rts/Capability.c

   1 /* ---------------------------------------------------------------------------
   2  *
   3  * (c) The GHC Team, 2003-2006
   4  *
   5  * Capabilities
   6  *
   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
  11  * register (BaseReg; actually it is a pointer to cap->r).
  12  *
  13  * Only in an THREADED_RTS build will there be multiple capabilities,
  14  * for non-threaded builds there is only one global capability, namely
  15  * MainCapability.
  16  *
  17  * --------------------------------------------------------------------------*/
  18
  19 #include "PosixSource.h"
  20 #include "Rts.h"
  21 #include "RtsUtils.h"
  22 #include "RtsFlags.h"
  23 #include "STM.h"
  24 #include "OSThreads.h"
  25 #include "Capability.h"
  26 #include "Schedule.h"
  27 #include "Sparks.h"
  28 #include "Trace.h"
  29 #include "GC.h"
  30
  31 // one global capability, this is the Capability for non-threaded
  32 // builds, and for +RTS -N1
  33 Capability MainCapability;
  34
  35 nat n_capabilities;
  36 Capability *capabilities = NULL;
  37
  38 // Holds the Capability which last became free.  This is used so that
  39 // an in-call has a chance of quickly finding a free Capability.
  40 // Maintaining a global free list of Capabilities would require global
  41 // locking, so we don't do that.
  42 Capability *last_free_capability;
  43
  44 /* GC indicator, in scope for the scheduler, init'ed to false */
  45 volatile StgWord waiting_for_gc = 0;
  46
  47 #if defined(THREADED_RTS)
  48 STATIC_INLINE rtsBool
  49 globalWorkToDo (void)
  50 {
  51     return blackholes_need_checking
  52         || sched_state >= SCHED_INTERRUPTING
  53         ;
  54 }
  55 #endif
  56
  57 #if defined(THREADED_RTS)
  58 StgClosure *
  59 findSpark (Capability *cap)
  60 {
  61   Capability *robbed;
  62   StgClosurePtr spark;
  63   rtsBool retry;
  64   nat i = 0;
  65
  66   if (!emptyRunQueue(cap)) {
  67       // If there are other threads, don't try to run any new
  68       // sparks: sparks might be speculative, we don't want to take
  69       // resources away from the main computation.
  70       return 0;
  71   }
  72
  73   // first try to get a spark from our own pool.
  74   // We should be using reclaimSpark(), because it works without
  75   // needing any atomic instructions:
  76   //   spark = reclaimSpark(cap->sparks);
  77   // However, measurements show that this makes at least one benchmark
  78   // slower (prsa) and doesn't affect the others.
  79   spark = tryStealSpark(cap);
  80   if (spark != NULL) {
  81       cap->sparks_converted++;
  82       return spark;
  83   }
  84
  85   if (n_capabilities == 1) { return NULL; } // makes no sense...
  86
  87   debugTrace(DEBUG_sched,
  88              "cap %d: Trying to steal work from other capabilities",
  89              cap->no);
  90
  91   do {
  92       retry = rtsFalse;
  93
  94       /* visit cap.s 0..n-1 in sequence until a theft succeeds. We could
  95       start at a random place instead of 0 as well.  */
  96       for ( i=0 ; i < n_capabilities ; i++ ) {
  97           robbed = &capabilities[i];
  98           if (cap == robbed)  // ourselves...
  99               continue;
 100
 101           if (emptySparkPoolCap(robbed)) // nothing to steal here
 102               continue;
 103
 104           spark = tryStealSpark(robbed);
 105           if (spark == NULL && !emptySparkPoolCap(robbed)) {
 106               // we conflicted with another thread while trying to steal;
 107               // try again later.
 108               retry = rtsTrue;
 109           }
 110
 111           if (spark != NULL) {
 112               debugTrace(DEBUG_sched,
 113                  "cap %d: Stole a spark from capability %d",
 114                          cap->no, robbed->no);
 115               cap->sparks_converted++;
 116               return spark;
 117           }
 118           // otherwise: no success, try next one
 119       }
 120   } while (retry);
 121
 122   debugTrace(DEBUG_sched, "No sparks stolen");
 123   return NULL;
 124 }
 125
 126 // Returns True if any spark pool is non-empty at this moment in time
 127 // The result is only valid for an instant, of course, so in a sense
 128 // is immediately invalid, and should not be relied upon for
 129 // correctness.
