1 /* ---------------------------------------------------------------------------
2 * (c) The GHC Team, 2003
6 * A Capability represent the token required to execute STG code,
7 * and all the state an OS thread/task needs to run Haskell code:
8 * its STG registers, a pointer to its TSO, a nursery etc. During
9 * STG execution, a pointer to the capabilitity is kept in a
12 * Only in an SMP build will there be multiple capabilities, for
13 * the threaded RTS and other non-threaded builds, there is only
14 * one global capability, namely MainCapability.
16 * --------------------------------------------------------------------------*/
18 #include "PosixSource.h"
22 #include "OSThreads.h"
23 #include "Capability.h"
24 #include "Schedule.h" /* to get at EMPTY_RUN_QUEUE() */
25 #include "Signals.h" /* to get at handleSignalsInThisThread() */
28 Capability MainCapability; /* for non-SMP, we have one global capability */
31 #if defined(RTS_SUPPORTS_THREADS)
33 nat rts_n_free_capabilities;
35 /* returning_worker_cond: when a worker thread returns from executing an
36 * external call, it needs to wait for an RTS Capability before passing
37 * on the result of the call to the Haskell thread that made it.
39 * returning_worker_cond is signalled in Capability.releaseCapability().
42 Condition returning_worker_cond = INIT_COND_VAR;
45 * To avoid starvation of threads blocked on worker_thread_cond,
46 * the task(s) that enter the Scheduler will check to see whether
47 * there are one or more worker threads blocked waiting on
48 * returning_worker_cond.
50 nat rts_n_waiting_workers = 0;
52 /* thread_ready_cond: when signalled, a thread has become runnable for a
55 * In the non-SMP case, it also implies that the thread that is woken up has
56 * exclusive access to the RTS and all its data structures (that are not
57 * locked by the Scheduler's mutex).
59 * thread_ready_cond is signalled whenever noCapabilities doesn't hold.
62 Condition thread_ready_cond = INIT_COND_VAR;
65 * To be able to make an informed decision about whether or not
66 * to create a new task when making an external call, keep track of
67 * the number of tasks currently blocked waiting on thread_ready_cond.
68 * (if > 0 => no need for a new task, just unblock an existing one).
70 * waitForWorkCapability() takes care of keeping it up-to-date;
71 * Task.startTask() uses its current value.
73 nat rts_n_waiting_tasks = 0;
75 static Condition *passTarget = NULL;
76 static rtsBool passingCapability = rtsFalse;
80 #define UNUSED_IF_NOT_SMP
82 #define UNUSED_IF_NOT_SMP STG_UNUSED
85 /* ----------------------------------------------------------------------------
87 ------------------------------------------------------------------------- */
90 initCapability( Capability *cap )
92 cap->f.stgGCEnter1 = (F_)__stg_gc_enter_1;
93 cap->f.stgGCFun = (F_)__stg_gc_fun;
97 static void initCapabilities_(nat n);
100 /* ---------------------------------------------------------------------------
101 * Function: initCapabilities()
103 * Purpose: set up the Capability handling. For the SMP build,
104 * we keep a table of them, the size of which is
105 * controlled by the user via the RTS flag RtsFlags.ParFlags.nNodes
107 * ------------------------------------------------------------------------- */
109 initCapabilities( void )
112 initCapabilities_(RtsFlags.ParFlags.nNodes);
114 initCapability(&MainCapability);
117 #if defined(RTS_SUPPORTS_THREADS)
118 initCondition(&returning_worker_cond);
119 initCondition(&thread_ready_cond);
120 rts_n_free_capabilities = 1;
127 /* Free capability list. */
128 static Capability *free_capabilities; /* Available capabilities for running threads */
129 static Capability *returning_capabilities;
130 /* Capabilities being passed to returning worker threads */
133 /* ----------------------------------------------------------------------------
134 grabCapability( Capability** )
136 (only externally visible when !RTS_SUPPORTS_THREADS. In the
137 threaded RTS, clients must use waitFor*Capability()).
