[Attention] Tune CUTLASS MLA num_splits (vllm-project#26846)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>
Signed-off-by: xuebwang-amd <xuebwang@amd.com>

Author: MatthewBonanni

csrc/attention/mla/cutlass_sm100_mla/device/sm100_mla.hpp

@@ -125,32 +125,37 @@ class MLA {
   }
 
   static void set_split_kv (KernelArguments& args) {
-    // printf("set_split_kv start");
     if (args.split_kv >= 1) return;
     auto [H, K, D, B] = args.problem_shape;
-    // std::cout << H << " " << K << " " << D << " " << B << "\n";      
     int sm_count = args.hw_info.sm_count;
-    // printf("    sm_count = %d\n", sm_count);
-    int max_splits = ceil_div(K, 128);
-    max_splits = min(16, max_splits);
-
-    // TODO: This avoids a hang when the batch size larger than 1 and 
-    // there is more than 1 kv_splits. 
-    // Discuss with NVIDIA how this can be fixed.
-    if (B > 1) {
-      max_splits = min(1, max_splits);
+    float seq_length_k = static_cast<float>(K) / 1024.0f;
+    int max_splits = 1;
+
+    if (B <= 4 && seq_length_k >= 16) {
+      max_splits = 16;
+    }
+    else if (B <= 8 && seq_length_k >= 4) {
+      max_splits = 8;
+    }
+    else if ((B <= 16 && seq_length_k >= 8) ||
+             (B == 48 && seq_length_k >= 32)) {
+      max_splits = 4;
+    }
+    else if ((B <= 32 && seq_length_k >= 16) ||
+             (B == 96 && seq_length_k >= 16)) {
+      max_splits = 2;
     }
-    
-    // printf("    max_splits = %d\n", max_splits);
+    else {
+      max_splits = 1;
+    }
+
+    // Wave-aware scheduling: ensure integer number of waves in K dimension
     int sms_per_batch = max(1, sm_count / B);
-    // printf("    sms_per_batch = %d\n", sms_per_batch);
     int split_heur = min(max_splits, sms_per_batch);
     int waves = ceil_div(B * split_heur, sm_count);
     int k_waves = ceil_div(max_splits, split_heur);
     int split_wave_aware = ceil_div(max_splits, k_waves);
     args.split_kv = split_wave_aware;
-    // printf("    args.split_kv = %d\n", args.split_kv);
-
   }
 
   /// Determines whether the GEMM can execute the given problem.