raise ValueError(
                 f"Invalid attention backend: {self.server_args.attention_backend}"

python/sglang/srt/server_args.py

@@ -323,6 +323,11 @@ class ServerArgs:
             self.sampling_backend = "pytorch"
 
         # Set kernel backends
+        if self.device == "cpu":
+            if self.attention_backend is None:
+                self.attention_backend = "intel_amx"
+            self.sampling_backend = "pytorch"
+
         if self.sampling_backend is None:
             self.sampling_backend = (
                 "flashinfer" if is_flashinfer_available() else "pytorch"
@@ -993,6 +998,7 @@ class ServerArgs:
                 "fa3",
                 "flashmla",
                 "cutlass_mla",
+                "intel_amx",
             ],
             default=ServerArgs.attention_backend,
             help="Choose the kernels for attention layers.",

python/sglang/srt/utils.py

@@ -2225,3 +2225,21 @@ def bind_or_assign(target, source):
         return target
     else:
         return source
+
+
+def support_triton(backend: str) -> bool:
+    return backend not in ["torch_native", "intel_amx"]
+
+
+try:
+    import sgl_kernel
+
+    is_intel_amx_backend_available = hasattr(
+        torch.ops.sgl_kernel, "convert_weight_packed"
+    )
+except:
+    is_intel_amx_backend_available = False
+
+
+def cpu_has_amx_support():
+    return torch._C._cpu._is_amx_tile_supported() and is_intel_amx_backend_available