[BugFix] [Kernel] Add Cutlass2x fallback kernels (vllm-project#5744)

Co-authored-by: Varun Sundar Rabindranath <varun@neuralmagic.com>

csrc/quantization/cutlass_w8a8/common.hpp

@@ -17,3 +17,11 @@ inline uint32_t next_pow_2(uint32_t const num) {
   return 1 << (CHAR_BIT * sizeof(num) - __builtin_clz(num - 1));
 }
 
+inline int get_cuda_max_shared_memory_per_block_opt_in(int const device) {
+  int max_shared_mem_per_block_opt_in = 0;
+  cudaDeviceGetAttribute(&max_shared_mem_per_block_opt_in,
+                        cudaDevAttrMaxSharedMemoryPerBlockOptin,
+                        device);
+  return max_shared_mem_per_block_opt_in;
+}
+

csrc/quantization/cutlass_w8a8/scaled_mm_c2x.cu

@@ -250,12 +250,39 @@ void cutlass_gemm_caller(torch::Tensor& out, torch::Tensor const& a,
   CUTLASS_CHECK(status);
 }
 
+template <typename Gemm, typename FallbackGemm, typename... EpilogueArgs>
+void fallback_cutlass_gemm_caller(torch::Tensor& out, torch::Tensor const& a,
+                                  torch::Tensor const& b,
+                                  EpilogueArgs&&... args) {
+  // In some cases, the GPU isn't able to accommodate the
+  // shared memory requirements of the Gemm. In such cases, use
+  // the FallbackGemm instead.
+  static const int max_shared_mem_per_block_opt_in =
+      get_cuda_max_shared_memory_per_block_opt_in(0);
+
+  size_t const gemm_shared_mem_size =
+      sizeof(typename Gemm::KernelType::SharedStorage);
+  size_t const fallback_gemm_shared_mem_size =
+      sizeof(typename FallbackGemm::KernelType::SharedStorage);
+
+  if (gemm_shared_mem_size <= max_shared_mem_per_block_opt_in) {
+    return cutlass_gemm_caller<Gemm>(out, a, b,
+                                     std::forward<EpilogueArgs>(args)...);
+  } else {
+    TORCH_CHECK(fallback_gemm_shared_mem_size <=
+                max_shared_mem_per_block_opt_in);
+    return cutlass_gemm_caller<FallbackGemm>(
+        out, a, b, std::forward<EpilogueArgs>(args)...);
+  }
+}
+
 template <typename InType, typename OutType,
           template <typename, typename> typename Epilogue>
 struct sm80_config_default {
   // This config is used in 2 cases,
   //  - M in (128, inf)
   //  - M in (64, 128] and N >= 8192
+  // Shared Memory required by this Gemm - 81920 bytes
   static_assert(std::is_same<InType, int8_t>());
   using TileShape = typename cutlass::gemm::GemmShape<128, 128, 64>;
   using WarpShape = typename cutlass::gemm::GemmShape<64, 64, 64>;
@@ -271,6 +298,7 @@ struct sm80_config_M64 {
   // This config is used in 2 cases,
   // - M in (32, 64]
   // - M in (64, 128] and N < 8192
+  // Shared Memory required by this Gemm - 122880 bytes
   static_assert(std::is_same<InType, int8_t>());
   using TileShape = typename cutlass::gemm::GemmShape<64, 128, 128>;
   using WarpShape = typename cutlass::gemm::GemmShape<64, 64, 64>;
@@ -284,6 +312,7 @@ template <typename InType, typename OutType,
           template <typename, typename> typename Epilogue>
 struct sm80_config_M32 {
   // M in (16, 32]
+  // Shared Memory required by this Gemm - 61440 bytes
   static_assert(std::is_same<InType, int8_t>());
   using TileShape = typename cutlass::gemm::GemmShape<32, 64, 128>;
   using WarpShape = typename cutlass::gemm::GemmShape<32, 64, 64>;
@@ -297,6 +326,7 @@ template <typename InType, typename OutType,
           template <typename, typename> typename Epilogue>
 struct sm80_config_M16 {
   // M in [1, 16]
+  // Shared Memory required by this Gemm - 51200 bytes
   static_assert(std::is_same<InType, int8_t>());
   using TileShape = typename cutlass::gemm::GemmShape<16, 64, 128>;
   using WarpShape = typename cutlass::gemm::GemmShape<16, 64, 64>;
@@ -331,35 +361,45 @@ void cutlass_gemm_sm80_dispatch(torch::Tensor& out, torch::Tensor const& a,
   using Cutlass2xGemmM16 =
       typename sm80_config_M16<InType, OutType, Epilogue>::Cutlass2xGemm;
 
+  // Due to shared memory requirements, some Gemms may fail to run on some
+  // GPUs. As the name indicates, the Fallback Gemm is used as an alternative
+  // in such cases.
+  // sm80_config_M16 has the least shared-memory requirement. However,
+  // based on some profiling, we select sm80_config_M32 as a better alternative
+  // performance wise.
+  using FallbackGemm =
+      typename sm80_config_M32<InType, OutType, Epilogue>::Cutlass2xGemm;
+
   uint32_t const m = a.size(0);
   uint32_t const mp2 =
       std::max(static_cast<uint32_t>(16), next_pow_2(m));  // next power of 2
   if (mp2 <= 16) {
     // M in [1, 16]
-    return cutlass_gemm_caller<Cutlass2xGemmM16>(
+    return fallback_cutlass_gemm_caller<Cutlass2xGemmM16, FallbackGemm>(
         out, a, b, std::forward<EpilogueArgs>(args)...);
   } else if (mp2 <= 32) {
     // M in (16, 32]
-    return cutlass_gemm_caller<Cutlass2xGemmM32>(
+    return fallback_cutlass_gemm_caller<Cutlass2xGemmM32, FallbackGemm>(
         out, a, b, std::forward<EpilogueArgs>(args)...);
   } else if (mp2 <= 64) {
     // M in (32, 64]
-    return cutlass_gemm_caller<Cutlass2xGemmM64>(
+    return fallback_cutlass_gemm_caller<Cutlass2xGemmM64, FallbackGemm>(
         out, a, b, std::forward<EpilogueArgs>(args)...);
   } else if (mp2 <= 128) {
     // M in (64, 128]
     uint32_t const n = out.size(1);
     bool const small_n = n < 8192;
     if (small_n) {
-      return cutlass_gemm_caller<Cutlass2xGemmM128SmallN>(
+      return fallback_cutlass_gemm_caller<Cutlass2xGemmM128SmallN,
+                                          FallbackGemm>(
           out, a, b, std::forward<EpilogueArgs>(args)...);
     } else {
-      return cutlass_gemm_caller<Cutlass2xGemmM128BigN>(
+      return fallback_cutlass_gemm_caller<Cutlass2xGemmM128BigN, FallbackGemm>(
           out, a, b, std::forward<EpilogueArgs>(args)...);
     }
   } else {
     // M in (128, inf)
-    return cutlass_gemm_caller<Cutlass2xGemmDefault>(
+    return fallback_cutlass_gemm_caller<Cutlass2xGemmDefault, FallbackGemm>(
         out, a, b, std::forward<EpilogueArgs>(args)...);
   }
 }