[Bugfix] Fix potentially unsafe custom allreduce synchronization

csrc/custom_all_reduce.cuh

@@ -6,6 +6,7 @@
 #include <cuda_runtime.h>
 
 #include <iostream>
+#include <array>
 #include <limits>
 #include <map>
 #include <unordered_map>
@@ -23,17 +24,23 @@
 
 namespace vllm {
 
-constexpr int kMaxBlocks = 64;
-// note: we don't want to use atomics for signals because peer atomics are no
-// supported on PCIe links
+constexpr int kMaxBlocks = 36;
+// Counter may overflow, but it's fine since unsigned int overflow is
+// well-defined behavior.
+using FlagType = uint32_t;
 struct Signal {
-  alignas(128) uint32_t start[kMaxBlocks][8];
-  alignas(128) uint32_t end[kMaxBlocks][8];
+  alignas(128) FlagType self_counter[kMaxBlocks][8];
+  // Two sets of peer counters are needed for two syncs. The reason is that
+  // it's possible for peer GPU block to arrive at the second sync point while
+  // the current GPU block haven't passed the first sync point. Thus, peer GPU
+  // may write counter+1 while current GPU is busy waiting for counter. We use
+  // alternating counter array to avoid this possibility.
+  alignas(128) FlagType peer_counter[2][kMaxBlocks][8];
 };
 
 struct __align__(16) RankData { const void* __restrict__ ptrs[8]; };
 
-struct __align__(16) RankSignals { volatile Signal* signals[8]; };
+struct __align__(16) RankSignals { Signal* signals[8]; };
 
 // like std::array, but aligned
 template <typename T, int sz>
@@ -123,47 +130,60 @@ DINLINE O downcast(array_t<float, O::size> val) {
   }
 }
 
-// This function is meant to be used as the first synchronization in the all
-// reduce kernel. Thus, it doesn't need to make any visibility guarantees for
-// prior memory accesses. Note: volatile writes will not be reordered against
-// other volatile writes.
-template <int ngpus>
-DINLINE void start_sync(const RankSignals& sg, volatile Signal* self_sg,
-                        int rank) {
-  if (threadIdx.x < ngpus) {
-    // reset flag for next time
-    self_sg->end[blockIdx.x][threadIdx.x] = 0;
-    // simultaneously write to the corresponding flag of all ranks.
-    // Latency = 1 p2p write
-    sg.signals[threadIdx.x]->start[blockIdx.x][rank] = 1;
-    // wait until we got true from all ranks
-    while (!self_sg->start[blockIdx.x][threadIdx.x]);
-  }
-  __syncthreads();
+static DINLINE void st_flag_release(FlagType* flag_addr, FlagType flag) {
+  asm volatile("st.release.sys.global.u32 [%1], %0;" ::"r"(flag),
+               "l"(flag_addr));
+}
+
+static DINLINE FlagType ld_flag_acquire(FlagType* flag_addr) {
+  FlagType flag;
+  asm volatile("ld.acquire.sys.global.u32 %0, [%1];"
+               : "=r"(flag)
+               : "l"(flag_addr));
+  return flag;
+}
+
+static DINLINE void st_flag_volatile(FlagType* flag_addr, FlagType flag) {
+  asm volatile("st.volatile.global.u32 [%1], %0;" ::"r"(flag), "l"(flag_addr));
+}
+
+static DINLINE FlagType ld_flag_volatile(FlagType* flag_addr) {
+  FlagType flag;
+  asm volatile("ld.volatile.global.u32 %0, [%1];"
+               : "=r"(flag)
+               : "l"(flag_addr));
+  return flag;
 }
 
