Support MHA with chunked prefix cache for flashinfer/flashmla backend, support page size > 1 for MHA chunked prefix (#8616)

Co-authored-by: xuyongfei.xyf <xuyongfei.xyf@antgroup.com>

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

@@ -776,14 +776,13 @@ class FlashAttentionBackend(AttentionBackend):
                 o = result
         else:
             if (
-                not global_server_args_dict["disable_chunked_prefix_cache"]
+                forward_batch.attn_attend_prefix_cache is not None
-                and forward_batch.attn_attend_prefix_cache is not None
                 and not forward_batch.forward_mode.is_target_verify()
                 and not forward_batch.forward_mode.is_draft_extend()
             ):
                 # Do multi-head attention with chunked prefix cache
-
                 if forward_batch.attn_attend_prefix_cache:
+                    assert not global_server_args_dict["disable_chunked_prefix_cache"]
                     # MHA for chunked prefix kv cache when running model with MLA
                     assert forward_batch.prefix_chunk_idx is not None
                     assert forward_batch.prefix_chunk_cu_seq_lens is not None
@@ -792,7 +791,8 @@ class FlashAttentionBackend(AttentionBackend):
                     chunk_idx = forward_batch.prefix_chunk_idx
                     assert chunk_idx >= 0
 
-                    output, lse, *rest = flash_attn_varlen_func(
+                    assert forward_batch.mha_return_lse
+                    output = flash_attn_varlen_func(
                         q=q.view(-1, layer.tp_q_head_num, layer.head_dim),
                         k=k.view(-1, layer.tp_k_head_num, layer.head_dim).to(q.dtype),
                         v=v.view(-1, layer.tp_k_head_num, layer.v_head_dim).to(q.dtype),
@@ -806,7 +806,7 @@ class FlashAttentionBackend(AttentionBackend):
                     )
                 else:
                     # MHA for extend part of sequence without attending prefix kv cache
-                    output, lse, *rest = flash_attn_varlen_func(
+                    output = flash_attn_varlen_func(
                         q=q.view(-1, layer.tp_q_head_num, layer.head_dim),
                         k=k.view(-1, layer.tp_k_head_num, layer.head_dim).to(q.dtype),
                         v=v.view(-1, layer.tp_k_head_num, layer.v_head_dim).to(q.dtype),
@@ -816,9 +816,13 @@ class FlashAttentionBackend(AttentionBackend):
                         max_seqlen_k=metadata.max_seq_len_q,
                         softmax_scale=layer.scaling,
                         causal=True,
-                        return_softmax_lse=True,
+                        return_softmax_lse=forward_batch.mha_return_lse,
                     )
-                return output, lse
+                if forward_batch.mha_return_lse:
+                    output, lse, *rest = output
+                    lse = torch.transpose(lse, 0, 1).contiguous()
+                    return output, lse
+                return output
             else:
                 # Do absorbed multi-latent attention
                 kv_cache = forward_batch.token_to_kv_pool.get_key_buffer(

