sim: Decouple draining from the SimObject hierarchy

Draining is currently done by traversing the SimObject graph and
calling drain()/drainResume() on the SimObjects. This is not ideal
when non-SimObjects (e.g., ports) need draining since this means that
SimObjects owning those objects need to be aware of this.

This changeset moves the responsibility for finding objects that need
draining from SimObjects and the Python-side of the simulator to the
DrainManager. The DrainManager now maintains a set of all objects that
need draining. To reduce the overhead in classes owning non-SimObjects
that need draining, objects inheriting from Drainable now
automatically register with the DrainManager. If such an object is
destroyed, it is automatically unregistered. This means that drain()
and drainResume() should never be called directly on a Drainable
object.

While implementing the new functionality, the DrainManager has now
been made thread safe. In practice, this means that it takes a lock
whenever it manipulates the set of Drainable objects since SimObjects
in different threads may create Drainable objects
dynamically. Similarly, the drain counter is now an atomic_uint, which
ensures that it is manipulated correctly when objects signal that they
are done draining.

A nice side effect of these changes is that it makes the drain state
changes stricter, which the simulation scripts can exploit to avoid
redundant drains.

2 files changed, 37 insertions, 64 deletions

diff --git a/src/python/m5/simulate.py b/src/python/m5/simulate.py
index 2aee677bd..66344bc79 100644
--- a/src/python/m5/simulate.py
+++ b/src/python/m5/simulate.py
@@ -66,6 +66,8 @@ _memory_modes = {
     "atomic_noncaching" : objects.params.atomic_noncaching,
     }
 
+_drain_manager = internal.drain.DrainManager.instance()
+
 # The final hook to generate .ini files.  Called from the user script
 # once the config is built.
 def instantiate(ckpt_dir=None):
@@ -129,10 +131,10 @@ def instantiate(ckpt_dir=None):
 
     # Restore checkpoint (if any)
     if ckpt_dir:
+        _drain_manager.preCheckpointRestore()
         ckpt = internal.core.getCheckpoint(ckpt_dir)
         internal.core.unserializeGlobals(ckpt);
         for obj in root.descendants(): obj.loadState(ckpt)
-        need_resume.append(root)
     else:
         for obj in root.descendants(): obj.initState()
 
@@ -140,10 +142,9 @@ def instantiate(ckpt_dir=None):
     # a checkpoint, If so, this call will shift them to be at a valid time.
     updateStatEvents()
 
-need_resume = []
 need_startup = True
 def simulate(*args, **kwargs):
-    global need_resume, need_startup
+    global need_startup
 
     if need_startup:
         root = objects.Root.getInstance()
@@ -160,9 +161,8 @@ def simulate(*args, **kwargs):
         # Reset to put the stats in a consistent state.
         stats.reset()
 
-    for root in need_resume:
-        resume(root)
-    need_resume = []
+    if _drain_manager.isDrained():
+        _drain_manager.resume()
 
     return internal.event.simulate(*args, **kwargs)
 
@@ -170,33 +170,40 @@ def simulate(*args, **kwargs):
 def curTick():
     return internal.core.curTick()
 
-# Drain the system in preparation of a checkpoint or memory mode
-# switch.
-def drain(root):
+def drain():
+    """Drain the simulator in preparation of a checkpoint or memory mode
+    switch.
+
+    This operation is a no-op if the simulator is already in the
+    Drained state.
+
+    """
+
     # Try to drain all objects. Draining might not be completed unless
     # all objects return that they are drained on the first call. This
     # is because as objects drain they may cause other objects to no
     # longer be drained.
     def _drain():
-        all_drained = False
-        dm = internal.drain.createDrainManager()
-        unready_objs = sum(obj.drain(dm) for obj in root.descendants())
-        # If we've got some objects that can't drain immediately, then simulate
-        if unready_objs > 0:
-            dm.setCount(unready_objs)
-            #WARNING: if a valid exit event occurs while draining, it will not
-            # get returned to the user script
+        # Try to drain the system. The drain is successful if all
+        # objects are done without simulation. We need to simulate
+        # more if not.
+        if _drain_manager.tryDrain():
+            return True
+
+        # WARNING: if a valid exit event occurs while draining, it
+        # will not get returned to the user script
+        exit_event = internal.event.simulate()
+        while exit_event.getCause() != 'Finished drain':
             exit_event = simulate()
-            while exit_event.getCause() != 'Finished drain':
-                exit_event = simulate()
-        else:
-            all_drained = True
-        internal.drain.cleanupDrainManager(dm)
-        return all_drained
 
