[libc] Use Atomics in GPU Benchmarks (#98842)

This PR replaces our old method of reducing the benchmark results by
using an array to using atomics instead. This should help us implement
single threaded benchmarks.

Author: jameshu15869

libc/benchmarks/gpu/CMakeLists.txt

@@ -43,6 +43,7 @@ add_unittest_framework_library(
     libc.src.__support.CPP.functional
     libc.src.__support.CPP.limits
     libc.src.__support.CPP.algorithm
+    libc.src.__support.CPP.atomic
     libc.src.__support.fixed_point.fx_rep
     libc.src.__support.macros.properties.types
     libc.src.__support.OSUtil.osutil

libc/benchmarks/gpu/LibcGpuBenchmark.cpp

@@ -1,6 +1,7 @@
 #include "LibcGpuBenchmark.h"
 #include "src/__support/CPP/algorithm.h"
 #include "src/__support/CPP/array.h"
+#include "src/__support/CPP/atomic.h"
 #include "src/__support/CPP/string.h"
 #include "src/__support/FPUtil/sqrt.h"
 #include "src/__support/GPU/utils.h"
@@ -12,61 +13,113 @@ namespace LIBC_NAMESPACE_DECL {
 namespace benchmarks {
 
 FixedVector<Benchmark *, 64> benchmarks;
-cpp::array<BenchmarkResult, 1024> results;
 
 void Benchmark::add_benchmark(Benchmark *benchmark) {
   benchmarks.push_back(benchmark);
 }
 
-BenchmarkResult
-reduce_results(const cpp::array<BenchmarkResult, 1024> &results) {
-  BenchmarkResult result;
-  uint64_t cycles_sum = 0;
-  double standard_deviation_sum = 0;
-  uint64_t min = UINT64_MAX;
-  uint64_t max = 0;
-  uint32_t samples_sum = 0;
-  uint32_t iterations_sum = 0;
-  clock_t time_sum = 0;
-  uint64_t num_threads = gpu::get_num_threads();
-  for (uint64_t i = 0; i < num_threads; i++) {
-    BenchmarkResult current_result = results[i];
-    cycles_sum += current_result.cycles;
-    standard_deviation_sum += current_result.standard_deviation;
-    min = cpp::min(min, current_result.min);
-    max = cpp::max(max, current_result.max);
-    samples_sum += current_result.samples;
-    iterations_sum += current_result.total_iterations;
-    time_sum += current_result.total_time;
+struct AtomicBenchmarkSums {
+  cpp::Atomic<uint64_t> cycles_sum = 0;
+  cpp::Atomic<uint64_t> standard_deviation_sum = 0;
+  cpp::Atomic<uint64_t> min = UINT64_MAX;
+  cpp::Atomic<uint64_t> max = 0;
+  cpp::Atomic<uint32_t> samples_sum = 0;
+  cpp::Atomic<uint32_t> iterations_sum = 0;
+  cpp::Atomic<clock_t> time_sum = 0;
+  cpp::Atomic<uint64_t> active_threads = 0;
+
+  void reset() {
+    cpp::atomic_thread_fence(cpp::MemoryOrder::RELEASE);
+    active_threads.store(0, cpp::MemoryOrder::RELAXED);
+    cycles_sum.store(0, cpp::MemoryOrder::RELAXED);
+    standard_deviation_sum.store(0, cpp::MemoryOrder::RELAXED);
+    min.store(UINT64_MAX, cpp::MemoryOrder::RELAXED);
+    max.store(0, cpp::MemoryOrder::RELAXED);
+    samples_sum.store(0, cpp::MemoryOrder::RELAXED);
+    iterations_sum.store(0, cpp::MemoryOrder::RELAXED);
+    time_sum.store(0, cpp::MemoryOrder::RELAXED);
+    cpp::atomic_thread_fence(cpp::MemoryOrder::RELEASE);
   }
-  result.cycles = cycles_sum / num_threads;
-  result.standard_deviation = standard_deviation_sum / num_threads;
-  result.min = min;
-  result.max = max;
-  result.samples = samples_sum / num_threads;
-  result.total_iterations = iterations_sum / num_threads;
-  result.total_time = time_sum / num_threads;
-  return result;
+
+  void update(const BenchmarkResult &result) {
+    cpp::atomic_thread_fence(cpp::MemoryOrder::RELEASE);
+    active_threads.fetch_add(1, cpp::MemoryOrder::RELAXED);
+
+    cycles_sum.fetch_add(result.cycles, cpp::MemoryOrder::RELAXED);
+    standard_deviation_sum.fetch_add(
+        static_cast<uint64_t>(result.standard_deviation),
+        cpp::MemoryOrder::RELAXED);
+
+    // Perform a CAS loop to atomically update the min
+    uint64_t orig_min = min.load(cpp::MemoryOrder::RELAXED);
+    while (!min.compare_exchange_strong(
+        orig_min, cpp::min(orig_min, result.min), cpp::MemoryOrder::ACQUIRE,
+        cpp::MemoryOrder::RELAXED))
+      ;
+
+    // Perform a CAS loop to atomically update the max
+    uint64_t orig_max = max.load(cpp::MemoryOrder::RELAXED);
+    while (!max.compare_exchange_strong(
+        orig_max, cpp::max(orig_max, result.max), cpp::MemoryOrder::ACQUIRE,
+        cpp::MemoryOrder::RELAXED))
+      ;
+
+    samples_sum.fetch_add(result.samples, cpp::MemoryOrder::RELAXED);
+    iterations_sum.fetch_add(result.total_iterations,
+                             cpp::MemoryOrder::RELAXED);
+    time_sum.fetch_add(result.total_time, cpp::MemoryOrder::RELAXED);
+    cpp::atomic_thread_fence(cpp::MemoryOrder::RELEASE);
+  }
+};
+
+AtomicBenchmarkSums all_results;
+
+void print_results(Benchmark *b) {
+  constexpr auto GREEN = "\033[32m";
+  constexpr auto RESET = "\033[0m";
+
+  BenchmarkResult result;
+  cpp::atomic_thread_fence(cpp::MemoryOrder::RELEASE);
+  int num_threads = all_results.active_threads.load(cpp::MemoryOrder::RELAXED);
+  result.cycles =
+      all_results.cycles_sum.load(cpp::MemoryOrder::RELAXED) / num_threads;
+  result.standard_deviation =
+      all_results.standard_deviation_sum.load(cpp::MemoryOrder::RELAXED) /
+      num_threads;
+  result.min = all_results.min.load(cpp::MemoryOrder::RELAXED);
+  result.max = all_results.max.load(cpp::MemoryOrder::RELAXED);
+  result.samples =
+      all_results.samples_sum.load(cpp::MemoryOrder::RELAXED) / num_threads;
+  result.total_iterations =
+      all_results.iterations_sum.load(cpp::MemoryOrder::RELAXED) / num_threads;
+  result.total_time =
+      all_results.time_sum.load(cpp::MemoryOrder::RELAXED) / num_threads;
+  cpp::atomic_thread_fence(cpp::MemoryOrder::RELEASE);
+
+  log << GREEN << "[ RUN      ] " << RESET << b->get_name() << '\n';
+  log << GREEN << "[       OK ] " << RESET << b->get_name() << ": "
+      << result.cycles << " cycles, " << result.min << " min, " << result.max
+      << " max, " << result.total_iterations << " iterations, "
+      << result.total_time << " ns, "
+      << static_cast<uint64_t>(result.standard_deviation)
+      << " stddev (num threads: " << num_threads << ")\n";
 }
 
