gdb/py-inferior: Keep inferior threads in a map

The python code maintains a list of threads for each inferior.  This
list is implemented as a linked list.  When the number of threads grows
high, this implementation can begin to be a performance bottleneck as
finding a particular thread_object in the list has a complexity of O(N).

We see this in ROCgdb[1], a downstream port of GDB for AMDGUP.  On
AMDGPU devices, the number of threads can get significantly higher than
on usual GDB workloads.

In some situations, we can reach the end of the inferior process with
GDB still having a substantial list of known threads.  While running
target_mourn_inferior, we end up in inferior::clear_thread_list which
iterates over all remaining threads and marks each thread exited.  This
fires the gdb::observers::thread_exit observer and eventually
py-inferior.c:set_thread_exited gets called.  This function searches in
the linked list with poor performances.

This patch proposes to change the linked list that keeps the per
inferior_object list of thread_objects into a std::unordered_map.  This
allows to have the search operation complexity be O(1) on average
instead of O(N).

With this patch, we can complete clear_thread_list in about 2.5 seconds
compared to 10 minutes without it.

Except for the performance change, no user visible change is expected.

Regression tested on Ubuntu-22.04 x86_64.

[1] https://github.com/ROCm-Developer-Tools/ROCgdb

gdb/python/py-inferior.c

@@ -30,17 +30,10 @@
 #include "gdbsupport/gdb_signals.h"
 #include "py-event.h"
 #include "py-stopevent.h"
+#include <unordered_map>
 
-struct threadlist_entry
-{
-  threadlist_entry (gdbpy_ref<thread_object> &&ref)
-    : thread_obj (std::move (ref))
-  {
-  }
-
-  gdbpy_ref<thread_object> thread_obj;
-  struct threadlist_entry *next;
-};
+using thread_map_t
+  = std::unordered_map<thread_info *, gdbpy_ref<thread_object>>;
 
 struct inferior_object
 {
@@ -49,12 +42,9 @@ struct inferior_object
   /* The inferior we represent.  */
   struct inferior *inferior;
 
-  /* thread_object instances under this inferior.  This list owns a
+  /* thread_object instances under this inferior.  This owns a
      reference to each object it contains.  */
-  struct threadlist_entry *threads;
-
-  /* Number of threads in the list.  */
-  int nthreads;
+  thread_map_t *threads;
 };
 
 extern PyTypeObject inferior_object_type
@@ -65,8 +55,6 @@ struct infpy_deleter
 {
   void operator() (inferior_object *obj)
   {
-    struct threadlist_entry *th_entry, *th_tmp;
-
     if (!gdb_python_initialized)
       return;
 
@@ -75,15 +63,7 @@ struct infpy_deleter
 
     inf_obj->inferior = NULL;
 
-    /* Deallocate threads list.  */
-    for (th_entry = inf_obj->threads; th_entry != NULL;)
-      {
-	th_tmp = th_entry;
-	th_entry = th_entry->next;
-	delete th_tmp;
-      }
-
-    inf_obj->nthreads = 0;
+    delete inf_obj->threads;
   }
 };
 
@@ -257,8 +237,7 @@ inferior_to_inferior_object (struct inferior *inferior)
 	return NULL;
 
       inf_obj->inferior = inferior;
-      inf_obj->threads = NULL;
-      inf_obj->nthreads = 0;
+      inf_obj->threads = new thread_map_t ();
 
       /* PyObject_New initializes the new object with a refcount of 1.  This
 	 counts for the reference we are keeping in the inferior data.  */
@@ -333,11 +312,10 @@ thread_to_thread_object (thread_info *thr)
   if (inf_obj == NULL)
     return NULL;
 
-  for (threadlist_entry *thread = inf_obj->threads;
-       thread != NULL;
-       thread = thread->next)
-    if (thread->thread_obj->thread == thr)
-      return gdbpy_ref<>::new_reference ((PyObject *) thread->thread_obj.get ());
+  auto thread_it = inf_obj->threads->find (thr);
+  if (thread_it != inf_obj->threads->end ())
+    return gdbpy_ref<>::new_reference
+      ((PyObject *) (thread_it->second.get ()));
 