 130 rtsBool
 131 anySparks (void)
 132 {
 133     nat i;
 134
 135     for (i=0; i < n_capabilities; i++) {
 136         if (!emptySparkPoolCap(&capabilities[i])) {
 137             return rtsTrue;
 138         }
 139     }
 140     return rtsFalse;
 141 }
 142 #endif
 143
 144 /* -----------------------------------------------------------------------------
 145  * Manage the returning_tasks lists.
 146  *
 147  * These functions require cap->lock
 148  * -------------------------------------------------------------------------- */
 149
 150 #if defined(THREADED_RTS)
 151 STATIC_INLINE void
 152 newReturningTask (Capability *cap, Task *task)
 153 {
 154     ASSERT_LOCK_HELD(&cap->lock);
 155     ASSERT(task->return_link == NULL);
 156     if (cap->returning_tasks_hd) {
 157         ASSERT(cap->returning_tasks_tl->return_link == NULL);
 158         cap->returning_tasks_tl->return_link = task;
 159     } else {
 160         cap->returning_tasks_hd = task;
 161     }
 162     cap->returning_tasks_tl = task;
 163 }
 164
 165 STATIC_INLINE Task *
 166 popReturningTask (Capability *cap)
 167 {
 168     ASSERT_LOCK_HELD(&cap->lock);
 169     Task *task;
 170     task = cap->returning_tasks_hd;
 171     ASSERT(task);
 172     cap->returning_tasks_hd = task->return_link;
 173     if (!cap->returning_tasks_hd) {
 174         cap->returning_tasks_tl = NULL;
 175     }
 176     task->return_link = NULL;
 177     return task;
 178 }
 179 #endif
 180
 181 /* ----------------------------------------------------------------------------
 182  * Initialisation
 183  *
 184  * The Capability is initially marked not free.
 185  * ------------------------------------------------------------------------- */
 186
 187 static void
 188 initCapability( Capability *cap, nat i )
 189 {
 190     nat g;
 191
 192     cap->no = i;
 193     cap->in_haskell        = rtsFalse;
 194     cap->in_gc             = rtsFalse;
 195
 196     cap->run_queue_hd      = END_TSO_QUEUE;
 197     cap->run_queue_tl      = END_TSO_QUEUE;
 198
 199 #if defined(THREADED_RTS)
 200     initMutex(&cap->lock);
 201     cap->running_task      = NULL; // indicates cap is free
 202     cap->spare_workers     = NULL;
 203     cap->suspended_ccalling_tasks = NULL;
 204     cap->returning_tasks_hd = NULL;
 205     cap->returning_tasks_tl = NULL;
 206     cap->wakeup_queue_hd    = END_TSO_QUEUE;
 207     cap->wakeup_queue_tl    = END_TSO_QUEUE;
 208     cap->sparks_created     = 0;
 209     cap->sparks_converted   = 0;
 210     cap->sparks_pruned      = 0;
 211 #endif
 212
 213     cap->f.stgEagerBlackholeInfo = (W_)&__stg_EAGER_BLACKHOLE_info;
 214     cap->f.stgGCEnter1     = (F_)__stg_gc_enter_1;
 215     cap->f.stgGCFun        = (F_)__stg_gc_fun;
 216
 217     cap->mut_lists  = stgMallocBytes(sizeof(bdescr *) *
 218                                      RtsFlags.GcFlags.generations,
 219                                      "initCapability");
 220     cap->saved_mut_lists = stgMallocBytes(sizeof(bdescr *) *
 221                                           RtsFlags.GcFlags.generations,
 222                                           "initCapability");
 223
 224     for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
 225         cap->mut_lists[g] = NULL;
 226     }
 227
 228     cap->free_tvar_watch_queues = END_STM_WATCH_QUEUE;
 229     cap->free_invariant_check_queues = END_INVARIANT_CHECK_QUEUE;
 230     cap->free_trec_chunks = END_STM_CHUNK_LIST;
 231     cap->free_trec_headers = NO_TREC;
 232     cap->transaction_tokens = 0;
 233     cap->context_switch = 0;
 234 }
 235
 236 /* ---------------------------------------------------------------------------
 237  * Function:  initCapabilities()
 238  *
 239  * Purpose:   set up the Capability handling. For the THREADED_RTS build,
 240  *            we keep a table of them, the size of which is
 241  *            controlled by the user via the RTS flag -N.
