[BugFix] Fix NaN errors in paged attention kernel (#936)

csrc/attention/attention_kernels.cu

@@ -246,6 +246,8 @@ __global__ void single_query_cached_kv_attention_kernel(
     accs[i] = 0.f;
   }
 
+  scalar_t zero_value;
+  zero(zero_value);
   for (int block_idx = warp_idx; block_idx < num_blocks; block_idx += NUM_WARPS) {
     const int physical_block_number = block_table[block_idx];
     const int physical_block_offset = (lane % NUM_V_VECS_PER_ROW) * V_VEC_SIZE;
@@ -261,6 +263,16 @@ __global__ void single_query_cached_kv_attention_kernel(
       if (row_idx < HEAD_SIZE) {
         const int offset = row_idx * BLOCK_SIZE + physical_block_offset;
         V_vec v_vec = *reinterpret_cast<const V_vec*>(v_ptr + offset);
+        if (block_idx == num_blocks - 1) {
+          // NOTE(woosuk): When v_vec contains the tokens that are out of the context,
+          // we should explicitly zero out the values since they may contain NaNs.
+          // See https://github.com/vllm-project/vllm/issues/641#issuecomment-1682544472
+          scalar_t* v_vec_ptr = reinterpret_cast<scalar_t*>(&v_vec);
+#pragma unroll
+          for (int j = 0; j <= V_VEC_SIZE; j++) {
+            v_vec_ptr[j] = token_idx + j < context_len ? v_vec_ptr[j] : zero_value;
+          }
+        }
         accs[i] += dot(logits_vec, v_vec);
       }
     }

csrc/attention/dtype_bfloat16.cuh

@@ -420,4 +420,14 @@ inline __device__ void from_float(bf16_8_t& dst, Float8_ src) {
 #endif
 }
 
+// Zero-out a variable.
+inline __device__ void zero(__nv_bfloat16& dst) {
+#if defined(__CUDA_ARCH__) && __CUDA_ARCH__ < 800
+  assert(false);
+#else
+  // Same as CUDART_ZERO_BF16 introduced in CUDA 12.2.
+  dst = __ushort_as_bfloat16((unsigned short)0x0000U);
+#endif
+}
+
 } // namespace vllm

csrc/attention/dtype_float16.cuh

@@ -390,11 +390,6 @@ inline __device__ float sum(uint4 v) {
   return sum(c);
 }
 
-// Zero-out a vector.
-inline __device__ void zero(uint16_t& dst) {
-  dst = uint16_t(0);
-}
-
 // From float32 to float16.
 inline __device__ void from_float(uint16_t& dst, float src) {
   dst = float_to_half(src);
@@ -441,4 +436,9 @@ inline __device__ Float8_ to_float(uint4 u) {
   return tmp;
 }
 
+// Zero-out a variable.
+inline __device__ void zero(uint16_t& dst) {
+  dst = uint16_t(0);
+}
+
 } // namespace vllm

csrc/attention/dtype_float32.cuh

@@ -265,4 +265,9 @@ inline __device__ Float8_ to_float(Float8_ u) {
   return u;
 }
 
+// Zero-out a variable.
+inline __device__ void zero(float& dst) {
+  dst = 0.f;
+}
+
 } // namespace vllm