[Kernel] Better inf handling for grouped topk cu (#24886)

Signed-off-by: lumina37 <[email protected]>

Author: lumina37

csrc/moe/grouped_topk_kernels.cu

@@ -21,14 +21,14 @@
 #include <torch/all.h>
 #include <cuda_fp16.h>
 #include <cuda_bf16.h>
+#include <cuda/std/limits>
 #include <cooperative_groups.h>
 #include <cooperative_groups/reduce.h>
 namespace cg = cooperative_groups;
 
 namespace vllm {
 namespace moe {
 
-constexpr float kNegInfinity = INFINITY * -1;
 constexpr unsigned FULL_WARP_MASK = 0xffffffff;
 constexpr int32_t WARP_SIZE = 32;
 constexpr int32_t BLOCK_SIZE = 512;
@@ -411,14 +411,21 @@ __device__ inline float cuda_cast<float, __nv_bfloat16>(__nv_bfloat16 val) {
   return __bfloat162float(val);
 }
 
+template <typename T>
+__device__ inline T neg_inf() {
+  // cuda::std::numeric_limits<T>::infinity() returns `0` for [T=bf16 or fp16]
+  // so we need to cast from fp32
+  return cuda_cast<T, float>(-cuda::std::numeric_limits<float>::infinity());
+}
+
 template <typename T>
 __device__ void topk_with_k2(T* output, T const* input,
                              cg::thread_block_tile<32> const& tile,
                              int32_t const lane_id,
                              int const num_experts_per_group) {
   // Get the top2 per thread
-  T largest = -INFINITY;
-  T second_largest = -INFINITY;
+  T largest = neg_inf<T>();
+  T second_largest = neg_inf<T>();
 
   if (num_experts_per_group > WARP_SIZE) {
     for (int i = lane_id; i < num_experts_per_group; i += WARP_SIZE) {
@@ -513,8 +520,8 @@ __global__ void group_idx_and_topk_idx_kernel(
       warp_id * topk;
   s_topk_idx += warp_id * topk;
 
-  T value = kNegInfinity;
-  T topk_group_value = kNegInfinity;
+  T value = neg_inf<T>();
+  T topk_group_value = neg_inf<T>();
   int32_t num_equalto_topkth_group;
 
 #if (defined(__CUDA_ARCH__) && (__CUDA_ARCH__ >= 900))
@@ -525,11 +532,8 @@ __global__ void group_idx_and_topk_idx_kernel(
   if (case_id < num_tokens) {
     // calculate group_idx
     int32_t target_num_min = WARP_SIZE - n_group + topk_group;
-    if (lane_id < n_group &&
-        (isfinite(cuda_cast<float, T>(
-            group_scores[lane_id]))))  // The check is necessary to avoid
-                                       // abnormal input
-    {
+    // The check is necessary to avoid abnormal input
+    if (lane_id < n_group && cuda::std::isfinite(group_scores[lane_id])) {
       value = group_scores[lane_id];
     }
 
@@ -540,23 +544,22 @@ __global__ void group_idx_and_topk_idx_kernel(
       __syncwarp();  // Ensure all threads have valid data before reduction
       topk_group_value = cg::reduce(tile, value, cg::greater<T>());
       if (value == topk_group_value) {
-        value = kNegInfinity;
+        value = neg_inf<T>();
       }
       pre_count_equal_to_top_value = count_equal_to_top_value;
-      count_equal_to_top_value = __popc(__ballot_sync(
-          FULL_WARP_MASK, (value == cuda_cast<T, float>(kNegInfinity))));
+      count_equal_to_top_value =
+          __popc(__ballot_sync(FULL_WARP_MASK, (value == neg_inf<T>())));
     }
     num_equalto_topkth_group = target_num_min - pre_count_equal_to_top_value;
   }
   __syncthreads();
 
   warp_topk::WarpSelect</*capability*/ WARP_SIZE, /*greater*/ true, T, int32_t,
                         /* is_stable */ true>
-      queue((int32_t)topk, -INFINITY);
+      queue((int32_t)topk, neg_inf<T>());
 
   int count_equalto_topkth_group = 0;
-  bool if_proceed_next_topk =
-      (topk_group_value != cuda_cast<T, float>(kNegInfinity));
+  bool if_proceed_next_topk = topk_group_value != neg_inf<T>();
   if (case_id < num_tokens && if_proceed_next_topk) {
     for (int i_group = 0; i_group < n_group; i_group++) {
       if ((group_scores[i_group] > topk_group_value) ||
@@ -566,10 +569,10 @@ __global__ void group_idx_and_topk_idx_kernel(
         for (int32_t i = lane_id; i < align_num_experts_per_group;
              i += WARP_SIZE) {
           T candidates =
-              (i < num_experts_per_group) && isfinite(cuda_cast<float, T>(
-                                                 scores_with_bias[offset + i]))
+              (i < num_experts_per_group) &&
+                      cuda::std::isfinite(scores_with_bias[offset + i])
                   ? scores_with_bias[offset + i]
-                  : cuda_cast<T, float>(kNegInfinity);
+                  : neg_inf<T>();
           queue.add(candidates, offset + i);
         }
         if (group_scores[i_group] == topk_group_value) {
@@ -598,7 +601,8 @@ __global__ void group_idx_and_topk_idx_kernel(
       if (i < topk) {
         s_topk_value[i] = value;
       }
-      topk_sum += reduce(tile, cuda_cast<float, T>(value), cg::plus<float>());
+      topk_sum +=
+          cg::reduce(tile, cuda_cast<float, T>(value), cg::plus<float>());
     }
   }