Bug 1883522 - When running with MOZ_USE_PERFORMANCE_MARKER_FILE=1, mmap the marker file to help out perf + simpleperf + samply. r=jrmuizel

Differential Revision: https://phabricator.services.mozilla.com/D203536

dom/performance/Performance.cpp

@@ -8,6 +8,11 @@
 
 #include <sstream>
 
+#if defined(XP_LINUX)
+#  include <fcntl.h>
+#  include <sys/mman.h>
+#endif
+
 #include "ETWTools.h"
 #include "GeckoProfiler.h"
 #include "nsRFPService.h"
@@ -593,14 +598,61 @@ std::pair<TimeStamp, TimeStamp> Performance::GetTimeStampsForMarker(
   return std::make_pair(startTimeStamp, endTimeStamp);
 }
 
+static FILE* MaybeOpenMarkerFile() {
+  if (!getenv("MOZ_USE_PERFORMANCE_MARKER_FILE")) {
+    return nullptr;
+  }
+
+#ifdef XP_LINUX
+  // We treat marker files similar to Jitdump files (see PerfSpewer.cpp) and
+  // mmap them if needed.
+  int fd = open(GetMarkerFilename().c_str(), O_CREAT | O_TRUNC | O_RDWR, 0666);
+  FILE* markerFile = fdopen(fd, "w+");
+
+  if (!markerFile) {
+    return nullptr;
+  }
+
+  // On Linux and Android, we need to mmap the file so that the path makes it
+  // into the perf.data file or into samply.
+  // On non-Android, make the mapping executable, otherwise the MMAP event may
+  // not be recorded by perf (see perf_event_open mmap_data).
+  // But on Android, don't make the mapping executable, because doing so can
+  // make the mmap call fail on some Android devices. It's also not required on
+  // Android because simpleperf sets mmap_data = 1 for unrelated reasons (it
+  // wants to know about vdex files for Java JIT profiling, see
+  // SetRecordNotExecutableMaps).
+  int protection = PROT_READ;
+#  ifndef ANDROID
+  protection |= PROT_EXEC;
+#  endif
+
+  // Mmap just the first page - that's enough to ensure the path makes it into
+  // the recording.
+  long page_size = sysconf(_SC_PAGESIZE);
+  void* mmap_address = mmap(nullptr, page_size, protection, MAP_PRIVATE, fd, 0);
+  if (mmap_address == MAP_FAILED) {
+    fclose(markerFile);
+    return nullptr;
+  }
+  return markerFile;
+#else
+  // On macOS, we just need to `open` or `fopen` the marker file, and samply
+  // will know its path because it hooks those functions - no mmap needed.
+  // On Windows, there's no need to use MOZ_USE_PERFORMANCE_MARKER_FILE because
+  // we have ETW trace events for UserTiming measures. Still, we want this code
+  // to compile successfully on Windows, so we use fopen rather than
+  // open+fdopen.
+  return fopen(GetMarkerFilename().c_str(), "w+");
+#endif
+}
+
 // This emits markers to an external marker-[pid].txt file for use by an
 // external profiler like samply or etw-gecko
 void Performance::MaybeEmitExternalProfilerMarker(
     const nsAString& aName, Maybe<const PerformanceMeasureOptions&> aOptions,
     Maybe<const nsAString&> aStartMark, const Optional<nsAString>& aEndMark) {
-  static FILE* markerFile = getenv("MOZ_USE_PERFORMANCE_MARKER_FILE")
-                                ? fopen(GetMarkerFilename().c_str(), "w+")
-                                : nullptr;
+  static FILE* markerFile = MaybeOpenMarkerFile();
   if (!markerFile) {
     return;
   }