-// This function is meant to be used as the second or the final synchronization
-// barrier in the all reduce kernel. If it's the final synchronization barrier,
-// we don't need to make any visibility guarantees for prior memory accesses.
-template <int ngpus, bool final_sync = false>
-DINLINE void end_sync(const RankSignals& sg, volatile Signal* self_sg,
-                      int rank) {
-  __syncthreads();
-  // eliminate the case that prior writes are not visible after signals become
-  // visible. Note that I did not managed to make this happen through a lot of
-  // testing. Might be the case that hardware provides stronger guarantee than
-  // the memory model.
-  if constexpr (!final_sync) __threadfence_system();
+// is_start: whether this is the very first synchronization barrier.
+// need_fence: whether a memory fence is needed. If true, a release-acquire
+// semantic is used to enforce memory access order before and after this
+// barrier.
+template <int ngpus, bool is_start, bool need_fence = false>
+DINLINE void multi_gpu_barrier(const RankSignals& sg, Signal* self_sg,
+                               int rank) {
+  if constexpr (!is_start) __syncthreads();
+  static_assert(
+      !(is_start && need_fence));  // Start barrier shouldn't need fence.
   if (threadIdx.x < ngpus) {
-    // reset flag for next time
-    self_sg->start[blockIdx.x][threadIdx.x] = 0;
-    // simultaneously write to the corresponding flag of all ranks.
-    // Latency = 1 p2p write
-    sg.signals[threadIdx.x]->end[blockIdx.x][rank] = 1;
-    // wait until we got true from all ranks
-    while (!self_sg->end[blockIdx.x][threadIdx.x]);
+    // Increment the counter. Technically we only need one counter, but we use
+    // multiple per block to eliminate the need to share the counter via smem.
+    auto val = self_sg->self_counter[blockIdx.x][threadIdx.x] += 1;
+    // Write the expected counter value to peer and wait for correct value from
+    // peer.
+    auto peer_counter_ptr =
+        &sg.signals[threadIdx.x]->peer_counter[val % 2][blockIdx.x][rank];
+    auto self_counter_ptr =
+        &self_sg->peer_counter[val % 2][blockIdx.x][threadIdx.x];
+    if constexpr (need_fence) {
+      st_flag_release(peer_counter_ptr, val);
+      while (ld_flag_acquire(self_counter_ptr) != val);
+    } else {
+      st_flag_volatile(peer_counter_ptr, val);
+      while (ld_flag_volatile(self_counter_ptr) != val);
+    }
   }
-  if constexpr (!final_sync) __syncthreads();
+  if constexpr (is_start || need_fence) __syncthreads();
 }
 
 template <typename P, int ngpus, typename A>
@@ -178,33 +198,31 @@ DINLINE P packed_reduce(const P* ptrs[], int idx) {
 
 template <typename T, int ngpus>
 __global__ void __launch_bounds__(512, 1)
-    cross_device_reduce_1stage(RankData* _dp, RankSignals sg,
-                               volatile Signal* self_sg, T* __restrict__ result,
-                               int rank, int size) {
+    cross_device_reduce_1stage(RankData* _dp, RankSignals sg, Signal* self_sg,
+                               T* __restrict__ result, int rank, int size) {
   using P = typename packed_t<T>::P;
   using A = typename packed_t<T>::A;
   // note: we don't reorder the address so the accumulation order is the same
   // for all ranks, ensuring bitwise identical results
   auto dp = *_dp;
-  start_sync<ngpus>(sg, self_sg, rank);
+  multi_gpu_barrier<ngpus, true>(sg, self_sg, rank);
   // do the actual reduction
   for (int idx = blockIdx.x * blockDim.x + threadIdx.x; idx < size;
        idx += gridDim.x * blockDim.x) {
     ((P*)result)[idx] = packed_reduce<P, ngpus, A>((const P**)&dp.ptrs[0], idx);
   }
-  end_sync<ngpus, true>(sg, self_sg, rank);
+  multi_gpu_barrier<ngpus, false>(sg, self_sg, rank);
 }
 
 template <typename P>
-DINLINE P* get_tmp_buf(volatile Signal* sg) {
+DINLINE P* get_tmp_buf(Signal* sg) {
   return (P*)(((Signal*)sg) + 1);
 }
 
