1 /* -----------------------------------------------------------------------------
3 * (c) The GHC Team 2001-2005
7 * -------------------------------------------------------------------------*/
16 A task is an OSThread that runs Haskell code. Every OSThread
17 created by the RTS for the purposes of running Haskell code is a
18 Task, and OS threads that enter the Haskell RTS for the purposes of
19 making a call-in are also Tasks.
21 The relationship between the number of tasks and capabilities, and
22 the runtime build (-threaded, -smp etc.) is summarised by the
25 build Tasks Capabilities
26 ---------------------------------
31 The non-threaded build has a single Task and a single global
34 The 'threaded' build has multiple Tasks, but a single Capability.
35 At any one time only one task executing STG code, other tasks are
36 either busy executing code outside the RTS (e.g., a C call) or
37 waiting for their turn to (again) evaluate some STG code. A task
38 relinquishes its RTS token when it is asked to evaluate an external
41 The SMP build allows multiple tasks and mulitple Capabilities.
42 Multiple Tasks may all be running Haskell code simultaneously.
44 In general, there may be multiple Tasks for an OS thread. This
45 happens if one Task makes a foreign call from Haskell, and
46 subsequently calls back in to create a new bound thread.
48 A particular Task structure can belong to more than one OS thread
49 over its lifetime. This is to avoid creating an unbounded number
50 of Task structures. The stats just accumulate.
55 The OS thread named in the Task structure has exclusive access to
56 the structure, as long as it is the running_task of its Capability.
57 That is, if (task->cap->running_task == task), then task->id owns
58 the Task. Otherwise the Task is owned by the owner of the parent
59 data structure on which it is sleeping; for example, if the task is
60 sleeping on spare_workers field of a Capability, then the owner of the
61 Capability has access to the Task.
63 When a task is migrated from sleeping on one Capability to another,
64 its task->cap field must be modified. When the task wakes up, it
65 will read the new value of task->cap to find out which Capability
66 it belongs to. Hence some synchronisation is required on
67 task->cap, and this is why we have task->lock.
69 If the Task is not currently owned by task->id, then the thread is
72 (a) waiting on the condition task->cond. The Task is either
73 (1) a bound Task, the TSO will be on a queue somewhere
74 (2) a worker task, on the spare_workers queue of task->cap.
76 (b) making a foreign call. The Task will be on the
77 suspended_ccalling_tasks list.
79 We re-establish ownership in each case by respectively
81 (a) the task is currently blocked in yieldCapability().
82 This call will return when we have ownership of the Task and
83 a Capability. The Capability we get might not be the same
84 as the one we had when we called yieldCapability().
86 (b) we must call resumeThread(task), which will safely establish
87 ownership of the Task and a Capability.
90 typedef struct Task_ {
91 #if defined(THREADED_RTS)
92 OSThreadId id; // The OS Thread ID of this task
95 // This points to the Capability that the Task "belongs" to. If
96 // the Task owns a Capability, then task->cap points to it. If
97 // the task does not own a Capability, then either (a) if the task
98 // is a worker, then task->cap points to the Capability it belongs
99 // to, or (b) it is returning from a foreign call, then task->cap
100 // points to the Capability with the returning_worker queue that this
103 // When a task goes to sleep, it may be migrated to a different
104 // Capability. Hence, we always check task->cap on wakeup. To
105 // syncrhonise between the migrater and the migratee, task->lock
106 // must be held when modifying task->cap.
107 struct Capability_ *cap;
109 rtsBool stopped; // this task has stopped or exited Haskell
110 StgTSO * suspended_tso; // the TSO is stashed here when we
111 // make a foreign call (NULL otherwise);
113 // The following 3 fields are used by bound threads:
114 StgTSO * tso; // the bound TSO (or NULL)
115 SchedulerStatus stat; // return status
116 StgClosure ** ret; // return value
118 #if defined(THREADED_RTS)
119 Condition cond; // used for sleeping & waking up this task
120 Mutex lock; // lock for the condition variable
122 // this flag tells the task whether it should wait on task->cond
123 // or just continue immediately. It's a workaround for the fact
124 // that signalling a condition variable doesn't do anything if the
125 // thread is already running, but we want it to be sticky.
