[OpenMP][OMPT] thread_num determination during execution of nested serialized parallel regions

[vladaindjic]

Summary

__ompt_get_task_info_internal function is adapted to support thread_num
determination during the execution of multiple nested serialized
parallel regions enclosed by a regular parallel region.

Consider the following program that contains parallel region R1 executed
by two threads. Let the worker thread T of region R1 executes serialized
parallel regions R2 that encloses another serialized parallel region R3.
Note that the thread T is the master thread of both R2 and R3 regions.

Assume that __ompt_get_task_info_internal function is called with the
argument “ancestor_level == 1” during the execution of region R3.
The function should determine the “thread_num” of the thread T inside
the team of region R2, whose implicit task is at level 1 inside the
hierarchy of active tasks. Since the thread T is the master thread of
region R2, one should expected that “thread_num” takes a value 0.
After the while loop finishes, the following stands: “lwt != NULL”,
“prev_lwt == NULL”, “prev_team” represents the team information about
the innermost serialized parallel region R3. This results in executing
the assignment “thread_num = prev_team->t.t_master_tid”. Note that
“prev_team->t.t_master_tid” was initialized at the moment of
R2’s creation and represents the “thread_num” of the thread T inside
the region R1 which encloses R2. Since the thread T is the worker thread
of the region R1, “the thread_num” takes value 1, which is a contradiction.

This patch proposes to use “lwt” instead of “prev_lwt” when determining
the “thread_num”. If “lwt” exists, the task at the requested level belongs
to the serialized parallel region. Since the serialized parallel region
is executed by one thread only, the “thread_num” takes value 0.

Similarly, assume that __ompt_get_task_info_internal function is called
with the argument “ancestor_level == 2” during the execution of region R3.
The function should determine the “thread_num” of the thread T inside the
team of region R1. Since the thread is the worker inside the region R1,
one should expected that “thread_num” takes value 1. After the loop finishes,
the following stands: “lwt == NULL”, “prev_lwt != NULL”, “prev_team” represents
the team information about the innermost serialized parallel region R3.
This leads to execution of the assignment “thread_num = 0”, which causes
a contradiction.

Ignoring the “prev_lwt” leads to executing the assignment
“thread_num = prev_team->t.t_master_tid” instead. From the previous explanation,
it is obvious that “thread_num” takes value 1.

Note that the “prev_lwt” variable is marked as unnecessary and thus removed.

This patch introduces the test case which represents the OpenMP program
described earlier in the summary.

[protze.joachim]

Overall looks good to me besides one inline comment.
I successfully tested with gcc/11, icc/19 and latest clang.

openmp/runtime/test/ompt/parallel/nested_lwt_thread_num.c

88

There is no guaranteed ordering of implicit task begin on primary thread and the output on the worker thread.
Adding a barrier at the beginning of the parallel region should make the test more robust.

[vladaindjic]

[OpenMP][OMPT] Barrier has been added inside nested_lwt_thread_num.c test case.

As @protze.joachim proposed, the barrier has been added at the beginning of the
regular (the outermost) parallel region.

[protze.joachim]

lgtm

[vladaindjic]

@protze.joachim Thank you for the revision. Could you please land this patch for me?

openmp/runtime/test/ompt/parallel/nested_lwt_thread_num.c

88

@protze.joachim Thanks for the comment. As you proposed, the explicit barrier has been added at the beginning of the parallel region. Does the test case look complete now?

openmp/runtime/src/ompt-specific.cpp

int __ompt_get_task_info_internal(int ancestor_level, int *type,
   if (thr) {
     kmp_taskdata_t *taskdata = thr->th.th_current_task;
     if (taskdata == NULL)
       return 0;
     kmp_team *team = thr->th.th_team, *prev_team = NULL;
     if (team == NULL)
       return 0;
     ompt_lw_taskteam_t *lwt = NULL,
-                       *next_lwt = LWT_FROM_TEAM(taskdata->td_team),
-                       *prev_lwt = NULL;
+                       *next_lwt = LWT_FROM_TEAM(taskdata->td_team);
 
     while (ancestor_level > 0) {
       // needed for thread_num
       prev_team = team;
-      prev_lwt = lwt;
       // next lightweight team (if any)
       if (lwt)
         lwt = lwt->parent;
 
