Manually track num_max_thread (#12380)

* use cached version of get thread max

* reserve core affects omp singleton

* omp_thread_max_ updated in one line

* remove enabled block

* add brackets

* re-add excluded reserved

* add missing var

* refactor macro

src/engine/openmp.cc

@@ -73,18 +73,14 @@ void OpenMP::set_reserve_cores(int cores) {
   CHECK_GE(cores, 0);
   reserve_cores_ = cores;
 #ifdef _OPENMP
-  if (reserve_cores_ >= omp_thread_max_) {
-    omp_set_num_threads(1);
-  } else {
-    omp_set_num_threads(omp_thread_max_ - reserve_cores_);
-  }
+  omp_thread_max_ = std::max(omp_thread_max_ - reserve_cores_, 1);
 #endif
 }
 
 int OpenMP::GetRecommendedOMPThreadCount(bool exclude_reserved) const {
 #ifdef _OPENMP
   if (omp_num_threads_set_in_environment_) {
-    return omp_get_max_threads();
+    return omp_thread_max_;
   }
   if (enabled_) {
     int thread_count = omp_get_max_threads();
@@ -101,10 +97,8 @@ int OpenMP::GetRecommendedOMPThreadCount(bool exclude_reserved) const {
     }
     return omp_thread_max_;
   }
-  return 1;
-#else
-  return 1;
 #endif
+  return 1;
 }
 
 OpenMP *__init_omp__ = OpenMP::Get();