1 /* ---------------------------------------------------------------------------
3 * (c) The GHC Team, 2002
7 * A Capability represent the token required to execute STG code,
8 * and all the state an OS thread/task needs to run Haskell code:
9 * its STG registers, a pointer to its TSO, a nursery etc. During
10 * STG execution, a pointer to the capabilitity is kept in a
13 * Only in an SMP build will there be multiple capabilities, for
14 * the threaded RTS and other non-threaded builds, there is only
15 * one global capability, namely MainCapability.
17 * --------------------------------------------------------------------------*/
18 #include "PosixSource.h"
21 #include "OSThreads.h"
22 #include "Capability.h"
23 #include "Schedule.h" /* to get at EMPTY_RUN_QUEUE() */
24 #include "Signals.h" /* to get at handleSignalsInThisThread() */
27 Capability MainCapability; /* for non-SMP, we have one global capability */
30 nat rts_n_free_capabilities;
32 #if defined(RTS_SUPPORTS_THREADS)
33 /* returning_worker_cond: when a worker thread returns from executing an
34 * external call, it needs to wait for an RTS Capability before passing
35 * on the result of the call to the Haskell thread that made it.
37 * returning_worker_cond is signalled in Capability.releaseCapability().
40 Condition returning_worker_cond = INIT_COND_VAR;
43 * To avoid starvation of threads blocked on worker_thread_cond,
44 * the task(s) that enter the Scheduler will check to see whether
45 * there are one or more worker threads blocked waiting on
46 * returning_worker_cond.
48 nat rts_n_waiting_workers = 0;
50 /* thread_ready_cond: when signalled, a thread has become runnable for a
53 * In the non-SMP case, it also implies that the thread that is woken up has
54 * exclusive access to the RTS and all its data structures (that are not
55 * locked by the Scheduler's mutex).
57 * thread_ready_cond is signalled whenever noCapabilities doesn't hold.
60 Condition thread_ready_cond = INIT_COND_VAR;
63 * To be able to make an informed decision about whether or not
64 * to create a new task when making an external call, keep track of
65 * the number of tasks currently blocked waiting on thread_ready_cond.
66 * (if > 0 => no need for a new task, just unblock an existing one).
68 * waitForWorkCapability() takes care of keeping it up-to-date;
69 * Task.startTask() uses its current value.
71 nat rts_n_waiting_tasks = 0;
73 static Condition *passTarget = NULL;
74 static rtsBool passingCapability = rtsFalse;
77 /* -----------------------------------------------------------------------------
79 -------------------------------------------------------------------------- */
82 initCapability( Capability *cap )
84 cap->f.stgGCEnter1 = (F_)__stg_gc_enter_1;
85 cap->f.stgGCFun = (F_)__stg_gc_fun;
89 static void initCapabilities_(nat n);
93 * Function: initCapabilities()
95 * Purpose: set up the Capability handling. For the SMP build,
96 * we keep a table of them, the size of which is
97 * controlled by the user via the RTS flag RtsFlags.ParFlags.nNodes
99 * Pre-conditions: no locks assumed held.
104 #if defined(RTS_SUPPORTS_THREADS)
105 initCondition(&returning_worker_cond);
106 initCondition(&thread_ready_cond);
110 initCapabilities_(RtsFlags.ParFlags.nNodes);
112 initCapability(&MainCapability);
113 rts_n_free_capabilities = 1;
120 /* Free capability list. */
121 static Capability *free_capabilities; /* Available capabilities for running threads */
122 static Capability *returning_capabilities;
123 /* Capabilities being passed to returning worker threads */
126 /* -----------------------------------------------------------------------------
127 Acquiring capabilities
128 -------------------------------------------------------------------------- */
131 * Function: grabCapability(Capability**)
133 * Purpose: the act of grabbing a capability is easy; just
134 * remove one from the free capabilities list (which
135 * may just have one entry). In threaded builds, worker
136 * threads are prevented from doing so willy-nilly
137 * via the condition variables thread_ready_cond and
138 * returning_worker_cond.
141 void grabCapability(Capability** cap)
144 ASSERT(rts_n_free_capabilities == 1);
145 rts_n_free_capabilities = 0;
146 *cap = &MainCapability;
147 handleSignalsInThisThread();
149 *cap = free_capabilities;
150 free_capabilities = (*cap)->link;
151 rts_n_free_capabilities--;
153 IF_DEBUG(scheduler, sched_belch("worker: got capability"));
157 * Function: releaseCapability(Capability*)
159 * Purpose: Letting go of a capability. Causes a
160 * 'returning worker' thread or a 'waiting worker'
161 * to wake up, in that order.
164 void releaseCapability(Capability* cap
169 { // Precondition: sched_mutex must be 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 ( !EMPTY_RUN_QUEUE() )*/ {
192 cap->link = free_capabilities;
193 free_capabilities = cap;
194 rts_n_free_capabilities++;
196 rts_n_free_capabilities = 1;
198 /* Signal that a capability is available */
199 signalCondition(&thread_ready_cond);
200 startSchedulerTaskIfNecessary(); // if there is more work to be done,
201 // we'll need a new thread
202 IF_DEBUG(scheduler, sched_belch("worker: released capability"));
208 #if defined(RTS_SUPPORTS_THREADS)
210 * When a native thread has completed the execution of an external
211 * call, it needs to communicate the result back. This is done
214 * - in resumeThread(), the thread calls grabReturnCapability().
215 * - If no capabilities are readily available, grabReturnCapability()
216 * increments a counter rts_n_waiting_workers, and blocks
217 * waiting for the condition returning_worker_cond to become
219 * - upon entry to the Scheduler, a worker thread checks the
220 * value of rts_n_waiting_workers. If > 0, the worker thread
221 * will yield its capability to let a returning worker thread
222 * proceed with returning its result -- this is done via
223 * yieldToReturningWorker().