       // next heavyweight team (if any) after
       // lightweight teams are exhausted
       if (!lwt && taskdata) {
         // first try scheduling parent (for explicit task scheduling)
if (task_frame) {
       *task_frame = info ? &info->frame : NULL;
     }
     if (parallel_data) {
       *parallel_data = team_info ? &(team_info->parallel_data) : NULL;
     }
     if (thread_num) {
       if (level == 0)
         *thread_num = __kmp_get_tid();
-      else if (prev_lwt)
+      else if (lwt)
         *thread_num = 0;
       else
         *thread_num = prev_team->t.t_master_tid;
       //        *thread_num = team->t.t_master_tid;
     }
     return info ? 2 : 0;
   }
   return 0;

openmp/runtime/test/ompt/parallel/nested_lwt_thread_num.c

+// RUN: %libomp-compile-and-run | FileCheck %s
+// REQUIRES: ompt
+
+#include "callback.h"
+#include <omp.h>
+
+
+__attribute__ ((noinline)) // workaround for bug in icc
+void print_task_info_at(int ancestor_level, int id)
+{
+#pragma omp critical
+  {
+    int task_type;
+    char buffer[2048];
+    ompt_data_t *parallel_data;
+    ompt_data_t *task_data;
+    int thread_num;
+    ompt_get_task_info(ancestor_level, &task_type, &task_data, NULL,
+                       &parallel_data, &thread_num);
+    format_task_type(task_type, buffer);
+    printf("%" PRIu64 ": ancestor_level=%d id=%d task_type=%s=%d "
+                      "parallel_id=%" PRIu64 " task_id=%" PRIu64
+                      " thread_num=%d\n",
+        ompt_get_thread_data()->value, ancestor_level, id, buffer,
+        task_type, parallel_data->value, task_data->value, thread_num);
+  }
+};
+
+__attribute__ ((noinline)) // workaround for bug in icc
+void print_innermost_task_info(int id)
+{
+  print_task_info_at(0, id);
+}
+
+
+int main()
+{
+
+#pragma omp parallel num_threads(2)
+  {
+    // sync threads before checking the output
+#pragma omp barrier
+    // region 0
+    if (omp_get_thread_num() == 1) {
+      // executed by worker thread only
+      // assert that thread_num is 1
+      print_innermost_task_info(1);
+
+#pragma omp parallel num_threads(1)
+      {
+        // serialized region 1
+        // assert that thread_num is 0
+        print_innermost_task_info(2);
+
+#pragma omp parallel num_threads(1)
+        {
+          // serialized region 2
+          // assert that thread_num is 0
+          print_innermost_task_info(3);
+
+          // Check the value of thread_num while iterating over the hierarchy
+          // of active tasks.
+          print_task_info_at(0, 3);
+          print_task_info_at(1, 2);
+          print_task_info_at(2, 1);
+
+        }
+
+      }
+    }
+  }
+
+
+  // Check if libomp supports the callbacks for this test.
+  // CHECK-NOT: {{^}}0: Could not register callback 'ompt_callback_task_create'
+  // CHECK-NOT: {{^}}0: Could not register callback 'ompt_callback_implicit_task'
+
+
+  // CHECK: {{^}}0: NULL_POINTER=[[NULL:.*$]]
