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;
74 /* -----------------------------------------------------------------------------
76 -------------------------------------------------------------------------- */
79 initCapability( Capability *cap )
81 cap->f.stgGCEnter1 = (F_)__stg_gc_enter_1;
82 cap->f.stgGCFun = (F_)__stg_gc_fun;
86 static void initCapabilities_(nat n);
90 * Function: initCapabilities()
92 * Purpose: set up the Capability handling. For the SMP build,
93 * we keep a table of them, the size of which is
94 * controlled by the user via the RTS flag RtsFlags.ParFlags.nNodes
96 * Pre-conditions: no locks assumed held.
101 #if defined(RTS_SUPPORTS_THREADS)
102 initCondition(&returning_worker_cond);
103 initCondition(&thread_ready_cond);
107 initCapabilities_(RtsFlags.ParFlags.nNodes);
109 initCapability(&MainCapability);
110 rts_n_free_capabilities = 1;
117 /* Free capability list. */
118 static Capability *free_capabilities; /* Available capabilities for running threads */
119 static Capability *returning_capabilities;
120 /* Capabilities being passed to returning worker threads */
123 /* -----------------------------------------------------------------------------
124 Acquiring capabilities
125 -------------------------------------------------------------------------- */
128 * Function: grabCapability(Capability**)
130 * Purpose: the act of grabbing a capability is easy; just
131 * remove one from the free capabilities list (which
132 * may just have one entry). In threaded builds, worker
133 * threads are prevented from doing so willy-nilly
134 * via the condition variables thread_ready_cond and
135 * returning_worker_cond.
138 void grabCapability(Capability** cap)
140 ASSERT(rts_n_free_capabilities > 0);
142 rts_n_free_capabilities = 0;
143 *cap = &MainCapability;
144 handleSignalsInThisThread();
146 *cap = free_capabilities;
147 free_capabilities = (*cap)->link;
148 rts_n_free_capabilities--;
153 * Function: releaseCapability(Capability*)
155 * Purpose: Letting go of a capability. Causes a
156 * 'returning worker' thread or a 'waiting worker'
157 * to wake up, in that order.
160 void releaseCapability(Capability* cap
165 { // Precondition: sched_mutex must be held
166 #if defined(RTS_SUPPORTS_THREADS)
168 ASSERT(rts_n_free_capabilities == 0);
170 /* Check to see whether a worker thread can be given
171 the go-ahead to return the result of an external call..*/
172 if (rts_n_waiting_workers > 0) {
173 /* Decrement the counter here to avoid livelock where the
174 * thread that is yielding its capability will repeatedly
175 * signal returning_worker_cond.
178 // SMP variant untested
179 cap->link = returning_capabilities;
180 returning_capabilities = cap;
183 rts_n_waiting_workers--;
184 signalCondition(&returning_worker_cond);
185 } else /*if ( !EMPTY_RUN_QUEUE() )*/ {
187 cap->link = free_capabilities;
188 free_capabilities = cap;
189 rts_n_free_capabilities++;
191 rts_n_free_capabilities = 1;
193 /* Signal that a capability is available */
194 signalCondition(&thread_ready_cond);
200 #if defined(RTS_SUPPORTS_THREADS)
202 * When a native thread has completed the execution of an external
203 * call, it needs to communicate the result back. This is done
206 * - in resumeThread(), the thread calls grabReturnCapability().
207 * - If no capabilities are readily available, grabReturnCapability()
208 * increments a counter rts_n_waiting_workers, and blocks
209 * waiting for the condition returning_worker_cond to become
211 * - upon entry to the Scheduler, a worker thread checks the
212 * value of rts_n_waiting_workers. If > 0, the worker thread
213 * will yield its capability to let a returning worker thread
214 * proceed with returning its result -- this is done via
215 * yieldToReturningWorker().
216 * - the worker thread that yielded its capability then tries
217 * to re-grab a capability and re-enter the Scheduler.
221 * Function: grabReturnCapability(Capability**)
223 * Purpose: when an OS thread returns from an external call,
224 * it calls grabReturnCapability() (via Schedule.resumeThread())
225 * to wait for permissions to enter the RTS & communicate the
226 * result of the external call back to the Haskell thread that
229 * Pre-condition: pMutex is held.
230 * Post-condition: pMutex is still held and a capability has
231 * been assigned to the worker thread.
234 grabReturnCapability(Mutex* pMutex, Capability** pCap)
237 fprintf(stderr,"worker (%ld): returning, waiting for lock.\n", osThreadId()));
239 fprintf(stderr,"worker (%ld): returning; workers waiting: %d\n",
240 osThreadId(), rts_n_waiting_workers));
241 if ( noCapabilities() ) {
242 rts_n_waiting_workers++;
243 wakeBlockedWorkerThread();
244 context_switch = 1; // make sure it's our turn soon
245 waitCondition(&returning_worker_cond, pMutex);
247 *pCap = returning_capabilities;
248 returning_capabilities = (*pCap)->link;
250 *pCap = &MainCapability;
251 ASSERT(rts_n_free_capabilities == 0);
252 handleSignalsInThisThread();
255 grabCapability(pCap);
261 /* -----------------------------------------------------------------------------
262 Yielding/waiting for capabilities
263 -------------------------------------------------------------------------- */
266 * Function: yieldToReturningWorker(Mutex*,Capability*)
268 * Purpose: when, upon entry to the Scheduler, an OS worker thread
269 * spots that one or more threads are blocked waiting for
270 * permission to return back their result, it gives up
273 * Pre-condition: pMutex is assumed held and the thread possesses
275 * Post-condition: pMutex is held and the Capability has
279 yieldToReturningWorker(Mutex* pMutex, Capability** pCap)
281 if ( rts_n_waiting_workers > 0 ) {
283 fprintf(stderr,"worker thread (%p): giving up RTS token\n", osThreadId()));
284 releaseCapability(*pCap);
285 /* And wait for work */
286 waitForWorkCapability(pMutex, pCap, rtsFalse);
288 fprintf(stderr,"worker thread (%p): got back RTS token (after yieldToReturningWorker)\n",
296 * Function: waitForWorkCapability(Mutex*, Capability**, rtsBool)
298 * Purpose: wait for a Capability to become available. In
299 * the process of doing so, updates the number
300 * of tasks currently blocked waiting for a capability/more
301 * work. That counter is used when deciding whether or
302 * not to create a new worker thread when an external
305 * Pre-condition: pMutex is held.
306 * Post-condition: pMutex is held and *pCap is held by the current thread
309 waitForWorkCapability(Mutex* pMutex, Capability** pCap, rtsBool runnable)
311 while ( noCapabilities() || (runnable && EMPTY_RUN_QUEUE()) ) {
312 rts_n_waiting_tasks++;
313 waitCondition(&thread_ready_cond, pMutex);
314 rts_n_waiting_tasks--;
316 grabCapability(pCap);
320 #endif /* RTS_SUPPORTS_THREADS */
324 * Function: initCapabilities_(nat)
326 * Purpose: upon startup, allocate and fill in table
327 * holding 'n' Capabilities. Only for SMP, since
328 * it is the only build that supports multiple
329 * capabilities within the RTS.
332 initCapabilities_(nat n)
335 Capability *cap, *prev;
338 for (i = 0; i < n; i++) {
339 cap = stgMallocBytes(sizeof(Capability), "initCapabilities");
344 free_capabilities = cap;
345 rts_n_free_capabilities = n;
346 returning_capabilities = NULL;
347 IF_DEBUG(scheduler,fprintf(stderr,"scheduler: Allocated %d capabilities\n", n_free_capabilities););