 242  *
 243  * ------------------------------------------------------------------------- */
 244 void
 245 initCapabilities( void )
 246 {
 247 #if defined(THREADED_RTS)
 248     nat i;
 249
 250 #ifndef REG_Base
 251     // We can't support multiple CPUs if BaseReg is not a register
 252     if (RtsFlags.ParFlags.nNodes > 1) {
 253         errorBelch("warning: multiple CPUs not supported in this build, reverting to 1");
 254         RtsFlags.ParFlags.nNodes = 1;
 255     }
 256 #endif
 257
 258     n_capabilities = RtsFlags.ParFlags.nNodes;
 259
 260     if (n_capabilities == 1) {
 261         capabilities = &MainCapability;
 262         // THREADED_RTS must work on builds that don't have a mutable
 263         // BaseReg (eg. unregisterised), so in this case
 264         // capabilities[0] must coincide with &MainCapability.
 265     } else {
 266         capabilities = stgMallocBytes(n_capabilities * sizeof(Capability),
 267                                       "initCapabilities");
 268     }
 269
 270     for (i = 0; i < n_capabilities; i++) {
 271         initCapability(&capabilities[i], i);
 272     }
 273
 274     debugTrace(DEBUG_sched, "allocated %d capabilities", n_capabilities);
 275
 276 #else /* !THREADED_RTS */
 277
 278     n_capabilities = 1;
 279     capabilities = &MainCapability;
 280     initCapability(&MainCapability, 0);
 281
 282 #endif
 283
 284     // There are no free capabilities to begin with.  We will start
 285     // a worker Task to each Capability, which will quickly put the
 286     // Capability on the free list when it finds nothing to do.
 287     last_free_capability = &capabilities[0];
 288 }
 289
 290 /* ----------------------------------------------------------------------------
 291  * setContextSwitches: cause all capabilities to context switch as
 292  * soon as possible.
 293  * ------------------------------------------------------------------------- */
 294
 295 void setContextSwitches(void)
 296 {
 297     nat i;
 298     for (i=0; i < n_capabilities; i++) {
 299         contextSwitchCapability(&capabilities[i]);
 300     }
 301 }
 302
 303 /* ----------------------------------------------------------------------------
 304  * Give a Capability to a Task.  The task must currently be sleeping
 305  * on its condition variable.
 306  *
 307  * Requires cap->lock (modifies cap->running_task).
 308  *
 309  * When migrating a Task, the migrater must take task->lock before
 310  * modifying task->cap, to synchronise with the waking up Task.
 311  * Additionally, the migrater should own the Capability (when
 312  * migrating the run queue), or cap->lock (when migrating
 313  * returning_workers).
 314  *
 315  * ------------------------------------------------------------------------- */
 316
 317 #if defined(THREADED_RTS)
 318 STATIC_INLINE void
 319 giveCapabilityToTask (Capability *cap USED_IF_DEBUG, Task *task)
 320 {
 321     ASSERT_LOCK_HELD(&cap->lock);
 322     ASSERT(task->cap == cap);
 323     debugTrace(DEBUG_sched, "passing capability %d to %s %p",
 324                cap->no, task->tso ? "bound task" : "worker",
 325                (void *)task->id);
 326     ACQUIRE_LOCK(&task->lock);
 327     task->wakeup = rtsTrue;
 328     // the wakeup flag is needed because signalCondition() doesn't
 329     // flag the condition if the thread is already runniing, but we want
 330     // it to be sticky.
 331     signalCondition(&task->cond);
 332     RELEASE_LOCK(&task->lock);
 333 }
 334 #endif
 335
 336 /* ----------------------------------------------------------------------------
 337  * Function:  releaseCapability(Capability*)
 338  *
 339  * Purpose:   Letting go of a capability. Causes a
 340  *            'returning worker' thread or a 'waiting worker'
 341  *            to wake up, in that order.
 342  * ------------------------------------------------------------------------- */
 343
 344 #if defined(THREADED_RTS)
 345 void
 346 releaseCapability_ (Capability* cap,
 347                     rtsBool always_wakeup)
 348 {
 349     Task *task;
 350
 351     task = cap->running_task;
 352
 353     ASSERT_PARTIAL_CAPABILITY_INVARIANTS(cap,task);
 354
 355     cap->running_task = NULL;
 356
 357     // Check to see whether a worker thread can be given
 358     // the go-ahead to return the result of an external call..
