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 #ifdef RTS_SUPPORTS_THREADS
141 ASSERT(rts_n_free_capabilities > 0);
144 rts_n_free_capabilities = 0;
145 *cap = &MainCapability;
146 handleSignalsInThisThread();
148 *cap = free_capabilities;
149 free_capabilities = (*cap)->link;
150 rts_n_free_capabilities--;
152 #ifdef RTS_SUPPORTS_THREADS
154 fprintf(stderr,"worker thread (%p): got capability\n",
160 * Function: releaseCapability(Capability*)
162 * Purpose: Letting go of a capability. Causes a
163 * 'returning worker' thread or a 'waiting worker'
164 * to wake up, in that order.
167 void releaseCapability(Capability* cap
172 { // Precondition: sched_mutex must be held
173 #if defined(RTS_SUPPORTS_THREADS)
175 ASSERT(rts_n_free_capabilities == 0);
177 /* Check to see whether a worker thread can be given
178 the go-ahead to return the result of an external call..*/
179 if (rts_n_waiting_workers > 0) {
180 /* Decrement the counter here to avoid livelock where the
181 * thread that is yielding its capability will repeatedly
182 * signal returning_worker_cond.
185 // SMP variant untested
186 cap->link = returning_capabilities;
187 returning_capabilities = cap;
190 rts_n_waiting_workers--;
191 signalCondition(&returning_worker_cond);
192 } else /*if ( !EMPTY_RUN_QUEUE() )*/ {
194 cap->link = free_capabilities;
195 free_capabilities = cap;
196 rts_n_free_capabilities++;
198 rts_n_free_capabilities = 1;
200 /* Signal that a capability is available */
201 signalCondition(&thread_ready_cond);
204 #ifdef RTS_SUPPORTS_THREADS
206 fprintf(stderr,"worker thread (%p): released capability\n",
212 #if defined(RTS_SUPPORTS_THREADS)
214 * When a native thread has completed the execution of an external
215 * call, it needs to communicate the result back. This is done
218 * - in resumeThread(), the thread calls grabReturnCapability().
219 * - If no capabilities are readily available, grabReturnCapability()
220 * increments a counter rts_n_waiting_workers, and blocks
221 * waiting for the condition returning_worker_cond to become
223 * - upon entry to the Scheduler, a worker thread checks the
224 * value of rts_n_waiting_workers. If > 0, the worker thread
225 * will yield its capability to let a returning worker thread
226 * proceed with returning its result -- this is done via
227 * yieldToReturningWorker().
228 * - the worker thread that yielded its capability then tries
229 * to re-grab a capability and re-enter the Scheduler.
233 * Function: grabReturnCapability(Capability**)
235 * Purpose: when an OS thread returns from an external call,
236 * it calls grabReturnCapability() (via Schedule.resumeThread())
237 * to wait for permissions to enter the RTS & communicate the
238 * result of the external call back to the Haskell thread that
241 * Pre-condition: pMutex is held.
242 * Post-condition: pMutex is still held and a capability has
243 * been assigned to the worker thread.
246 grabReturnCapability(Mutex* pMutex, Capability** pCap)
249 fprintf(stderr,"worker (%p): returning, waiting for lock.\n", osThreadId()));
251 fprintf(stderr,"worker (%p): returning; workers waiting: %d\n",
252 osThreadId(), rts_n_waiting_workers));
253 if ( noCapabilities() ) {
254 rts_n_waiting_workers++;
255 wakeBlockedWorkerThread();
256 context_switch = 1; // make sure it's our turn soon
257 waitCondition(&returning_worker_cond, pMutex);
259 *pCap = returning_capabilities;
260 returning_capabilities = (*pCap)->link;
262 *pCap = &MainCapability;
263 ASSERT(rts_n_free_capabilities == 0);
264 handleSignalsInThisThread();
267 grabCapability(pCap);
273 /* -----------------------------------------------------------------------------
274 Yielding/waiting for capabilities
275 -------------------------------------------------------------------------- */
278 * Function: yieldToReturningWorker(Mutex*,Capability*,Condition*)
280 * Purpose: when, upon entry to the Scheduler, an OS worker thread
281 * spots that one or more threads are blocked waiting for
282 * permission to return back their result, it gives up
284 * Immediately afterwards, it tries to reaquire the Capabilty
285 * using waitForWorkCapability.
