[llvm-exegesis] Add support for pinning benchmarking process to a CPU (…

…#85168)

This patch adds in support for pinning a benchmarking process to a
specific CPU (in the subprocess benchmarking mode on Linux). This is
intended to be used in environments where a certain set of CPUs is
isolated from the scheduler using something like cgroups and thus should
present less potential for noise than normal. This also opens up the
door for doing multithreaded benchmarking as we can now pin benchmarking
processes to specific CPUs that we know won't interfere with each other.

llvm/test/tools/llvm-exegesis/X86/latency/cpu-pinning-execution-mode.s

@@ -0,0 +1,5 @@
+# REQUIRES: exegesis-can-measure-latency, x86_64-linux
+
+# RUN: not llvm-exegesis -mtriple=x86_64-unknown-unknown -mode=latency -opcode-name=ADD64rr -execution-mode=inprocess --benchmark-process-cpu=0 2>&1 | FileCheck %s
+
+# CHECK: llvm-exegesis error: The inprocess execution mode does not support benchmark core pinning.

llvm/test/tools/llvm-exegesis/X86/latency/cpu-pinning.s

@@ -0,0 +1,5 @@
+# REQUIRES: exegesis-can-measure-latency, x86_64-linux
+
+# RUN: llvm-exegesis -mtriple=x86_64-unknown-unknown -mode=latency -opcode-name=ADD64rr -execution-mode=subprocess | FileCheck %s
+
+# CHECK: - { key: latency, value: {{[0-9.]*}}, per_snippet_value: {{[0-9.]*}}

llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp

@@ -98,7 +98,8 @@ class InProcessFunctionExecutorImpl : public BenchmarkRunner::FunctionExecutor {
 public:
   static Expected<std::unique_ptr<InProcessFunctionExecutorImpl>>
   create(const LLVMState &State, object::OwningBinary<object::ObjectFile> Obj,
-         BenchmarkRunner::ScratchSpace *Scratch) {
+         BenchmarkRunner::ScratchSpace *Scratch,
+         std::optional<int> BenchmarkProcessCPU) {
     Expected<ExecutableFunction> EF =
         ExecutableFunction::create(State.createTargetMachine(), std::move(Obj));
 
@@ -190,27 +191,31 @@ class SubProcessFunctionExecutorImpl
 public:
   static Expected<std::unique_ptr<SubProcessFunctionExecutorImpl>>
   create(const LLVMState &State, object::OwningBinary<object::ObjectFile> Obj,
-         const BenchmarkKey &Key) {
+         const BenchmarkKey &Key, std::optional<int> BenchmarkProcessCPU) {
     Expected<ExecutableFunction> EF =
         ExecutableFunction::create(State.createTargetMachine(), std::move(Obj));
     if (!EF)
       return EF.takeError();
 
     return std::unique_ptr<SubProcessFunctionExecutorImpl>(
-        new SubProcessFunctionExecutorImpl(State, std::move(*EF), Key));
+        new SubProcessFunctionExecutorImpl(State, std::move(*EF), Key,
+                                           BenchmarkProcessCPU));
   }
 
 private:
   SubProcessFunctionExecutorImpl(const LLVMState &State,
                                  ExecutableFunction Function,
-                                 const BenchmarkKey &Key)
-      : State(State), Function(std::move(Function)), Key(Key) {}
+                                 const BenchmarkKey &Key,
+                                 std::optional<int> BenchmarkCPU)
+      : State(State), Function(std::move(Function)), Key(Key),
+        BenchmarkProcessCPU(BenchmarkCPU) {}
 
   enum ChildProcessExitCodeE {
     CounterFDReadFailed = 1,
     RSeqDisableFailed,
     FunctionDataMappingFailed,
-    AuxiliaryMemorySetupFailed
+    AuxiliaryMemorySetupFailed,
+    SetCPUAffinityFailed
   };
 