   PyErr_SetString (PyExc_SystemError,
 		   _("could not find gdb thread object"));
@@ -348,7 +326,6 @@ static void
 add_thread_object (struct thread_info *tp)
 {
   inferior_object *inf_obj;
-  struct threadlist_entry *entry;
 
   if (!gdb_python_initialized)
     return;
@@ -364,18 +341,19 @@ add_thread_object (struct thread_info *tp)
 
   inf_obj = (inferior_object *) thread_obj->inf_obj;
 
-  entry = new threadlist_entry (std::move (thread_obj));
-  entry->next = inf_obj->threads;
+  auto ins_result = inf_obj->threads->emplace
+    (thread_map_t::value_type (tp, std::move (thread_obj)));
 
-  inf_obj->threads = entry;
-  inf_obj->nthreads++;
+  if (!ins_result.second)
+    return;
 
   if (evregpy_no_listeners_p (gdb_py_events.new_thread))
     return;
 
-  gdbpy_ref<> event = create_thread_event_object (&new_thread_event_object_type,
-						  (PyObject *)
-						  entry->thread_obj.get ());
+  gdbpy_ref<> event = create_thread_event_object
+    (&new_thread_event_object_type,
+     (PyObject *) ins_result.first->second.get ());
+
   if (event == NULL
       || evpy_emit_event (event.get (), gdb_py_events.new_thread) < 0)
     gdbpy_print_stack ();
@@ -384,8 +362,6 @@ add_thread_object (struct thread_info *tp)
 static void
 delete_thread_object (struct thread_info *tp, int ignore)
 {
-  struct threadlist_entry **entry, *tmp;
-
   if (!gdb_python_initialized)
     return;
 
@@ -395,29 +371,22 @@ delete_thread_object (struct thread_info *tp, int ignore)
   if (inf_obj == NULL)
     return;
 
-  /* Find thread entry in its inferior's thread_list.  */
-  for (entry = &inf_obj->threads; *entry != NULL; entry =
-	 &(*entry)->next)
-    if ((*entry)->thread_obj->thread == tp)
-      break;
-
-  if (!*entry)
-    return;
-
-  tmp = *entry;
-  tmp->thread_obj->thread = NULL;
-
-  *entry = (*entry)->next;
-  inf_obj->nthreads--;
-
-  delete tmp;
+  auto it = inf_obj->threads->find (tp);
+  if (it != inf_obj->threads->end ())
+    {
+      /* Some python code can still hold a reference to the thread_object
+	 instance.   Make sure to remove the link to the associated
+	 thread_info object as it will be freed soon.  This makes the python
+	 object invalid (i.e. gdb.InfThread.is_valid returns False).  */
+      it->second->thread = nullptr;
+      inf_obj->threads->erase (it);
+    }
 }
 
 static PyObject *
 infpy_threads (PyObject *self, PyObject *args)
 {
-  int i;
-  struct threadlist_entry *entry;
+  int i = 0;
   inferior_object *inf_obj = (inferior_object *) self;
   PyObject *tuple;
 
@@ -432,16 +401,16 @@ infpy_threads (PyObject *self, PyObject *args)
       GDB_PY_HANDLE_EXCEPTION (except);
     }
 
-  tuple = PyTuple_New (inf_obj->nthreads);
+  tuple = PyTuple_New (inf_obj->threads->size ());
   if (!tuple)
     return NULL;
 
-  for (i = 0, entry = inf_obj->threads; i < inf_obj->nthreads;
-       i++, entry = entry->next)
+  for (const thread_map_t::value_type &entry : *inf_obj->threads)
     {
-      PyObject *thr = (PyObject *) entry->thread_obj.get ();
+      PyObject *thr = (PyObject *) entry.second.get ();
       Py_INCREF (thr);
       PyTuple_SET_ITEM (tuple, i, thr);
+      i = i + 1;
     }
 
   return tuple;