+  // CHECK: {{^}}[[MASTER_ID:[0-9]+]]: ompt_event_initial_task_begin: parallel_id=[[PARALLEL_ID_0:[0-9]+]], task_id=[[TASK_ID_0:[0-9]+]], actual_parallelism=1, index=1, flags=1
+
+  // region 0
+  // CHECK: {{^}}[[MASTER_ID]]: ompt_event_parallel_begin: parent_task_id=[[TASK_ID_0]],
+  // CHECK-SAME: parallel_id=[[PARALLEL_ID_1:[0-9]+]]
+  // CHECK-DAG: {{^}}[[MASTER_ID]]: ompt_event_implicit_task_begin: parallel_id=[[PARALLEL_ID_1]], task_id=[[TASK_ID_1:[0-9]+]]
+  // CHECK-DAG: {{^}}[[WORKER_ID:[0-9]+]]: ompt_event_implicit_task_begin: parallel_id=[[PARALLEL_ID_1]], task_id=[[TASK_ID_2:[0-9]+]]
+  // assert some info about implicit task executed by worker thread
+  // thread_num is the most important
+  // CHECK: {{^}}[[WORKER_ID]]: ancestor_level=0 id=1
+  // CHECK-SAME: parallel_id=[[PARALLEL_ID_1]] task_id=[[TASK_ID_2]]
+  // CHECK-SAME: thread_num=1
+
+  // serialized region 1
+  // CHECK: {{^}}[[WORKER_ID]]: ompt_event_parallel_begin: parent_task_id=[[TASK_ID_2]],
+  // CHECK-SAME: parallel_id=[[PARALLEL_ID_2:[0-9]+]]
+  // CHECK-DAG: {{^}}[[WORKER_ID]]: ompt_event_implicit_task_begin: parallel_id=[[PARALLEL_ID_2]], task_id=[[TASK_ID_3:[0-9]+]]
+  // assert some information about the implicit task of the serialized region 1
+  // pay attention that thread_num should take value 0
+  // CHECK: {{^}}[[WORKER_ID]]: ancestor_level=0 id=2
+  // CHECK-SAME: parallel_id=[[PARALLEL_ID_2]] task_id=[[TASK_ID_3]]
+  // CHECK-SAME: thread_num=0
+
+  // serialized region 2
+  // CHECK: {{^}}[[WORKER_ID]]: ompt_event_parallel_begin: parent_task_id=[[TASK_ID_3]],
+  // CHECK-SAME: parallel_id=[[PARALLEL_ID_3:[0-9]+]]
+  // CHECK-DAG: {{^}}[[WORKER_ID]]: ompt_event_implicit_task_begin: parallel_id=[[PARALLEL_ID_3]], task_id=[[TASK_ID_4:[0-9]+]]
+  // assert some information about the implicit task of the serialized region 2
+  // pay attention that thread_num should take value 0
+  // CHECK: {{^}}[[WORKER_ID]]: ancestor_level=0 id=3
+  // CHECK-SAME: parallel_id=[[PARALLEL_ID_3]] task_id=[[TASK_ID_4]]
+  // CHECK-SAME: thread_num=0
+
+  // Check the value of thread_num argument while iterating over the hierarchy
+  // of active tasks. The expected is that thread_num takes the value checked
+  // above in the test case (0, 0, 1 - respectively).
+
+  // Thread is the master thread of the region 2, so thread_num should be 0.
+  // CHECK: {{^}}[[WORKER_ID]]: ancestor_level=0 id=3
+  // CHECK-SAME: parallel_id=[[PARALLEL_ID_3]] task_id=[[TASK_ID_4]]
+  // CHECK-SAME: thread_num=0
+
+  // Thread is the master thread of the region 1, so thread_num should be 0.
+  // CHECK: {{^}}[[WORKER_ID]]: ancestor_level=1 id=2
+  // CHECK-SAME: parallel_id=[[PARALLEL_ID_2]] task_id=[[TASK_ID_3]]
+  // CHECK-SAME: thread_num=0
+
+  // Thread is the worker thread of the region 0, so thread_num should be 1.
+  // CHECK: {{^}}[[WORKER_ID]]: ancestor_level=2 id=1
+  // CHECK-SAME: parallel_id=[[PARALLEL_ID_1]] task_id=[[TASK_ID_2]]
+  // CHECK-SAME: thread_num=1
+
+  return 0;
+}