[llvm-exegesis] Error Out If Perf Counter is Not Fully Enabled (#132892)

Perf counters can be multiplexed if there are too many that need to be
scheduled on a core at the same time (and they exceed the available
PMUs). Other processes (especially system ones in certain environments,
not commonly on Desktop Linux from what I've seen) can also interfere.
This will impact the measurement fidelity as the counter is not actually
counting cycles/uops the entire time. This patch makes it so that we
error out in these cases so the user gets a visible indication things
have gone wrong rather than things failing silently.

Author: boomanaiden154

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

@@ -49,5 +49,15 @@ void SnippetSignal::log(raw_ostream &OS) const {
 #endif // LLVM_ON_UNIX
 }
 
+char PerfCounterNotFullyEnabled::ID;
+
+std::error_code PerfCounterNotFullyEnabled::convertToErrorCode() const {
+  return inconvertibleErrorCode();
+}
+
+void PerfCounterNotFullyEnabled::log(raw_ostream &OS) const {
+  OS << "The perf counter was not scheduled on the CPU the entire time.";
+}
+
 } // namespace exegesis
 } // namespace llvm

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

@@ -76,6 +76,18 @@ class SnippetSignal : public SnippetExecutionFailure {
   int SignalNumber;
 };
 
+// A class representing a case where a perf counter was only partially
+// scheduled, most likely due to perf counter contention.
+struct PerfCounterNotFullyEnabled
+    : public ErrorInfo<PerfCounterNotFullyEnabled> {
+  static char ID;
+  PerfCounterNotFullyEnabled() {}
+
+  void log(raw_ostream &OS) const override;
+
+  std::error_code convertToErrorCode() const override;
+};
+
 } // namespace exegesis
 } // namespace llvm
 

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

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "PerfHelper.h"
+#include "Error.h"
 #include "llvm/Config/config.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/Error.h"
@@ -117,6 +118,8 @@ void ConfiguredEvent::initRealEvent(const pid_t ProcessID, const int GroupFD) {
   const int CPU = -1;
   const uint32_t Flags = 0;
   perf_event_attr AttrCopy = *Event.attribute();
+  AttrCopy.read_format =
+      PERF_FORMAT_TOTAL_TIME_ENABLED | PERF_FORMAT_TOTAL_TIME_RUNNING;
   FileDescriptor = perf_event_open(&AttrCopy, ProcessID, CPU, GroupFD, Flags);
   if (FileDescriptor == -1) {
     errs() << "Unable to open event. ERRNO: " << strerror(errno)
@@ -132,15 +135,20 @@ void ConfiguredEvent::initRealEvent(const pid_t ProcessID, const int GroupFD) {
 
 Expected<SmallVector<int64_t>>
 ConfiguredEvent::readOrError(StringRef /*unused*/) const {
-  int64_t Count = 0;
-  ssize_t ReadSize = ::read(FileDescriptor, &Count, sizeof(Count));
+  int64_t EventInfo[3] = {0, 0, 0};
+  ssize_t ReadSize = ::read(FileDescriptor, &EventInfo, sizeof(EventInfo));
 
-  if (ReadSize != sizeof(Count))
+  if (ReadSize != sizeof(EventInfo))
     return make_error<StringError>("Failed to read event counter",
                                    errc::io_error);
 
+  int64_t EventTimeEnabled = EventInfo[1];
+  int64_t EventTimeRunning = EventInfo[2];
+  if (EventTimeEnabled != EventTimeRunning)
+    return make_error<PerfCounterNotFullyEnabled>();
+
   SmallVector<int64_t, 1> Result;
-  Result.push_back(Count);
+  Result.push_back(EventInfo[0]);
   return Result;
 }