-    all_drained = _drain()
-    while (not all_drained):
-        all_drained = _drain()
+        return False
+
+    # Don't try to drain a system that is already drained
+    is_drained = _drain_manager.isDrained()
+    while not is_drained:
+        is_drained = _drain()
+
+    assert _drain_manager.isDrained(), "Drain state inconsistent"
 
 def memWriteback(root):
     for obj in root.descendants():
@@ -206,18 +213,15 @@ def memInvalidate(root):
     for obj in root.descendants():
         obj.memInvalidate()
 
-def resume(root):
-    for obj in root.descendants(): obj.drainResume()
-
 def checkpoint(dir):
     root = objects.Root.getInstance()
     if not isinstance(root, objects.Root):
         raise TypeError, "Checkpoint must be called on a root object."
-    drain(root)
+
+    drain()
     memWriteback(root)
     print "Writing checkpoint"
     internal.core.serializeAll(dir)
-    resume(root)
 
 def _changeMemoryMode(system, mode):
     if not isinstance(system, (objects.Root, objects.System)):
@@ -228,15 +232,9 @@ def _changeMemoryMode(system, mode):
     else:
         print "System already in target mode. Memory mode unchanged."
 
-def switchCpus(system, cpuList, do_drain=True, verbose=True):
+def switchCpus(system, cpuList, verbose=True):
     """Switch CPUs in a system.
 
-    By default, this method drains and resumes the system. This
-    behavior can be disabled by setting the keyword argument
-    'do_drain' to false, which might be desirable if multiple
-    operations requiring a drained system are going to be performed in
-    sequence.
-
     Note: This method may switch the memory mode of the system if that
     is required by the CPUs. It may also flush all caches in the
     system.
@@ -244,9 +242,6 @@ def switchCpus(system, cpuList, do_drain=True, verbose=True):
     Arguments:
       system -- Simulated system.
       cpuList -- (old_cpu, new_cpu) tuples
-
-    Keyword Arguments:
-      do_drain -- Perform a drain/resume of the system when switching.
     """
 
     if verbose:
@@ -292,8 +287,7 @@ def switchCpus(system, cpuList, do_drain=True, verbose=True):
     except KeyError:
         raise RuntimeError, "Invalid memory mode (%s)" % memory_mode_name
 
-    if do_drain:
-        drain(system)
+    drain()
 
     # Now all of the CPUs are ready to be switched out
     for old_cpu, new_cpu in cpuList:
@@ -314,7 +308,4 @@ def switchCpus(system, cpuList, do_drain=True, verbose=True):
     for old_cpu, new_cpu in cpuList:
         new_cpu.takeOverFrom(old_cpu)
 
-    if do_drain:
-        resume(system)
-
 from internal.core import disableAllListeners
diff --git a/src/python/swig/drain.i b/src/python/swig/drain.i
index 4442db207..59474f190 100644
--- a/src/python/swig/drain.i
+++ b/src/python/swig/drain.i
@@ -46,21 +46,3 @@
 %nodefaultctor Drainable;
 
 %include "sim/drain.hh"
-
-%inline %{
-
-DrainManager *
-createDrainManager()
-{
-    return new DrainManager();
-}
-
-void
-cleanupDrainManager(DrainManager *drain_manager)
-{
-    assert(drain_manager);
-    assert(drain_manager->getCount() == 0);
-    delete drain_manager;
-}
-
-%}