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

@@ -59,6 +59,115 @@ class PrefillMetadata:
 global_workspace_buffer = None
 
 
+class FlashInferMhaChunkKVRunner:
+    def __init__(
+        self, model_runner: ModelRunner, attn_backend: "FlashInferMlaAttnBackend"
+    ):
+        # Parse Constants
+        self.num_local_heads = (
+            model_runner.model_config.num_attention_heads // get_attention_tp_size()
+        )
+        self.qk_nope_head_dim = model_runner.model_config.qk_nope_head_dim
+        self.qk_rope_head_dim = model_runner.model_config.qk_rope_head_dim
+        self.v_head_dim = model_runner.model_config.v_head_dim
+        self.data_type = model_runner.dtype
+        self.q_data_type = model_runner.dtype
+
+        # Buffers and wrappers
+        self.qo_indptr = attn_backend.qo_indptr
+        self.workspace_buffer = attn_backend.workspace_buffer
+        self.fmha_backend = attn_backend.fmha_backend
+
+        self.chunk_ragged_wrappers = []
+        self.ragged_wrapper = attn_backend.prefill_wrapper_ragged
+
+    def update_prefix_chunks(self, num_prefix_chunks: int):
+        while num_prefix_chunks > len(self.chunk_ragged_wrappers):
+            ragged_wrapper = BatchPrefillWithRaggedKVCacheWrapper(
+                self.workspace_buffer, "NHD", backend=self.fmha_backend
+            )
+            self.chunk_ragged_wrappers.append(ragged_wrapper)
+
+    def update_wrapper(
+        self,
+        forward_batch: ForwardBatch,
+    ):
+        assert forward_batch.num_prefix_chunks is not None
+        num_prefix_chunks = forward_batch.num_prefix_chunks
+        self.update_prefix_chunks(num_prefix_chunks)
+
+        prefix_lens = forward_batch.extend_prefix_lens
+        seq_lens = forward_batch.seq_lens
+
+        bs = len(seq_lens)
+        qo_indptr = self.qo_indptr
+        qo_indptr[1 : bs + 1] = torch.cumsum(seq_lens - prefix_lens, dim=0)
+        qo_indptr = qo_indptr[: bs + 1]
+
+        for chunk_idx in range(forward_batch.num_prefix_chunks):
+            # MHA for chunked prefix kv cache when running model with MLA
+            assert forward_batch.prefix_chunk_idx is not None
+            assert forward_batch.prefix_chunk_cu_seq_lens is not None
+            assert forward_batch.prefix_chunk_max_seq_lens is not None
+
+            kv_indptr = forward_batch.prefix_chunk_cu_seq_lens[chunk_idx]
+            wrapper = self.chunk_ragged_wrappers[chunk_idx]
+            wrapper.begin_forward(
+                qo_indptr=qo_indptr,
+                kv_indptr=kv_indptr,
+                num_qo_heads=self.num_local_heads,
+                num_kv_heads=self.num_local_heads,
+                head_dim_qk=self.qk_nope_head_dim + self.qk_rope_head_dim,
+                head_dim_vo=self.v_head_dim,
+                q_data_type=self.q_data_type,
+                causal=False,
+            )
+        # ragged prefill
+        self.ragged_wrapper.begin_forward(
+            qo_indptr=qo_indptr,
+            kv_indptr=qo_indptr,
+            num_qo_heads=self.num_local_heads,
+            num_kv_heads=self.num_local_heads,
+            head_dim_qk=self.qk_nope_head_dim + self.qk_rope_head_dim,
+            head_dim_vo=self.v_head_dim,
+            q_data_type=self.q_data_type,
+            causal=True,
+        )
+
+    def forward(
+        self,
+        q: torch.Tensor,
+        k: torch.Tensor,
+        v: torch.Tensor,
+        layer: RadixAttention,
+        forward_batch: ForwardBatch,
+    ):
+        logits_soft_cap = layer.logit_cap
+        if forward_batch.attn_attend_prefix_cache:
+            chunk_idx = forward_batch.prefix_chunk_idx
+            assert chunk_idx >= 0
+            wrapper = self.chunk_ragged_wrappers[chunk_idx]
+            o1, s1 = wrapper.forward_return_lse(
+                q.view(-1, layer.tp_q_head_num, layer.head_dim),
+                k.view(-1, layer.tp_k_head_num, layer.head_dim).to(q.dtype),
+                v.view(-1, layer.tp_v_head_num, layer.v_head_dim).to(q.dtype),
+                causal=False,
+                sm_scale=layer.scaling,
+                logits_soft_cap=logits_soft_cap,
+            )
+        else:
+            o1, s1 = self.ragged_wrapper.forward_return_lse(
+                q.view(-1, layer.tp_q_head_num, layer.head_dim),
+                k.view(-1, layer.tp_k_head_num, layer.head_dim).to(q.dtype),
+                v.view(-1, layer.tp_v_head_num, layer.v_head_dim).to(q.dtype),
+                causal=True,
+                sm_scale=layer.scaling,
+                logits_soft_cap=logits_soft_cap,
+            )
+
+        return o1, s1
+
+
 class FlashInferMLAAttnBackend(AttentionBackend):
     """Flashinfer attention kernels."""
 
