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/*
* Copyright (c) 2012, 2015, 2017 ARM Limited
* All rights reserved
*
* The license below extends only to copyright in the software and shall
* not be construed as granting a license to any other intellectual
* property including but not limited to intellectual property relating
* to a hardware implementation of the functionality of the software
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*
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* this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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#ifndef __SIM_DRAIN_HH__
#define __SIM_DRAIN_HH__
#include <atomic>
#include <mutex>
#include <vector>
namespace gem5
{
class Drainable;
/**
* Object drain/handover states
*
* An object starts out in the Running state. When the simulator
* prepares to take a snapshot or prepares a CPU for handover, it
* calls the drain() method to transfer the object into the Draining
* or Drained state. If any object enters the Draining state
* (Drainable::drain() returning >0), simulation continues until it
* all objects have entered the Drained state.
*
* Before resuming simulation, the simulator calls resume() to
* transfer the object to the Running state. This in turn results in a
* call to drainResume() for all Drainable objects in the
* simulator. New Drainable objects may be created while resuming. In
* such cases, the new objects will be created in the Resuming state
* and later resumed.
*
* \note Even though the state of an object (visible to the rest of
* the world through Drainable::getState()) could be used to determine
* if all objects have entered the Drained state, the protocol is
* actually a bit more elaborate. See Drainable::drain() for details.
*
* @ingroup api_drain
*/
enum class DrainState
{
Running, /**< Running normally */
Draining, /**< Draining buffers pending serialization/handover */
Drained, /**< Buffers drained, ready for serialization/handover */
Resuming, /**< Transient state while the simulator is resuming */
};
class DrainManager
{
private:
DrainManager();
DrainManager(DrainManager &) = delete;
~DrainManager();
public:
/** Get the singleton DrainManager instance */
static DrainManager &instance() { return _instance; }
/**
* Try to drain the system.
*
* Try to drain the system and return true if all objects are in a
* the Drained state at which point the whole simulator is in a
* consistent state and ready for checkpointing or CPU
* handover. The simulation script must continue simulating until
* the simulation loop returns "Finished drain", at which point
* this method should be called again. This cycle should continue
* until this method returns true.
*
* @return true if all objects were drained successfully, false if
* more simulation is needed.
*
* @ingroup api_drain
*/
bool tryDrain();
/**
* Resume normal simulation in a Drained system.
*
* @ingroup api_drain
*/
void resume();
/**
* Run state fixups before a checkpoint restore operation.
*
* This is called before restoring the checkpoint and to make
* sure that everything has been set to drained.
*
* When restoring from a checkpoint, this function should be called
* first before calling the resume() function. And also before
* calling loadstate() on any object.
*
* The drain state of an object isn't stored in a checkpoint since
* the whole system is always going to be in the Drained state
* when the checkpoint is created. When the checkpoint is restored
* at a later stage, recreated objects will be in the Running
* state since the state isn't stored in checkpoints. This method
* performs state fixups on all Drainable objects and the
* DrainManager itself.
*
* @ingroup api_drain
*/
void preCheckpointRestore();
/**
* Check if the system is drained
*
* @ingroup api_drain
*/
bool isDrained() const { return _state == DrainState::Drained; }
/**
* Get the simulators global drain state
*
* @ingroup api_drain
*/
DrainState state() const { return _state; }
/**
* Notify the DrainManager that a Drainable object has finished
* draining.
*
* @ingroup api_drain
*/
void signalDrainDone();
public:
void registerDrainable(Drainable *obj);
void unregisterDrainable(Drainable *obj);
private:
/**
* Helper function to check if all Drainable objects are in a
* specific state.
*/
bool allInState(DrainState state) const;
/**
* Thread-safe helper function to get the number of Drainable
* objects in a system.
*/
size_t drainableCount() const;
/** Lock protecting the set of drainable objects */
mutable std::mutex globalLock;
/** Set of all drainable objects */
std::vector<Drainable *> _allDrainable;
/**
* Number of objects still draining. This is flagged atomic since
* it can be manipulated by SimObjects living in different
* threads.
*/
std::atomic_uint _count;
/** Global simulator drain state */
DrainState _state;
/** Singleton instance of the drain manager */
static DrainManager _instance;
};
/**
* Interface for objects that might require draining before
* checkpointing.