224 * - the worker thread that yielded its capability then tries
225 * to re-grab a capability and re-enter the Scheduler.
229 * Function: grabReturnCapability(Capability**)
231 * Purpose: when an OS thread returns from an external call,
232 * it calls grabReturnCapability() (via Schedule.resumeThread())
233 * to wait for permissions to enter the RTS & communicate the
234 * result of the external call back to the Haskell thread that
237 * Pre-condition: pMutex is held.
238 * Post-condition: pMutex is still held and a capability has
239 * been assigned to the worker thread.
242 grabReturnCapability(Mutex* pMutex, Capability** pCap)
245 sched_belch("worker: returning; workers waiting: %d",
246 rts_n_waiting_workers));
247 if ( noCapabilities() || passingCapability ) {
248 rts_n_waiting_workers++;
249 wakeBlockedWorkerThread();
250 context_switch = 1; // make sure it's our turn soon
251 waitCondition(&returning_worker_cond, pMutex);
253 *pCap = returning_capabilities;
254 returning_capabilities = (*pCap)->link;
256 *pCap = &MainCapability;
257 ASSERT(rts_n_free_capabilities == 0);
258 handleSignalsInThisThread();
261 grabCapability(pCap);
267 /* -----------------------------------------------------------------------------
268 Yielding/waiting for capabilities
269 -------------------------------------------------------------------------- */
272 * Function: yieldToReturningWorker(Mutex*,Capability*,Condition*)
274 * Purpose: when, upon entry to the Scheduler, an OS worker thread
275 * spots that one or more threads are blocked waiting for
276 * permission to return back their result, it gives up
278 * Immediately afterwards, it tries to reaquire the Capabilty
279 * using waitForWorkCapability.
282 * Pre-condition: pMutex is assumed held and the thread possesses
284 * Post-condition: pMutex is held and the thread possesses
288 yieldToReturningWorker(Mutex* pMutex, Capability** pCap, Condition* pThreadCond)
290 if ( rts_n_waiting_workers > 0 ) {
291 IF_DEBUG(scheduler, sched_belch("worker: giving up capability"));
292 releaseCapability(*pCap);
293 /* And wait for work */
294 waitForWorkCapability(pMutex, pCap, pThreadCond);
296 sched_belch("worker: got back capability (after yieldToReturningWorker)"));
303 * Function: waitForWorkCapability(Mutex*, Capability**, Condition*)
305 * Purpose: wait for a Capability to become available. In
306 * the process of doing so, updates the number
307 * of tasks currently blocked waiting for a capability/more
308 * work. That counter is used when deciding whether or
309 * not to create a new worker thread when an external
311 * If pThreadCond is not NULL, a capability can be specifically
312 * passed to this thread using passCapability.
314 * Pre-condition: pMutex is held.
315 * Post-condition: pMutex is held and *pCap is held by the current thread
319 waitForWorkCapability(Mutex* pMutex, Capability** pCap, Condition* pThreadCond)
322 #error SMP version not implemented
324 while ( noCapabilities() || (passingCapability && passTarget != pThreadCond)) {
326 sched_belch("worker: wait for capability (cond: %p)",
330 waitCondition(pThreadCond, pMutex);
331 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);
347 * Function: passCapability(Mutex*, Capability*, Condition*)
349 * Purpose: Let go of the capability and make sure the thread associated
350 * with the Condition pTargetThreadCond gets it next.
352 * Pre-condition: pMutex is held and cap is held by the current thread
353 * Post-condition: pMutex is held; cap will be grabbed by the "target"
354 * thread when pMutex is released.
358 passCapability(Mutex* pMutex, Capability* cap, Condition *pTargetThreadCond)
361 #error SMP version not implemented
363 ASSERT(rts_n_free_capabilities == 0);
364 rts_n_free_capabilities = 1;
365 signalCondition(pTargetThreadCond);
366 passTarget = pTargetThreadCond;
367 passingCapability = rtsTrue;
368 IF_DEBUG(scheduler, sched_belch("worker: passCapability"));
372 * Function: passCapabilityToWorker(Mutex*, Capability*)
374 * Purpose: Let go of the capability and make sure that a
375 * "plain" worker thread (not a bound thread) gets it next.
377 * Pre-condition: pMutex is held and cap is held by the current thread
378 * Post-condition: pMutex is held; cap will be grabbed by the "target"
379 * thread when pMutex is released.
383 passCapabilityToWorker(Mutex* pMutex, Capability* cap)
386 #error SMP version not implemented
388 rts_n_free_capabilities = 1;
389 signalCondition(&thread_ready_cond);
390 startSchedulerTaskIfNecessary();
392 passingCapability = rtsTrue;
393 IF_DEBUG(scheduler, sched_belch("worker: passCapabilityToWorker"));
398 #endif /* RTS_SUPPORTS_THREADS */
402 * Function: initCapabilities_(nat)
404 * Purpose: upon startup, allocate and fill in table
405 * holding 'n' Capabilities. Only for SMP, since
406 * it is the only build that supports multiple
407 * capabilities within the RTS.
410 initCapabilities_(nat n)
413 Capability *cap, *prev;
416 for (i = 0; i < n; i++) {
417 cap = stgMallocBytes(sizeof(Capability), "initCapabilities");
422 free_capabilities = cap;
423 rts_n_free_capabilities = n;
424 returning_capabilities = NULL;
426 sched_belch("allocated %d capabilities", n_free_capabilities));