@@ -74,6 +183,12 @@ class FlashInferMLAAttnBackend(AttentionBackend):
         self.max_context_len = model_runner.model_config.context_len
         self.device = model_runner.device
         self.skip_prefill = skip_prefill
+        self.enable_chunk_kv = (
+            not skip_prefill
+            and global_server_args_dict["disaggregation_mode"] != "decode"
+            and not global_server_args_dict["disable_chunked_prefix_cache"]
+            and not global_server_args_dict["flashinfer_mla_disable_ragged"]
+        )
         self.page_size = model_runner.page_size
 
         # Allocate buffers
@@ -117,11 +232,11 @@ class FlashInferMLAAttnBackend(AttentionBackend):
         else:
             self.q_indptr_decode = q_indptr_decode_buf
 
-        fmha_backend = "auto"
+        self.fmha_backend = "auto"
         if is_sm100_supported():
-            fmha_backend = "cutlass"
+            self.fmha_backend = "cutlass"
         self.prefill_wrapper_ragged = BatchPrefillWithRaggedKVCacheWrapper(
-            self.workspace_buffer, "NHD", backend=fmha_backend
+            self.workspace_buffer, "NHD", backend=self.fmha_backend
         )
 
         if not self.skip_prefill:
@@ -145,6 +260,8 @@ class FlashInferMLAAttnBackend(AttentionBackend):
             self.indices_updater_prefill = FlashInferMLAIndicesUpdaterPrefill(
                 model_runner, self
             )
+            if self.enable_chunk_kv:
+                self.mha_chunk_kv_cache = FlashInferMhaChunkKVRunner(model_runner, self)
 
         self.indices_updater_decode = FlashInferMLAIndicesUpdaterDecode(
             model_runner, self
@@ -373,6 +490,10 @@ class FlashInferMLAAttnBackend(AttentionBackend):
     def get_cuda_graph_seq_len_fill_value(self):
         return 1
 
+    def init_mha_chunk_metadata(self, forward_batch: ForwardBatch):
+        """Init the metadata for a forward pass."""
+        self.mha_chunk_kv_cache.update_wrapper(forward_batch)
+
     def forward_extend(
         self,
         q: torch.Tensor,
@@ -384,6 +505,16 @@ class FlashInferMLAAttnBackend(AttentionBackend):
         q_rope: Optional[torch.Tensor] = None,
         k_rope: Optional[torch.Tensor] = None,
     ):
+        if (
+            forward_batch.attn_attend_prefix_cache is not None
+            and forward_batch.mha_return_lse
+        ):  # MHA Chunk
+            assert self.enable_chunk_kv
+            assert q_rope is None
+            assert k_rope is None
+            o1, s1 = self.mha_chunk_kv_cache.forward(q, k, v, layer, forward_batch)
+            return o1, s1
+
         cache_loc = forward_batch.out_cache_loc
         logits_soft_cap = layer.logit_cap
         prefill_wrapper_paged = self.forward_metadata.prefill_wrapper
@@ -412,8 +543,8 @@ class FlashInferMLAAttnBackend(AttentionBackend):
                 k = torch.cat([k, k_rope], dim=-1)
             o = self.prefill_wrapper_ragged.forward(
                 qall,
-                k.view(-1, layer.tp_k_head_num, layer.head_dim),
+                k.view(-1, layer.tp_k_head_num, layer.head_dim).to(q.dtype),
-                v.view(-1, layer.tp_k_head_num, layer.v_head_dim),
+                v.view(-1, layer.tp_k_head_num, layer.v_head_dim).to(q.dtype),
                 causal=True,
                 sm_scale=layer.scaling,
                 logits_soft_cap=logits_soft_cap,
@@ -732,6 +863,7 @@ class FlashInferMLAIndicesUpdaterPrefill:
                 head_dim_qk=self.qk_nope_head_dim + self.qk_rope_head_dim,
                 head_dim_vo=self.v_head_dim,
                 q_data_type=self.q_data_type,
+                causal=True,
             )
         else:
             # mla paged prefill

python/sglang/srt/managers/schedule_batch.py

@@ -106,6 +106,7 @@ GLOBAL_SERVER_ARGS_KEYS = [
     "enable_symm_mem",
     "quantization",
     "enable_custom_logit_processor",
+    "disaggregation_mode",
 ]
 
