Minor gtrace timestamp changes

PErfetto seems to have changed how it interprets flow event time stamps.
This was detected by testing with the new support for the Iris tasking
runtime.
For that reason, we now add 0.250 us to the start of the flow event
and subtract 0.250 us from the end of the flow event.
the timestamps are adjusted to meet their definition of "enclosing
slice" and "begin >= timestamp of the flow".

Here is what the documentation says:

```
Each flow event phase has a specific type of binding point:
* Flow start (ph='s'): binding point is always "enclosing slice"
* Flow step (ph='t'): binding point is "enclosing slice" (not used by APEX)
* Flow end (ph='f'): binding point is "next slice". If the event contains
bp="e" then the binding point is "enclosing slice."
A bit of detail on "next slice" binding rules: Next slice is defined as the next slice to begin >= the timestamp of the flow. In the case of multiple slices having the same timestamp as the flow event, the earliest event in the trace buffer is chosen.
```

this commit also includes a change to optionally include timers around
the lifetime of the pthread. it's not really necessary so off by default.

src/apex/trace_event_listener.cpp

@@ -185,14 +185,15 @@ uint64_t get_flow_id() {
 }
 
 void write_flow_event(std::stringstream& ss, double ts, char ph,
-    std::string cat, uint64_t id, uint64_t pid, uint64_t tid, std::string name) {
+    std::string cat, uint64_t id, uint64_t pid, uint64_t tid, std::string parent_name,
+    std::string child_name) {
     ss << "{\"ts\":" << ts
        << ",\"ph\":\"" << ph
        << "\",\"cat\":\"" << cat
        << "\",\"id\":" << id
        << ",\"pid\":" << pid
        << ",\"tid\":" << tid
-       << ",\"name\":\"" << name << "\"},\n";
+       << ",\"name\":\"" << parent_name << " -> " << child_name << "\"},\n";
 }
 
 inline void trace_event_listener::_common_stop(std::shared_ptr<profiler> &p) {
@@ -218,10 +219,10 @@ inline void trace_event_listener::_common_stop(std::shared_ptr<profiler> &p) {
             ) {
             //std::cout << "FLOWING!" << std::endl;
             uint64_t flow_id = reversed_node_id + get_flow_id();
-            write_flow_event(ss, p->tt_ptr->parent->get_flow_us()+0.250, 's', "ControlFlow", flow_id,
-                saved_node_id, p->tt_ptr->parent->thread_id, p->tt_ptr->parent->task_id->get_name());
-            write_flow_event(ss, p->get_start_us(), 'f', "ControlFlow", flow_id,
-                saved_node_id, _tid, p->tt_ptr->parent->task_id->get_name());
+            write_flow_event(ss, p->tt_ptr->parent->get_flow_us()+0.25, 's', "ControlFlow", flow_id,
+                saved_node_id, p->tt_ptr->parent->thread_id, p->tt_ptr->parent->task_id->get_name(), p->get_task_id()->get_name());
+            write_flow_event(ss, p->get_start_us()-0.25, 'f', "ControlFlow", flow_id,
+                saved_node_id, _tid, p->tt_ptr->parent->task_id->get_name(), p->get_task_id()->get_name());
         }
         if (p->tt_ptr->explicit_trace_start) {
             ss << "{\"name\":\"" << p->get_task_id()->get_name()
@@ -378,13 +379,13 @@ void trace_event_listener::on_async_event(base_thread_node &node,
         if (data.reverse_flow) {
             double begin_ts = (p->get_stop_us() + p->get_start_us()) * 0.5;
             double end_ts = std::min(p->get_stop_us(), data.parent_ts_stop);
-            write_flow_event(ss, begin_ts, 's', data.cat, flow_id, saved_node_id, atol(tid.c_str()), data.name);
-            write_flow_event(ss, end_ts, 't', data.cat, flow_id, saved_node_id, data.parent_tid, data.name);
+            write_flow_event(ss, begin_ts, 's', data.cat, flow_id, saved_node_id, atol(tid.c_str()), data.name, p->get_task_id()->get_name());
+            write_flow_event(ss, end_ts, 't', data.cat, flow_id, saved_node_id, data.parent_tid, data.name, p->get_task_id()->get_name());
         } else {
             double begin_ts = std::min(p->get_start_us(), ((data.parent_ts_stop + data.parent_ts_start) * 0.5));
             double end_ts = p->get_start_us();
-            write_flow_event(ss, begin_ts, 's', data.cat, flow_id, saved_node_id, data.parent_tid, data.name);
-            write_flow_event(ss, end_ts, 't', data.cat, flow_id, saved_node_id, atol(tid.c_str()), data.name);
+            write_flow_event(ss, begin_ts, 's', data.cat, flow_id, saved_node_id, data.parent_tid, data.name, p->get_task_id()->get_name());
+            write_flow_event(ss, end_ts, 't', data.cat, flow_id, saved_node_id, atol(tid.c_str()), data.name, p->get_task_id()->get_name());
         }
         }
         write_to_trace(ss);

src/scripts/apex_exec

@@ -62,7 +62,8 @@ where APEX options are zero or more of:
     --apex:kokkos-tuning          enable Kokkos runtime autotuning support
     --apex:kokkos-fence           enable Kokkos fences for async kernels
     --apex:raja                   enable RAJA support
-    --apex:pthread                enable pthread wrapper support
+    --apex:pthread                enable pthread wrapper support (default: off)
+    --apex:track-pthread          track pthread lifetime (forces --apex:pthread on)
     --apex:gpu-memory             enable GPU memory wrapper support
     --apex:cpu-memory             enable CPU memory wrapper support
     --apex:delay-memory           delay memory wrapper support until explicitly enabled
@@ -187,6 +188,10 @@ while (( "$#" )); do
       shift
       ;;
     --apex:pthread)
+      pthread=yes
+      shift
+      ;;
+    --apex:track-pthread)
       pthread=yes
       export APEX_TIME_TOP_LEVEL_OS_THREADS=1
       shift

src/wrappers/pthread_wrapper_internal.cpp

@@ -106,8 +106,14 @@ class apex_wrapper {
   apex_wrapper(start_routine_p func, std::shared_ptr<apex::task_wrapper> parent) :
     _wrapped(false), _twp(nullptr), _timing(false) {
     apex::register_thread("APEX pthread wrapper", parent);
-    //_twp = apex::new_task((apex_function_address)func, UINTMAX_MAX, parent);
-    _twp = apex::new_task((apex_function_address)func);
+    // if we are tracking thread lifetimes, the top level timer for the thread
+    // will have the flow event/parent of the pthread_create call, from the
+    // register_thread timer, above.
+    if (apex::apex_options::top_level_os_threads()) {
+        _twp = apex::new_task((apex_function_address)func);
+    } else {
+        _twp = apex::new_task((apex_function_address)func, UINTMAX_MAX, parent);
+    }
     _twp->explicit_trace_start = true;
   }
   ~apex_wrapper() {