 template <typename T, int ngpus>
 __global__ void __launch_bounds__(512, 1)
-    cross_device_reduce_2stage(RankData* _dp, RankSignals sg,
-                               volatile Signal* self_sg, T* __restrict__ result,
-                               int rank, int size) {
+    cross_device_reduce_2stage(RankData* _dp, RankSignals sg, Signal* self_sg,
+                               T* __restrict__ result, int rank, int size) {
   int tid = blockIdx.x * blockDim.x + threadIdx.x;
   int stride = gridDim.x * blockDim.x;
   using P = typename packed_t<T>::P;
@@ -222,12 +240,12 @@ __global__ void __launch_bounds__(512, 1)
     tmps[i] = get_tmp_buf<P>(sg.signals[target]);
   }
   auto tmp_out = tmps[0];
-  start_sync<ngpus>(sg, self_sg, rank);
+  multi_gpu_barrier<ngpus, true>(sg, self_sg, rank);
   // stage 1: reduce scatter
   for (int idx = start + tid; idx < end; idx += stride) {
     tmp_out[idx - start] = packed_reduce<P, ngpus, A>(ptrs, idx);
   }
-  end_sync<ngpus>(sg, self_sg, rank);
+  multi_gpu_barrier<ngpus, false, true>(sg, self_sg, rank);
 
   // stage 2: allgather. Note: it's important to match the tid between
   // the two stages, because visibility across devices is only guaranteed
@@ -437,6 +455,8 @@ class CustomAllreduce {
 #define KL(ngpus, name)                                                       \
   name<T, ngpus><<<blocks, threads, 0, stream>>>(ptrs, sg_, self_sg_, output, \
                                                  rank_, size);
+    // TODO(hanzhi713): Threshold is different for A100 and H100.
+    // Add per device threshold.
 #define REDUCE_CASE(ngpus)                            \
   case ngpus: {                                       \
     if (world_size_ == 2) {                           \

csrc/custom_all_reduce_test.cu

@@ -1,15 +1,15 @@
 /**
  * This is a standalone test for custom allreduce.
  * To compile, make sure you have MPI and NCCL installed in your system.
- * export MPI_HOME=XXX
+ * export MPI_HOME=xxx
  * nvcc -O2 -arch=native -std=c++17 custom_all_reduce_test.cu -o
- * custom_all_reduce_test -lnccl -I${MPI_HOME}/include -lmpi
+ * custom_all_reduce_test -lnccl -I${MPI_HOME} -lmpi
  *
  * Warning: this C++ test is not designed to be very readable and was used
  * during the rapid prototyping process.
  *
  * To run:
- * mpirun -np 8 ./custom_all_reduce_test
+ * mpirun --allow-run-as-root -np 8 ./custom_all_reduce_test
  */
 #include <cuda.h>
 #include <curand_kernel.h>
@@ -302,15 +302,19 @@ int main(int argc, char** argv) {
 
   bool performance_test = true;
   cudaProfilerStart();
-  // for (int threads : {256, 512}) {
+  // Uncomment to scan through different block size configs.
+  // for (int threads : {256, 512, 1024}) {
   //   for (int block_limit = 16; block_limit < 112; block_limit += 4) {
-  //     run<half>(myRank, nRanks, comm, threads, block_limit, 4096 * 1024);
+  //     run<half>(myRank, nRanks, comm, threads, block_limit, 1024 * 1024,
+  //     performance_test);
   //   }
   // }
+  // Scan through different sizes to test performance.
   for (int sz = 512; sz <= (8 << 20); sz *= 2) {
     run<half>(myRank, nRanks, comm, 512, 36, sz + 8 * 47, performance_test);
   }
 
   cudaProfilerStop();
+  MPICHECK(MPI_Finalize());
   return EXIT_SUCCESS;
 }