   StringRef childProcessExitCodeToString(int ExitCode) const {
@@ -223,6 +228,8 @@ class SubProcessFunctionExecutorImpl
       return "Failed to map memory for assembled snippet";
     case ChildProcessExitCodeE::AuxiliaryMemorySetupFailed:
       return "Failed to setup auxiliary memory";
+    case ChildProcessExitCodeE::SetCPUAffinityFailed:
+      return "Failed to set CPU affinity of the benchmarking process";
     default:
       return "Child process returned with unknown exit code";
     }
@@ -384,6 +391,29 @@ class SubProcessFunctionExecutorImpl
     return make_error<SnippetSignal>(ChildSignalInfo.si_signo);
   }
 
+  static void setCPUAffinityIfRequested(int CPUToUse) {
+    // Set the CPU affinity for the child process, so that we ensure that if
+    // the user specified a CPU the process should run on, the benchmarking
+    // process is running on that CPU.
+    cpu_set_t CPUMask;
+    CPU_ZERO(&CPUMask);
+    CPU_SET(CPUToUse, &CPUMask);
+    // TODO(boomanaiden154): Rewrite this to use LLVM primitives once they
+    // are available.
+    int SetAffinityReturn = sched_setaffinity(0, sizeof(CPUMask), &CPUMask);
+    if (SetAffinityReturn == -1) {
+      exit(ChildProcessExitCodeE::SetCPUAffinityFailed);
+    }
+
+    // Check (if assertions are enabled) that we are actually running on the
+    // CPU that was specified by the user.
+    unsigned int CurrentCPU;
+    assert(getcpu(&CurrentCPU, nullptr) == 0 &&
+           "Expected getcpu call to succeed.");
+    assert(static_cast<int>(CurrentCPU) == CPUToUse &&
+           "Expected current CPU to equal the CPU requested by the user");
+  }
+
   Error createSubProcessAndRunBenchmark(
       StringRef CounterName, SmallVectorImpl<int64_t> &CounterValues,
       ArrayRef<const char *> ValidationCounters,
@@ -416,6 +446,10 @@ class SubProcessFunctionExecutorImpl
     }
 
     if (ParentOrChildPID == 0) {
+      if (BenchmarkProcessCPU.has_value()) {
+        setCPUAffinityIfRequested(*BenchmarkProcessCPU);
+      }
+
       // We are in the child process, close the write end of the pipe.
       close(PipeFiles[1]);
       // Unregister handlers, signal handling is now handled through ptrace in
@@ -538,6 +572,7 @@ class SubProcessFunctionExecutorImpl
   const LLVMState &State;
   const ExecutableFunction Function;
   const BenchmarkKey &Key;
+  const std::optional<int> BenchmarkProcessCPU;
 };
 #endif // __linux__
 } // namespace
@@ -615,11 +650,15 @@ BenchmarkRunner::getRunnableConfiguration(
 Expected<std::unique_ptr<BenchmarkRunner::FunctionExecutor>>
 BenchmarkRunner::createFunctionExecutor(
     object::OwningBinary<object::ObjectFile> ObjectFile,
-    const BenchmarkKey &Key) const {
+    const BenchmarkKey &Key, std::optional<int> BenchmarkProcessCPU) const {
   switch (ExecutionMode) {
   case ExecutionModeE::InProcess: {
+    if (BenchmarkProcessCPU.has_value())
+      return make_error<Failure>("The inprocess execution mode does not "
+                                 "support benchmark core pinning.");
+
     auto InProcessExecutorOrErr = InProcessFunctionExecutorImpl::create(
-        State, std::move(ObjectFile), Scratch.get());
+        State, std::move(ObjectFile), Scratch.get(), BenchmarkProcessCPU);
     if (!InProcessExecutorOrErr)
       return InProcessExecutorOrErr.takeError();
 