 # Put some global args for easy access

python/sglang/srt/model_executor/forward_batch_info.py

@@ -241,6 +241,9 @@ class ForwardBatch:
     prefix_chunk_num_tokens: Optional[List[int]] = None
     # KV Indices for each chunk
     prefix_chunk_kv_indices: Optional[List[torch.Tensor]] = None
+    # For MLA chunked prefix cache used in chunked prefill
+    # Tell attention backend whether lse needs to be returned
+    mha_return_lse: Optional[bool] = None
 
     # For multimodal
     mm_inputs: Optional[List[MultimodalInputs]] = None

python/sglang/srt/model_executor/model_runner.py

@@ -518,9 +518,6 @@ class ModelRunner:
 
         if not self.use_mla_backend:
             server_args.disable_chunked_prefix_cache = True
-        elif self.page_size > 1:
-            logger.info("Disable chunked prefix cache when page size > 1.")
-            server_args.disable_chunked_prefix_cache = True
 
         if not server_args.disable_chunked_prefix_cache:
             logger.info("Chunked prefix cache is turned on.")

python/sglang/srt/models/deepseek_v2.py

@@ -995,29 +995,31 @@ class DeepseekV2AttentionMLA(nn.Module):
 
         if attention_backend == "ascend":
             return AttnForwardMethod.MLA
-        elif attention_backend == "flashinfer":
+        elif (
+            attention_backend == "flashinfer"
+            or attention_backend == "fa3"
+            or attention_backend == "flashmla"
+        ):
+            # Use MHA with chunked KV cache when prefilling on long sequences.
+            sum_extend_prefix_lens = (
+                sum(forward_batch.extend_prefix_lens_cpu)
+                if forward_batch.extend_prefix_lens_cpu is not None
+                else 0
+            )
             # Flashinfer MLA: Do not absorb when enabling ragged prefill
+            disable_ragged = (
+                attention_backend == "flashinfer" or attention_backend == "flashmla"
+            ) and self.flashinfer_mla_disable_ragged
             if (
-                not self.flashinfer_mla_disable_ragged
+                not disable_ragged
                 and forward_batch.forward_mode.is_extend()
                 and not forward_batch.forward_mode.is_target_verify()
                 and not forward_batch.forward_mode.is_draft_extend()
-                and sum(forward_batch.extend_prefix_lens_cpu) == 0
-            ):
-                return AttnForwardMethod.MHA
-            else:
-                return _dispatch_mla_subtype()
-        elif attention_backend == "fa3":
-            # Flash Attention: Use MHA with chunked KV cache when prefilling on long sequences.
-            if forward_batch.extend_prefix_lens_cpu is not None:
-                sum_extend_prefix_lens = sum(forward_batch.extend_prefix_lens_cpu)
-            if (
-                forward_batch.forward_mode.is_extend()
-                and not self.disable_chunked_prefix_cache
-                and not forward_batch.forward_mode.is_target_verify()
-                and not forward_batch.forward_mode.is_draft_extend()
                 and (
-                    sum_extend_prefix_lens >= self.chunked_prefix_cache_threshold
+                    (
+                        sum_extend_prefix_lens >= self.chunked_prefix_cache_threshold
+                        and not self.disable_chunked_prefix_cache
+                    )
                     or sum_extend_prefix_lens == 0
                 )
             ):
@@ -1685,7 +1687,6 @@ class DeepseekV2AttentionMLA(nn.Module):
             k[..., self.qk_nope_head_dim :] = k_pe
 
             output, lse = self.attn_mha(q, k, v, forward_batch, save_kv_cache=False)
-            lse = torch.transpose(lse, 0, 1).contiguous()
             tmp_output = torch.empty_like(accum_output)
             tmp_lse = torch.empty_like(accum_lse)
             merge_state_v2(output, lse, accum_output, accum_lse, tmp_output, tmp_lse)
@@ -1707,55 +1708,26 @@ class DeepseekV2AttentionMLA(nn.Module):
         # will be helpful for understanding the purpose of this function.
 