*
* An object's internal state needs to be drained when creating a
* checkpoint, switching between CPU models, or switching between
* timing models. Once the internal state has been drained from
* <i>all</i> objects in the simulator, the objects are serialized to
* disc or the configuration change takes place. The process works as
* follows (see simulate.py for details):
*
* <ol>
* <li>DrainManager::tryDrain() calls Drainable::drain() for every
* object in the system. Draining has completed if all of them
* return true. Otherwise, the drain manager keeps track of the
* objects that requested draining and waits for them to signal
* that they are done draining using the signalDrainDone() method.
*
* <li>Continue simulation. When an object has finished draining its
* internal state, it calls DrainManager::signalDrainDone() on the
* manager. The drain manager keeps track of the objects that
* haven't drained yet, simulation stops when the set of
* non-drained objects becomes empty.
*
* <li>Check if any object still needs draining
* (DrainManager::tryDrain()), if so repeat the process above.
*
* <li>Serialize objects, switch CPU model, or change timing model.
*
* <li>Call DrainManager::resume(), which in turn calls
* Drainable::drainResume() for all objects, and then continue the
* simulation.
* </ol>
*
*/
class Drainable
{
/**
* This class coordinates draining of a System.
*
* When draining the simulator, we need to make sure that all
* Drainable objects within the system have ended up in the drained
* state before declaring the operation to be successful. This class
* keeps track of how many objects are still in the process of
* draining. Once it determines that all objects have drained their
* state, it exits the simulation loop.
*
* @note A System might not be completely drained even though the
* DrainManager has caused the simulation loop to exit. Draining needs
* to be restarted until all Drainable objects declare that they don't
* need further simulation to be completely drained. See Drainable for
* more information.
*/
friend class DrainManager;
protected:
Drainable();
virtual ~Drainable();
/**
* Draining is the process of clearing out the states of
* SimObjects.These are the SimObjects that are partially
* executed or are partially in flight. Draining is mostly
* used before forking and creating a check point.
*
* This function notifies an object that it needs to drain its state.
*
* If the object does not need further simulation to drain
* internal buffers, it returns DrainState::Drained and
* automatically switches to the Drained state. If the object
* needs more simulation, it returns DrainState::Draining and
* automatically enters the Draining state. Other return values
* are invalid.
*
* @note An object that has entered the Drained state can be
* disturbed by other objects in the system and consequently stop
* being drained. These perturbations are not visible in the drain
* state. The simulator therefore repeats the draining process
* until all objects return DrainState::Drained on the first call
* to drain().
*
* @return DrainState::Drained if the object is drained at this
* point in time, DrainState::Draining if it needs further
* simulation.
*
* @ingroup api_drain
*/
virtual DrainState drain() = 0;
/**
* Resume execution after a successful drain.
*
* @ingroup api_drain
*/
virtual void drainResume() {};
/**
* Signal that an object is drained
*
* This method is designed to be called whenever an object enters
* into a state where it is ready to be drained. The method is
* safe to call multiple times and there is no need to check that
* draining has been requested before calling this method.
*
* @ingroup api_drain
*/
void signalDrainDone() const {
switch (_drainState) {
case DrainState::Running:
case DrainState::Drained:
case DrainState::Resuming:
return;
case DrainState::Draining:
_drainState = DrainState::Drained;
_drainManager.signalDrainDone();
return;
}
}
public:
/**
* Return the current drain state of an object.
*
* @ingroup api_drain
*/
DrainState drainState() const { return _drainState; }
/**
* Notify a child process of a fork. SimObjects are told that the
* process is going to be forked.
*
* Forking is a process of splitting a process in to two
* processes, which is then used for multiprocessing.
*
* When calling fork in gem5, we need to ensure that resources
* shared between the parent and the child are consistent. This
* method is intended to be overloaded to handle that. For
* example, an object could use this method to re-open input files
* to get a separate file description with a private file offset.
*
* This method is only called in the child of the fork. The call
* takes place in a drained system.
*
* @ingroup api_drain
*/
virtual void notifyFork() {};
private:
/** DrainManager interface to request a drain operation */
DrainState dmDrain();
/** DrainManager interface to request a resume operation */
void dmDrainResume();
/** Convenience reference to the drain manager */
DrainManager &_drainManager;
/**
* Current drain state of the object. Needs to be mutable since
* objects need to be able to signal that they have transitioned
* into a Drained state even if the calling method is const.
*/
mutable DrainState _drainState;
};
} // namespace gem5
#endif