[OpenMP] libomp: honor passive wait policy requested with tasking

Currently the library ignores requested wait policy in the presence
of tasking. Threads always actively spin. The patch fixes this problem
making the wait policy passive if this explicitly requested by user.

Differential Revision: https://reviews.llvm.org/D123044

openmp/runtime/src/kmp.h

@@ -3160,6 +3160,7 @@ extern int __kmp_tp_cached; /* whether threadprivate cache has been created
                                (__kmpc_threadprivate_cached()) */
 extern int __kmp_dflt_blocktime; /* number of milliseconds to wait before
                                     blocking (env setting) */
+extern bool __kmp_wpolicy_passive; /* explicitly set passive wait policy */
 #if KMP_USE_MONITOR
 extern int
     __kmp_monitor_wakeups; /* number of times monitor wakes up per second */

openmp/runtime/src/kmp_global.cpp

@@ -154,6 +154,7 @@ int __kmp_hier_threads_per[kmp_hier_layer_e::LAYER_LAST + 1];
 kmp_hier_sched_env_t __kmp_hier_scheds = {0, 0, NULL, NULL, NULL};
 #endif
 int __kmp_dflt_blocktime = KMP_DEFAULT_BLOCKTIME;
+bool __kmp_wpolicy_passive = false;
 #if KMP_USE_MONITOR
 int __kmp_monitor_wakeups = KMP_MIN_MONITOR_WAKEUPS;
 int __kmp_bt_intervals = KMP_INTERVALS_FROM_BLOCKTIME(KMP_DEFAULT_BLOCKTIME,

openmp/runtime/src/kmp_settings.cpp

@@ -812,6 +812,7 @@ static void __kmp_stg_parse_wait_policy(char const *name, char const *value,
       }
     } else if (__kmp_str_match("PASSIVE", 1, value)) {
       __kmp_library = library_throughput;
+      __kmp_wpolicy_passive = true; /* allow sleep while active tasking */
       if (blocktime_str == NULL) {
         // KMP_BLOCKTIME not specified, so set default to 0.
         __kmp_dflt_blocktime = 0;

openmp/runtime/src/kmp_tasking.cpp

@@ -1909,8 +1909,21 @@ kmp_int32 __kmp_omp_task(kmp_int32 gtid, kmp_task_t *new_task,
     if (serialize_immediate)
       new_taskdata->td_flags.task_serial = 1;
     __kmp_invoke_task(gtid, new_task, current_task);
+  } else if (__kmp_dflt_blocktime != KMP_MAX_BLOCKTIME &&
+             __kmp_wpolicy_passive) {
+    kmp_info_t *this_thr = __kmp_threads[gtid];
+    kmp_team_t *team = this_thr->th.th_team;
+    kmp_int32 nthreads = this_thr->th.th_team_nproc;
+    for (int i = 0; i < nthreads; ++i) {
+      kmp_info_t *thread = team->t.t_threads[i];
+      if (thread == this_thr)
+        continue;
+      if (thread->th.th_sleep_loc != NULL) {
+        __kmp_null_resume_wrapper(thread);
+        break; // awake one thread at a time
+      }
+    }
   }
-
   return TASK_CURRENT_NOT_QUEUED;
 }
 
@@ -4282,6 +4295,17 @@ void __kmpc_give_task(kmp_task_t *ptask, kmp_int32 start = 0) {
       pass = pass << 1;
 
   } while (!__kmp_give_task(thread, k, ptask, pass));
+
+  if (__kmp_dflt_blocktime != KMP_MAX_BLOCKTIME && __kmp_wpolicy_passive) {
+    // awake at least one thread to execute given task
+    for (int i = 0; i < nthreads; ++i) {
+      thread = team->t.t_threads[i];
+      if (thread->th.th_sleep_loc != NULL) {
+        __kmp_null_resume_wrapper(thread);
+        break;
+      }
+    }
+  }
 }
 
 /*!

openmp/runtime/src/kmp_wait_release.h

@@ -609,7 +609,8 @@ final_spin=FALSE)
       continue;
 
     // Don't suspend if there is a likelihood of new tasks being spawned.
-    if ((task_team != NULL) && TCR_4(task_team->tt.tt_found_tasks))
+    if (task_team != NULL && TCR_4(task_team->tt.tt_found_tasks) &&
+        !__kmp_wpolicy_passive)
       continue;
 
 #if KMP_USE_MONITOR