Add interface to system trace clock.

Add TimeTicks::NowFromSystemTraceTime to return the current system trace
event clock (falling back to TimeTicks::HighResNow when not supported).
On systems where the system trace clock is available this allows
system and chrome events to be stamped with a common clock (eliminating
post-processing time shift adjustment that can be hard).

BUG=chromium-os:27809
TEST=unit tests + collect trace data on chrome os w/ kernel that supports clock_gettime(CLOCK_SYSTEM_TRACE)

Change-Id: Ia6a500bd13a6f09c49052a1dd1421027960d8dea


Review URL: http://codereview.chromium.org/10257020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@134772 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 28 insertions, 0 deletions

diff --git a/base/time_posix.cc b/base/time_posix.cc
index 829c000..b8f1385 100644
--- a/base/time_posix.cc
+++ b/base/time_posix.cc
@@ -228,6 +228,34 @@ TimeTicks TimeTicks::HighResNow() {
   return Now();
 }
 
+#if defined(OS_POSIX) && defined(CLOCK_SYSTEM_TRACE)
+
+// static
+TimeTicks TimeTicks::NowFromSystemTraceTime() {
+  uint64_t absolute_micro;
+
+  struct timespec ts;
+  if (clock_gettime(CLOCK_SYSTEM_TRACE, &ts) != 0) {
+    NOTREACHED() << "clock_gettime(CLOCK_SYSTEM_TRACE) failed.";
+    return HighResNow();
+  }
+
+  absolute_micro =
+      (static_cast<int64>(ts.tv_sec) * Time::kMicrosecondsPerSecond) +
+      (static_cast<int64>(ts.tv_nsec) / Time::kNanosecondsPerMicrosecond);
+
+  return TimeTicks(absolute_micro);
+}
+
+#else // !(defined(OS_POSIX) && defined(CLOCK_SYSTEM_TRACE))
+
+// static
+TimeTicks TimeTicks::NowFromSystemTraceTime() {
+  return HighResNow();
+}
+
+#endif // defined(OS_POSIX) && defined(CLOCK_SYSTEM_TRACE)
+
 #endif  // !OS_MACOSX
 
 struct timeval Time::ToTimeVal() const {