port: updated PhysicalCoreID()

Summary:
Updated PhysicalCoreID() to use sched_getcpu() on x86_64 for glibc >= 2.22.  Added a new
function named GetCPUID() that calls sched_getcpu(), to avoid repeated code. This change is done as per the comments of PR: https://github.com/facebook/rocksdb/pull/2230

Signed-off-by: Jos Collin <jcollin@redhat.com>
Closes https://github.com/facebook/rocksdb/pull/2260

Differential Revision: D5025734

Pulled By: ajkr

fbshipit-source-id: f4cca68c12573cafcf8531e7411a1e733bbf8eef

port/port_posix.cc

@@ -137,23 +137,28 @@ void RWMutex::ReadUnlock() { PthreadCall("read unlock", pthread_rwlock_unlock(&m
 
 void RWMutex::WriteUnlock() { PthreadCall("write unlock", pthread_rwlock_unlock(&mu_)); }
 
+int GetCPUID() {
+  int cpuno = sched_getcpu();
+  if (cpuno < 0) {
+    return -1;
+  }
+  else {
+    return cpuno;
+  }
+}
+
 int PhysicalCoreID() {
 #if defined(__i386__) || defined(__x86_64__)
-  // if you ever find that this function is hot on Linux, you can go from
-  // ~200 nanos to ~20 nanos by adding the machinery to use __vdso_getcpu
+#if defined(__x86_64__) && (__GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 22))
+  return GetCPUID();
+#endif
   unsigned eax, ebx = 0, ecx, edx;
   if (!__get_cpuid(1, &eax, &ebx, &ecx, &edx)) {
     return -1;
   }
   return ebx >> 24;
 #else
-  int cpuno = sched_getcpu();
-  if (cpuno < 0) {
-    return -1;
-  }
-  else {
-    return cpuno;
-  }
+  return GetCPUID();
 #endif
 }