@@ -628,7 +667,7 @@ BenchmarkRunner::createFunctionExecutor(
   case ExecutionModeE::SubProcess: {
 #ifdef __linux__
     auto SubProcessExecutorOrErr = SubProcessFunctionExecutorImpl::create(
-        State, std::move(ObjectFile), Key);
+        State, std::move(ObjectFile), Key, BenchmarkProcessCPU);
     if (!SubProcessExecutorOrErr)
       return SubProcessExecutorOrErr.takeError();
 
@@ -643,8 +682,8 @@ BenchmarkRunner::createFunctionExecutor(
 }
 
 std::pair<Error, Benchmark> BenchmarkRunner::runConfiguration(
-    RunnableConfiguration &&RC,
-    const std::optional<StringRef> &DumpFile) const {
+    RunnableConfiguration &&RC, const std::optional<StringRef> &DumpFile,
+    std::optional<int> BenchmarkProcessCPU) const {
   Benchmark &BenchmarkResult = RC.BenchmarkResult;
   object::OwningBinary<object::ObjectFile> &ObjectFile = RC.ObjectFile;
 
@@ -665,7 +704,8 @@ std::pair<Error, Benchmark> BenchmarkRunner::runConfiguration(
   }
 
   Expected<std::unique_ptr<BenchmarkRunner::FunctionExecutor>> Executor =
-      createFunctionExecutor(std::move(ObjectFile), RC.BenchmarkResult.Key);
+      createFunctionExecutor(std::move(ObjectFile), RC.BenchmarkResult.Key,
+                             BenchmarkProcessCPU);
   if (!Executor)
     return {Executor.takeError(), std::move(BenchmarkResult)};
   auto NewMeasurements = runMeasurements(**Executor);

llvm/tools/llvm-exegesis/lib/BenchmarkRunner.h

@@ -68,7 +68,8 @@ class BenchmarkRunner {
 
   std::pair<Error, Benchmark>
   runConfiguration(RunnableConfiguration &&RC,
-                   const std::optional<StringRef> &DumpFile) const;
+                   const std::optional<StringRef> &DumpFile,
+                   std::optional<int> BenchmarkProcessCPU) const;
 
   // Scratch space to run instructions that touch memory.
   struct ScratchSpace {
@@ -135,7 +136,8 @@ class BenchmarkRunner {
 
   Expected<std::unique_ptr<FunctionExecutor>>
   createFunctionExecutor(object::OwningBinary<object::ObjectFile> Obj,
-                         const BenchmarkKey &Key) const;
+                         const BenchmarkKey &Key,
+                         std::optional<int> BenchmarkProcessCPU) const;
 };
 
 } // namespace exegesis

llvm/tools/llvm-exegesis/llvm-exegesis.cpp

@@ -269,6 +269,11 @@ static cl::list<ValidationEvent> ValidationCounters(
         "counter to validate benchmarking assumptions"),
     cl::CommaSeparated, cl::cat(BenchmarkOptions), ValidationEventOptions());
 
+static cl::opt<int> BenchmarkProcessCPU(
+    "benchmark-process-cpu",
+    cl::desc("The CPU number that the benchmarking process should executon on"),
+    cl::cat(BenchmarkOptions), cl::init(-1));
+
 static ExitOnError ExitOnErr("llvm-exegesis error: ");
 
 // Helper function that logs the error(s) and exits.
@@ -418,8 +423,12 @@ static void runBenchmarkConfigurations(
         std::optional<StringRef> DumpFile;
         if (DumpObjectToDisk.getNumOccurrences())
           DumpFile = DumpObjectToDisk;
+        const std::optional<int> BenchmarkCPU =
+            BenchmarkProcessCPU == -1
+                ? std::nullopt
+                : std::optional(BenchmarkProcessCPU.getValue());
         auto [Err, BenchmarkResult] =
-            Runner.runConfiguration(std::move(RC), DumpFile);
+            Runner.runConfiguration(std::move(RC), DumpFile, BenchmarkCPU);
         if (Err) {
           // Errors from executing the snippets are fine.
           // All other errors are a framework issue and should fail.