         # First do normal mha forward to get output for extended part
-        if self.q_lora_rank is not None:
+        return self.forward_normal_prepare(
-            q, latent_cache = self.fused_qkv_a_proj_with_mqa(hidden_states)[0].split(
+            positions, hidden_states, forward_batch, zero_allocator
-                [self.q_lora_rank, self.kv_lora_rank + self.qk_rope_head_dim], dim=-1
-            )
-            q = self.q_a_layernorm(q)
-            q = self.q_b_proj(q)[0].view(-1, self.num_local_heads, self.qk_head_dim)
-        else:
-            q = self.q_proj(hidden_states)[0].view(
-                -1, self.num_local_heads, self.qk_head_dim
-            )
-            latent_cache = self.kv_a_proj_with_mqa(hidden_states)[0]
-        _, q_pe = q.split([self.qk_nope_head_dim, self.qk_rope_head_dim], dim=-1)
-        kv_a, _ = latent_cache.split([self.kv_lora_rank, self.qk_rope_head_dim], dim=-1)
-        latent_cache = latent_cache.unsqueeze(1)
-        kv_a = self.kv_a_layernorm(kv_a)
-        kv = self.kv_b_proj(kv_a)[0]
-        kv = kv.view(-1, self.num_local_heads, self.qk_nope_head_dim + self.v_head_dim)
-        k_nope = kv[..., : self.qk_nope_head_dim]
-        v = kv[..., self.qk_nope_head_dim :]
-        k_pe = latent_cache[:, :, self.kv_lora_rank :]
-
-        q_pe, k_pe = self.rotary_emb(positions, q_pe, k_pe)
-        q[..., self.qk_nope_head_dim :] = q_pe
-        k = torch.empty_like(q)
-        k[..., : self.qk_nope_head_dim] = k_nope
-        k[..., self.qk_nope_head_dim :] = k_pe
-
-        latent_cache[:, :, : self.kv_lora_rank] = kv_a.unsqueeze(1)
-        latent_cache[:, :, self.kv_lora_rank :] = k_pe
-
-        # Save latent cache
-        forward_batch.token_to_kv_pool.set_kv_buffer(
-            self.attn_mha, forward_batch.out_cache_loc, latent_cache, None
         )
 
-        return q, k, v, forward_batch
-
     def forward_normal_chunked_kv_core(self, q, k, v, forward_batch):
+        has_extend_prefix = any(forward_batch.extend_prefix_lens_cpu)
+        # Only initialize the info once
+        if has_extend_prefix and forward_batch.num_prefix_chunks is None:
+            forward_batch.prepare_chunked_prefix_cache_info(q.device)
+            if hasattr(forward_batch.attn_backend, "init_mha_chunk_metadata"):
+                forward_batch.attn_backend.init_mha_chunk_metadata(forward_batch)
+
+        forward_batch.mha_return_lse = has_extend_prefix
         # Do mha for extended part without prefix
         forward_batch.set_attn_attend_prefix_cache(False)
-        attn_output, lse = self.attn_mha(q, k, v, forward_batch, save_kv_cache=False)
+        attn_output = self.attn_mha(q, k, v, forward_batch, save_kv_cache=False)
-        lse = torch.transpose(lse, 0, 1).contiguous()
 
         # Do mha attention with chunked prefix cache if there are any sequence with prefix
-        if any(forward_batch.extend_prefix_lens_cpu):
+        if has_extend_prefix:
-            # Only initialize the info once
+            attn_output, lse = attn_output
-            if forward_batch.num_prefix_chunks is None:
-                forward_batch.prepare_chunked_prefix_cache_info(q.device)
-
             forward_batch.set_attn_attend_prefix_cache(True)
             attn_output = self._chunked_prefix_attn_mha(
                 q=q,