[Openmp-commits] [PATCH] D123044: [OpenMP] libomp: honor passive wait policy requested in the presence of tasking

[Andrey Churbanov via Phabricator via Openmp-commits]

AndreyChurbanov created this revision.
AndreyChurbanov added reviewers: hbae, jlpeyton, tlwilmar.
AndreyChurbanov added a project: OpenMP.
Herald added subscribers: guansong, yaxunl.
Herald added a project: All.
AndreyChurbanov requested review of this revision.
Herald added a reviewer: jdoerfert.
Herald added subscribers: openmp-commits, sstefan1.

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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D123044

Files:
  openmp/runtime/src/kmp.h
  openmp/runtime/src/kmp_global.cpp
  openmp/runtime/src/kmp_settings.cpp
  openmp/runtime/src/kmp_tasking.cpp
  openmp/runtime/src/kmp_wait_release.h


Index: openmp/runtime/src/kmp_wait_release.h
===================================================================
--- openmp/runtime/src/kmp_wait_release.h
+++ openmp/runtime/src/kmp_wait_release.h
@@ -609,7 +609,8 @@
       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
Index: openmp/runtime/src/kmp_tasking.cpp
===================================================================
--- openmp/runtime/src/kmp_tasking.cpp
+++ openmp/runtime/src/kmp_tasking.cpp
@@ -1895,8 +1895,21 @@
     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;
 }
 
@@ -4268,6 +4281,17 @@
       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;
+      }
+    }
+  }
 }
 
 /*!
Index: openmp/runtime/src/kmp_settings.cpp
===================================================================
--- openmp/runtime/src/kmp_settings.cpp
+++ openmp/runtime/src/kmp_settings.cpp
@@ -812,6 +812,7 @@
       }
     } 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;
Index: openmp/runtime/src/kmp_global.cpp
===================================================================
--- openmp/runtime/src/kmp_global.cpp
+++ openmp/runtime/src/kmp_global.cpp
@@ -154,6 +154,7 @@
 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,
Index: openmp/runtime/src/kmp.h
===================================================================
--- openmp/runtime/src/kmp.h
+++ openmp/runtime/src/kmp.h
@@ -3168,6 +3168,7 @@
                                (__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 */


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