 void Benchmark::run_benchmarks() {
   uint64_t id = gpu::get_thread_id();
   gpu::sync_threads();
 
   for (Benchmark *b : benchmarks) {
-    results[id] = b->run();
+    if (id == 0)
+      all_results.reset();
+
     gpu::sync_threads();
-    if (id == 0) {
-      BenchmarkResult all_results = reduce_results(results);
-      constexpr auto GREEN = "\033[32m";
-      constexpr auto RESET = "\033[0m";
-      log << GREEN << "[ RUN      ] " << RESET << b->get_name() << '\n';
-      log << GREEN << "[       OK ] " << RESET << b->get_name() << ": "
-          << all_results.cycles << " cycles, " << all_results.min << " min, "
-          << all_results.max << " max, " << all_results.total_iterations
-          << " iterations, " << all_results.total_time << " ns, "
-          << static_cast<long>(all_results.standard_deviation) << " stddev\n";
-    }
+    auto current_result = b->run();
+    all_results.update(current_result);
+    gpu::sync_threads();
+
+    if (id == 0)
+      print_results(b);
   }
   gpu::sync_threads();
 }

libc/benchmarks/gpu/LibcGpuBenchmark.h

@@ -88,6 +88,7 @@ class Benchmark {
   }
 
   static void run_benchmarks();
+  const cpp::string_view get_name() const { return name; }
 
 protected:
   static void add_benchmark(Benchmark *benchmark);
@@ -97,13 +98,12 @@ class Benchmark {
     BenchmarkOptions options;
     return benchmark(options, func);
   }
-  const cpp::string_view get_name() const { return name; }
 };
 } // namespace benchmarks
 } // namespace LIBC_NAMESPACE_DECL
 
 #define BENCHMARK(SuiteName, TestName, Func)                                   \
   LIBC_NAMESPACE::benchmarks::Benchmark SuiteName##_##TestName##_Instance(     \
-      Func, #SuiteName "." #TestName);
+      Func, #SuiteName "." #TestName)
 
 #endif