138 ------------------------------------------------------------------------- */
141 grabCapability( Capability** cap )
144 #if defined(RTS_SUPPORTS_THREADS)
145 ASSERT(rts_n_free_capabilities == 1);
146 rts_n_free_capabilities = 0;
148 *cap = &MainCapability;
149 handleSignalsInThisThread();
151 *cap = free_capabilities;
152 free_capabilities = (*cap)->link;
153 rts_n_free_capabilities--;
155 IF_DEBUG(scheduler, sched_belch("worker: got capability"));
158 /* ----------------------------------------------------------------------------
159 * Function: releaseCapability(Capability*)
161 * Purpose: Letting go of a capability. Causes a
162 * 'returning worker' thread or a 'waiting worker'
163 * to wake up, in that order.
164 * ------------------------------------------------------------------------- */
167 releaseCapability( Capability* cap UNUSED_IF_NOT_SMP )
169 // Precondition: sched_mutex is held.
170 #if defined(RTS_SUPPORTS_THREADS)
172 ASSERT(rts_n_free_capabilities == 0);
174 // Check to see whether a worker thread can be given
175 // the go-ahead to return the result of an external call..
176 if (rts_n_waiting_workers > 0) {
177 // Decrement the counter here to avoid livelock where the
178 // thread that is yielding its capability will repeatedly
179 // signal returning_worker_cond.
182 // SMP variant untested
183 cap->link = returning_capabilities;
184 returning_capabilities = cap;
187 rts_n_waiting_workers--;
188 signalCondition(&returning_worker_cond);
189 IF_DEBUG(scheduler, sched_belch("worker: released capability to returning worker"));
190 } else if (passingCapability) {
191 if (passTarget == NULL) {
192 signalCondition(&thread_ready_cond);
193 startSchedulerTaskIfNecessary();
195 signalCondition(passTarget);
197 rts_n_free_capabilities = 1;
198 IF_DEBUG(scheduler, sched_belch("worker: released capability, passing it"));
202 cap->link = free_capabilities;
203 free_capabilities = cap;
204 rts_n_free_capabilities++;
206 rts_n_free_capabilities = 1;
208 // Signal that a capability is available
209 signalCondition(&thread_ready_cond);
210 startSchedulerTaskIfNecessary();
211 IF_DEBUG(scheduler, sched_belch("worker: released capability"));
217 #if defined(RTS_SUPPORTS_THREADS)
219 * When a native thread has completed the execution of an external
220 * call, it needs to communicate the result back. This is done
223 * - in resumeThread(), the thread calls waitForReturnCapability().
224 * - If no capabilities are readily available, waitForReturnCapability()
225 * increments a counter rts_n_waiting_workers, and blocks
226 * waiting for the condition returning_worker_cond to become
228 * - upon entry to the Scheduler, a worker thread checks the
229 * value of rts_n_waiting_workers. If > 0, the worker thread
230 * will yield its capability to let a returning worker thread
231 * proceed with returning its result -- this is done via
232 * yieldToReturningWorker().
233 * - the worker thread that yielded its capability then tries
234 * to re-grab a capability and re-enter the Scheduler.
237 /* ----------------------------------------------------------------------------
238 * waitForReturnCapability( Mutext *pMutex, Capability** )
240 * Purpose: when an OS thread returns from an external call,
241 * it calls grabReturnCapability() (via Schedule.resumeThread())
242 * to wait for permissions to enter the RTS & communicate the
243 * result of the external call back to the Haskell thread that
246 * ------------------------------------------------------------------------- */
249 waitForReturnCapability( Mutex* pMutex, Capability** pCap )
251 // Pre-condition: pMutex is held.
254 sched_belch("worker: returning; workers waiting: %d",
255 rts_n_waiting_workers));
257 if ( noCapabilities() || passingCapability ) {
258 rts_n_waiting_workers++;
259 wakeBlockedWorkerThread();
260 context_switch = 1; // make sure it's our turn soon
261 waitCondition(&returning_worker_cond, pMutex);
263 *pCap = returning_capabilities;
264 returning_capabilities = (*pCap)->link;
266 *pCap = &MainCapability;
267 ASSERT(rts_n_free_capabilities == 0);
268 handleSignalsInThisThread();
271 grabCapability(pCap);
274 // Post-condition: pMutex is held, pCap points to a capability
275 // which is now held by the current thread.
