[Perf] Optimize deepgemm experts initialization, 3.9% TTFT improvement (#30494)

Signed-off-by: yewentao256 <zhyanwentao@126.com>
Co-authored-by: li-jinpeng <3332126450@qq.com>
Co-authored-by: youkaichao <youkaichao@gmail.com>

vllm/model_executor/layers/fused_moe/deep_gemm_utils.py

@@ -84,10 +84,16 @@ def _fwd_kernel_ep_scatter_1(
     m_indices_start_ptr = m_indices + cur_expert_start
     off_expert = tl.arange(0, BLOCK_E)
 
+    # any rows in the per-expert aligned region that do not correspond to
+    # real tokens are left untouched here and should remain initialized to
+    # -1 so DeepGEMM can skip them
     for start_m in tl.range(0, cur_expert_token_num, BLOCK_E, num_stages=4):
+        offs = start_m + off_expert
+        mask = offs < cur_expert_token_num
         tl.store(
-            m_indices_start_ptr + start_m + off_expert,
+            m_indices_start_ptr + offs,
             cur_expert,
+            mask=mask,
         )
 
 
@@ -366,12 +372,17 @@ def deepgemm_moe_permute(
         (M_sum, H // block_k), device=device, dtype=torch.float32
     )
 
-    maybe_has_empty_blocks = (expert_tokens_meta is None) or (
-        expert_tokens_meta.expert_num_tokens_cpu is None
+    # DeepGEMM uses negative values in m_indices (here expert_ids) to mark
+    # completely invalid / padded blocks that should be skipped. We always
+    # initialize expert_ids to -1 so any row that is not explicitly written
+    # by the scatter kernel will be treated as invalid and skipped by
+    # DeepGEMM's scheduler.
+    expert_ids = torch.full(
+        (M_sum,),
+        fill_value=-1,
+        device=device,
+        dtype=torch.int32,
     )
-    expert_ids_init = torch.zeros if maybe_has_empty_blocks else torch.empty
-
-    expert_ids = expert_ids_init((M_sum), device=device, dtype=torch.int32)
     inv_perm = torch.empty(topk_ids.shape, device=device, dtype=torch.int32)
 
     expert_num_tokens = None