288 * Pre-condition: pMutex is assumed held and the thread possesses
290 * Post-condition: pMutex is held and the thread possesses
294 yieldToReturningWorker(Mutex* pMutex, Capability** pCap, Condition* pThreadCond)
296 if ( rts_n_waiting_workers > 0 ) {
298 fprintf(stderr,"worker thread (%p): giving up RTS token\n", osThreadId()));
299 releaseCapability(*pCap);
300 /* And wait for work */
301 waitForWorkCapability(pMutex, pCap, pThreadCond);
303 fprintf(stderr,"worker thread (%p): got back RTS token (after yieldToReturningWorker)\n",
311 * Function: waitForWorkCapability(Mutex*, Capability**, Condition*)
313 * Purpose: wait for a Capability to become available. In
314 * the process of doing so, updates the number
315 * of tasks currently blocked waiting for a capability/more
316 * work. That counter is used when deciding whether or
317 * not to create a new worker thread when an external
319 * If pThreadCond is not NULL, a capability can be specifically
320 * passed to this thread using passCapability.
322 * Pre-condition: pMutex is held.
323 * Post-condition: pMutex is held and *pCap is held by the current thread
326 static Condition *passTarget = NULL;
329 waitForWorkCapability(Mutex* pMutex, Capability** pCap, Condition* pThreadCond)
332 #error SMP version not implemented
335 fprintf(stderr,"worker thread (%p): wait for cap (cond: %p)\n",
336 osThreadId(),pThreadCond));
337 while ( noCapabilities() || (pThreadCond && passTarget != pThreadCond)
338 || (!pThreadCond && passTarget)) {
341 waitCondition(pThreadCond, pMutex);
343 fprintf(stderr,"worker thread (%p): get passed capability\n",
348 rts_n_waiting_tasks++;
349 waitCondition(&thread_ready_cond, pMutex);
350 rts_n_waiting_tasks--;
352 fprintf(stderr,"worker thread (%p): get normal capability\n",
357 grabCapability(pCap);
362 * Function: passCapability(Mutex*, Capability*, Condition*)
364 * Purpose: Let go of the capability and make sure the thread associated
365 * with the Condition pTargetThreadCond gets it next.
367 * Pre-condition: pMutex is held and cap is held by the current thread
368 * Post-condition: pMutex is held; cap will be grabbed by the "target"
369 * thread when pMutex is released.
373 passCapability(Mutex* pMutex, Capability* cap, Condition *pTargetThreadCond)
376 #error SMP version not implemented
378 rts_n_free_capabilities = 1;
379 signalCondition(pTargetThreadCond);
380 passTarget = pTargetThreadCond;
382 fprintf(stderr,"worker thread (%p): passCapability\n",
387 #endif /* RTS_SUPPORTS_THREADS */
391 * Function: initCapabilities_(nat)
393 * Purpose: upon startup, allocate and fill in table
394 * holding 'n' Capabilities. Only for SMP, since
395 * it is the only build that supports multiple
396 * capabilities within the RTS.
399 initCapabilities_(nat n)
402 Capability *cap, *prev;
405 for (i = 0; i < n; i++) {
406 cap = stgMallocBytes(sizeof(Capability), "initCapabilities");
411 free_capabilities = cap;
412 rts_n_free_capabilities = n;
413 returning_capabilities = NULL;
414 IF_DEBUG(scheduler,fprintf(stderr,"scheduler: Allocated %d capabilities\n", n_free_capabilities););