Allowing fast CPU sampling rates and supplying on-device CPU clock re…

…solution

Summary:
The clock tick from `sysconf(_SC_CLK_TCK)` is always 10ms which doesn't represent real jiffy interval configured for the kernel.  Cleaning up this limitation as redundant and getting the actual jiffy value from device logged as annotation `provider.stack_trace.cpu_timer_res_us`.
Turned out `CLOCK_REALTIME_COARSE` clock resolution matches kernel configuration frequency for jiffy (`CONFIG_HZ`).

Reviewed By: BurntBrunch

Differential Revision: D32772404

fbshipit-source-id: 1a11d995fcb081502c1af40f1c3385477e959fe5

cpp/profiler/jni.cpp

@@ -56,8 +56,8 @@ const char* StackTraceWhitelist =
 
 namespace {
 
-int32_t getSystemClockTickIntervalMs(facebook::jni::alias_ref<jobject>) {
-  return systemClockTickIntervalMs();
+int32_t getCpuClockResolutionMicros(facebook::jni::alias_ref<jobject>) {
+  return cpuClockResolutionMicros();
 }
 
 std::unordered_map<int32_t, std::shared_ptr<BaseTracer>> makeAvailableTracers(
@@ -197,8 +197,7 @@ JNIEXPORT jint JNI_OnLoad(JavaVM* vm, void*) {
         StackFrameThreadType,
         {
             makeNativeMethod(
-                "nativeSystemClockTickIntervalMs",
-                getSystemClockTickIntervalMs),
+                "nativeCpuClockResolutionMicros", getCpuClockResolutionMicros),
         });
     fbjni::registerNatives(
         StackTraceWhitelist,

cpp/util/common.cpp

@@ -20,6 +20,7 @@
 #include <fb/log.h>
 #include <pthread.h>
 #include <sys/stat.h>
+#include <time.h>
 #include <algorithm>
 #include <chrono>
 #include <sstream>
@@ -110,6 +111,17 @@ int32_t systemClockTickIntervalMs() {
 #error "Do not have systemClockTickIntervalMs implementation for this platform"
 #endif
 
+int32_t cpuClockResolutionMicros() {
+  timespec clock_res_timespec;
+  // It was empirically determined that this clock resolution is equal to actual
+  // size of kernel jiffy.
+  auto res = clock_getres(CLOCK_REALTIME_COARSE, &clock_res_timespec);
+  if (res != 0) {
+    return -1;
+  }
+  return clock_res_timespec.tv_nsec / 1000;
+}
+
 #if defined(ANDROID)
 #include <sys/system_properties.h>
 

cpp/util/common.h

@@ -26,9 +26,16 @@ namespace facebook {
 namespace profilo {
 
 int64_t monotonicTime();
+
 int32_t threadID();
+
 // Returns 0 if value was not found, and 1 if value <= 1, actual value otherwise
 int32_t systemClockTickIntervalMs();
+
+// Determines the kernel jiffy value and returns it's value in microseconds.
+// Returns -1 if unable to determine the actual value.
+int32_t cpuClockResolutionMicros();
+
 std::string get_system_property(const char* key);
 
 // Given a path, create the directory specified by it, along with all

java/main/com/facebook/profilo/provider/stacktrace/StackFrameThread.java

@@ -145,12 +145,6 @@ private synchronized boolean enableInternal(
           cpuClockModeEnabled = true;
           break;
       }
-      if (cpuClockModeEnabled) {
-        if (mSystemClockTimeIntervalMs == -1) {
-          mSystemClockTimeIntervalMs = nativeSystemClockTickIntervalMs();
-        }
-        sampleRateMs = Math.max(sampleRateMs, mSystemClockTimeIntervalMs);
-      }
     }
     // For now, we'll just keep an eye on the main thread. Eventually we
     // might want to pass a list of all the interesting threads.
@@ -255,6 +249,11 @@ protected void onTraceEnded(TraceContext context, ExtraDataFileProvider dataFile
           Integer.toBinaryString(
               providersToTracers(context.enabledProviders) & CPUProfiler.getAvailableTracers()));
     }
+    if ((context.enabledProviders & getSupportedProviders()) != 0) {
+      logAnnotation(
+          "provider.stack_trace.cpu_timer_res_us",
+          Integer.toString(nativeCpuClockResolutionMicros()));
+    }
   }
 
   private void logAnnotation(String key, String value) {
@@ -320,5 +319,5 @@ protected void onTraceStarted(TraceContext context, ExtraDataFileProvider dataFi
   }
 
   @DoNotStrip
-  private static native int nativeSystemClockTickIntervalMs();
+  private static native int nativeCpuClockResolutionMicros();
 }