280 /* ----------------------------------------------------------------------------
281 * yieldCapability( Mutex* pMutex, Capability** pCap )
282 * ------------------------------------------------------------------------- */
285 yieldCapability( Capability** pCap )
287 // Pre-condition: pMutex is assumed held, the current thread
288 // holds the capability pointed to by pCap.
290 if ( rts_n_waiting_workers > 0 || passingCapability ) {
291 IF_DEBUG(scheduler, sched_belch("worker: giving up capability"));
292 releaseCapability(*pCap);
296 // Post-condition: pMutex is assumed held, and either:
298 // 1. *pCap is NULL, in which case the current thread does not
299 // hold a capability now, or
300 // 2. *pCap is not NULL, in which case the current thread still
301 // holds the capability.
307 /* ----------------------------------------------------------------------------
308 * waitForCapability( Mutex*, Capability**, Condition* )
310 * Purpose: wait for a Capability to become available. In
311 * the process of doing so, updates the number
312 * of tasks currently blocked waiting for a capability/more
313 * work. That counter is used when deciding whether or
314 * not to create a new worker thread when an external
316 * If pThreadCond is not NULL, a capability can be specifically
317 * passed to this thread using passCapability.
318 * ------------------------------------------------------------------------- */
321 waitForCapability( Mutex* pMutex, Capability** pCap, Condition* pThreadCond )
323 // Pre-condition: pMutex is held.
325 while ( noCapabilities() ||
326 (passingCapability && passTarget != pThreadCond)) {
328 sched_belch("worker: wait for capability (cond: %p)",
331 if (pThreadCond != NULL) {
332 waitCondition(pThreadCond, pMutex);
333 IF_DEBUG(scheduler, sched_belch("worker: get passed capability"));
335 rts_n_waiting_tasks++;
336 waitCondition(&thread_ready_cond, pMutex);
337 rts_n_waiting_tasks--;
338 IF_DEBUG(scheduler, sched_belch("worker: get normal capability"));
341 passingCapability = rtsFalse;
342 grabCapability(pCap);
344 // Post-condition: pMutex is held and *pCap is held by the current thread
348 /* ----------------------------------------------------------------------------
349 passCapability, passCapabilityToWorker
350 ------------------------------------------------------------------------- */
353 passCapability( Condition *pTargetThreadCond )
355 // Pre-condition: pMutex is held and cap is held by the current thread
357 passTarget = pTargetThreadCond;
358 passingCapability = rtsTrue;
359 IF_DEBUG(scheduler, sched_belch("worker: passCapability"));
361 // Post-condition: pMutex is held; cap is still held, but will be
362 // passed to the target thread when next released.
366 passCapabilityToWorker( void )
368 // Pre-condition: pMutex is held and cap is held by the current thread
371 passingCapability = rtsTrue;
372 IF_DEBUG(scheduler, sched_belch("worker: passCapabilityToWorker"));
374 // Post-condition: pMutex is held; cap is still held, but will be
375 // passed to a worker thread when next released.
378 #endif /* RTS_SUPPORTS_THREADS */
380 /* ------------------------------------------------------------------------- */
384 * Function: initCapabilities_(nat)
386 * Purpose: upon startup, allocate and fill in table
387 * holding 'n' Capabilities. Only for SMP, since
388 * it is the only build that supports multiple
389 * capabilities within the RTS.
392 initCapabilities_(nat n)
395 Capability *cap, *prev;
398 for (i = 0; i < n; i++) {
399 cap = stgMallocBytes(sizeof(Capability), "initCapabilities");
404 free_capabilities = cap;
405 rts_n_free_capabilities = n;
406 returning_capabilities = NULL;
408 sched_belch("allocated %d capabilities", n_free_capabilities));