Use CLOCK_REALTIME_COARSE when available.

On systems that have CLOCK_REALTIME_COARSE with good enough resolution,
we can avoid making a system call to get the current time; it's serviced
from the vDSO.

This is v2 of the patch.  v1 can be found at [0] but was reverted in [1]
because of Chromium sandbox restrictions.  The necessary changes have
been applied upstream in [2].

[0] https://codereview.chromium.org/1125003002
[1] https://codereview.chromium.org/1130083003
[2] https://codereview.chromium.org/1133653002

BUG=
LOG=N

Review URL: https://codereview.chromium.org/1151283005

Cr-Commit-Position: refs/heads/master@{#28639}

src/base/platform/time.cc

@@ -20,10 +20,16 @@
 #include "src/base/lazy-instance.h"
 #include "src/base/win32-headers.h"
 #endif
+#include "src/base/atomicops.h"
 #include "src/base/cpu.h"
 #include "src/base/logging.h"
 #include "src/base/platform/platform.h"
 
+// CLOCK_REALTIME_COARSE was added in Linux 2.6.32.
+#if V8_OS_LINUX && !defined(CLOCK_REALTIME_COARSE)
+#define CLOCK_REALTIME_COARSE 5
+#endif
+
 namespace v8 {
 namespace base {
 
@@ -252,11 +258,40 @@ FILETIME Time::ToFiletime() const {
 #elif V8_OS_POSIX
 
 Time Time::Now() {
+#ifdef CLOCK_REALTIME_COARSE
+  struct timespec ts;
+  static const clockid_t kInvalidClockId = static_cast<clockid_t>(-1);
+  static clockid_t cached_clock_id = kInvalidClockId;
+  clockid_t clock_id = NoBarrier_Load(&cached_clock_id);
+  if (V8_UNLIKELY(clock_id == kInvalidClockId)) {
+    // Use CLOCK_REALTIME_COARSE when available (linux >= 2.6.32) and precise
+    // enough.  Unlike CLOCK_REALTIME, its coarse cousin is normally serviced
+    // from the vDSO, without making an actual system call.
+    //
+    // CLOCK_REALTIME_COARSE precision is determined by CONFIG_HZ, the number
+    // of ticks per second that the kernel runs at.  Granularity can be as low
+    // as one update every few hundred milliseconds so we need to ensure it is
+    // not _too_ coarse.  Most kernels are built with CONFIG_HZ=1000, providing
+    // a one millisecond precision that is good enough for our purposes.
+    if (clock_getres(CLOCK_REALTIME_COARSE, &ts) < 0 || ts.tv_sec > 0 ||
+        ts.tv_nsec > 1000 * 1000) {
+      clock_id = CLOCK_REALTIME;  // Not available or not suitable.
+    } else {
+      clock_id = CLOCK_REALTIME_COARSE;
+    }
+    NoBarrier_Store(&cached_clock_id, clock_id);
+  }
+  int result = clock_gettime(clock_id, &ts);
+  DCHECK_EQ(0, result);
+  USE(result);
+  return FromTimespec(ts);
+#else
   struct timeval tv;
   int result = gettimeofday(&tv, NULL);
   DCHECK_EQ(0, result);
   USE(result);
   return FromTimeval(tv);
+#endif
 }