Prevent after-fork number of OMP threads being bigger than 1.

This could happen if it was set in the environment. As we are setting engine::OpenMP::Get()->set_enabled(false) in initialize.cc in the child after forking, the behaviour goes back to what it was before apache#15762 was introduced.

Regions using omp get the threads count from GetRecommendedOMPThreadCount, so if omp is disabled they will get 1 thread and run serially

src/engine/openmp.cc

@@ -83,10 +83,10 @@ void OpenMP::set_reserve_cores(int cores) {
 
 int OpenMP::GetRecommendedOMPThreadCount(bool exclude_reserved) const {
 #ifdef _OPENMP
-  if (omp_num_threads_set_in_environment_) {
-    return omp_get_max_threads();
-  }
   if (enabled_) {
+    if (omp_num_threads_set_in_environment_) {
+      return omp_get_max_threads();
+    }
     int thread_count = omp_get_max_threads();
     if (exclude_reserved) {
       if (reserve_cores_ >= thread_count) {
@@ -100,8 +100,9 @@ int OpenMP::GetRecommendedOMPThreadCount(bool exclude_reserved) const {
       return thread_count;
     }
     return omp_thread_max_;
+  } else {
+    return 1;
   }
-  return 1;
 #else
   return 1;
 #endif

tests/python/unittest/test_engine.py

@@ -17,6 +17,9 @@
 
 import nose
 import mxnet as mx
+import os
+import unittest
+from mxnet.test_utils import EnvManager
 
 def test_bulk():
     with mx.engine.bulk(10):
@@ -30,6 +33,44 @@ def test_bulk():
             x += 1
     assert (x.asnumpy() == 104).all()
 
+@unittest.skip("OMP platform dependent")
+def test_engine_openmp_after_fork():
+    """
+    Test that the number of max threads in the child is 1. After forking we should not use a bigger
+    OMP thread pool.
+
+    With GOMP the child always has the same number when calling omp_get_max_threads, with LLVM OMP
+    the child respects the number of max threads set in the parent.
+    """
+    with EnvManager('OMP_NUM_THREADS', '42'):
+        r, w = os.pipe()
+        pid = os.fork()
+        if pid:
+            os.close(r)
+            wfd = os.fdopen(w, 'w')
+            wfd.write('a')
+            omp_max_threads = mx.base._LIB.omp_get_max_threads()
+            print("Parent omp max threads: {}".format(omp_max_threads))
+            try:
+                wfd.close()
+            except:
+                pass
+            try:
+                (cpid, status) = os.waitpid(pid, 0)
+                assert cpid == pid
+                exit_status = status >> 8
+                assert exit_status == 0
+            except:
+                pass
+        else:
+            os.close(w)
+            rfd = os.fdopen(r, 'r')
+            rfd.read(1)
+            omp_max_threads = mx.base._LIB.omp_get_max_threads()
+            print("Child omp max threads: {}".format(omp_max_threads))
+            assert omp_max_threads == 1
+
+
 
 if __name__ == '__main__':
     import nose