129 // Stats that we collect about this task
130 // ToDo: we probably want to put this in a separate TaskStats
131 // structure, so we can share it between multiple Tasks. We don't
132 // really want separate stats for each call in a nested chain of
133 // foreign->haskell->foreign->haskell calls, but we'll get a
134 // separate Task for each of the haskell calls.
135 long elapsedtimestart;
142 // Links tasks onto various lists. (ToDo: do we need double
147 // Links tasks on the returning_tasks queue of a Capability.
148 struct Task_ *return_link;
150 // Links tasks on the all_tasks list
151 struct Task_ *all_link;
153 // When a Haskell thread makes a foreign call that re-enters
154 // Haskell, we end up with another Task associated with the
155 // current thread. We have to remember the whole stack of Tasks
156 // associated with the current thread so that we can correctly
157 // save & restore the thread-local current task pointer.
158 struct Task_ *prev_stack;
161 INLINE_HEADER rtsBool
162 isBoundTask (Task *task)
164 return (task->tso != NULL);
168 // Linked list of all tasks.
170 extern Task *all_tasks;
172 // Start and stop the task manager.
173 // Requires: sched_mutex.
175 void initTaskManager (void);
176 void stopTaskManager (void);
178 // Create a new Task for a bound thread
179 // Requires: sched_mutex.
181 Task *newBoundTask (void);
183 // The current task is a bound task that is exiting.
184 // Requires: sched_mutex.
186 void boundTaskExiting (Task *task);
188 // This must be called when a new Task is associated with the current
189 // thread. It sets up the thread-local current task pointer so that
190 // myTask() can work.
191 INLINE_HEADER void taskEnter (Task *task);
193 // Notify the task manager that a task has stopped. This is used
194 // mainly for stats-gathering purposes.
195 // Requires: sched_mutex.
197 void taskStop (Task *task);
199 // Put the task back on the free list, mark it stopped. Used by
202 void discardTask (Task *task);
204 // Get the Task associated with the current OS thread (or NULL if none).
206 INLINE_HEADER Task *myTask (void);
208 // After a fork, the tasks are not carried into the child process, so
209 // we must tell the task manager.
210 // Requires: sched_mutex.
212 void resetTaskManagerAfterFork (void);
214 #if defined(THREADED_RTS)
216 // Workers are attached to the supplied Capability. This Capability
217 // should not currently have a running_task, because the new task
218 // will become the running_task for that Capability.
219 // Requires: sched_mutex.
221 void startWorkerTask (struct Capability_ *cap,
222 void OSThreadProcAttr (*taskStart)(Task *task));
224 #endif /* THREADED_RTS */
226 // -----------------------------------------------------------------------------
227 // INLINE functions... private from here on down:
229 // A thread-local-storage key that we can use to get access to the
230 // current thread's Task structure.
231 #if defined(THREADED_RTS)
232 extern ThreadLocalKey currentTaskKey;
234 extern Task *my_task;
238 // myTask() uses thread-local storage to find the Task associated with
239 // the current OS thread. If the current OS thread has multiple
240 // Tasks, because it has re-entered the RTS, then the task->prev_stack
241 // field is used to store the previous Task.
246 #if defined(THREADED_RTS)
247 return getThreadLocalVar(¤tTaskKey);
254 setMyTask (Task *task)
256 #if defined(THREADED_RTS)
257 setThreadLocalVar(¤tTaskKey,task);
263 // This must be called when a new Task is associated with the current
264 // thread. It sets up the thread-local current task pointer so that
265 // myTask() can work.
267 taskEnter (Task *task)
269 // save the current value, just in case this Task has been created
270 // as a result of re-entering the RTS (defaults to NULL):
271 task->prev_stack = myTask();