 359     if (cap->returning_tasks_hd != NULL) {
 360         giveCapabilityToTask(cap,cap->returning_tasks_hd);
 361         // The Task pops itself from the queue (see waitForReturnCapability())
 362         return;
 363     }
 364
 365     if (waiting_for_gc == PENDING_GC_SEQ) {
 366       last_free_capability = cap; // needed?
 367       debugTrace(DEBUG_sched, "GC pending, set capability %d free", cap->no);
 368       return;
 369     }
 370
 371
 372     // If the next thread on the run queue is a bound thread,
 373     // give this Capability to the appropriate Task.
 374     if (!emptyRunQueue(cap) && cap->run_queue_hd->bound) {
 375         // Make sure we're not about to try to wake ourselves up
 376         ASSERT(task != cap->run_queue_hd->bound);
 377         task = cap->run_queue_hd->bound;
 378         giveCapabilityToTask(cap,task);
 379         return;
 380     }
 381
 382     if (!cap->spare_workers) {
 383         // Create a worker thread if we don't have one.  If the system
 384         // is interrupted, we only create a worker task if there
 385         // are threads that need to be completed.  If the system is
 386         // shutting down, we never create a new worker.
 387         if (sched_state < SCHED_SHUTTING_DOWN || !emptyRunQueue(cap)) {
 388             debugTrace(DEBUG_sched,
 389                        "starting new worker on capability %d", cap->no);
 390             startWorkerTask(cap, workerStart);
 391             return;
 392         }
 393     }
 394
 395     // If we have an unbound thread on the run queue, or if there's
 396     // anything else to do, give the Capability to a worker thread.
 397     if (always_wakeup ||
 398         !emptyRunQueue(cap) || !emptyWakeupQueue(cap) ||
 399         !emptySparkPoolCap(cap) || globalWorkToDo()) {
 400         if (cap->spare_workers) {
 401             giveCapabilityToTask(cap,cap->spare_workers);
 402             // The worker Task pops itself from the queue;
 403             return;
 404         }
 405     }
 406
 407     last_free_capability = cap;
 408     debugTrace(DEBUG_sched, "freeing capability %d", cap->no);
 409 }
 410
 411 void
 412 releaseCapability (Capability* cap USED_IF_THREADS)
 413 {
 414     ACQUIRE_LOCK(&cap->lock);
 415     releaseCapability_(cap, rtsFalse);
 416     RELEASE_LOCK(&cap->lock);
 417 }
 418
 419 void
 420 releaseAndWakeupCapability (Capability* cap USED_IF_THREADS)
 421 {
 422     ACQUIRE_LOCK(&cap->lock);
 423     releaseCapability_(cap, rtsTrue);
 424     RELEASE_LOCK(&cap->lock);
 425 }
 426
 427 static void
 428 releaseCapabilityAndQueueWorker (Capability* cap USED_IF_THREADS)
 429 {
 430     Task *task;
 431
 432     ACQUIRE_LOCK(&cap->lock);
 433
 434     task = cap->running_task;
 435
 436     // If the current task is a worker, save it on the spare_workers
 437     // list of this Capability.  A worker can mark itself as stopped,
 438     // in which case it is not replaced on the spare_worker queue.
 439     // This happens when the system is shutting down (see
 440     // Schedule.c:workerStart()).
 441     // Also, be careful to check that this task hasn't just exited
 442     // Haskell to do a foreign call (task->suspended_tso).
 443     if (!isBoundTask(task) && !task->stopped && !task->suspended_tso) {
 444         task->next = cap->spare_workers;
 445         cap->spare_workers = task;
 446     }
 447     // Bound tasks just float around attached to their TSOs.
 448
 449     releaseCapability_(cap,rtsFalse);
 450
 451     RELEASE_LOCK(&cap->lock);
 452 }
 453 #endif
 454
 455 /* ----------------------------------------------------------------------------
 456  * waitForReturnCapability( Task *task )
 457  *
 458  * Purpose:  when an OS thread returns from an external call,
 459  * it calls waitForReturnCapability() (via Schedule.resumeThread())
 460  * to wait for permission to enter the RTS & communicate the
 461  * result of the external call back to the Haskell thread that
 462  * made it.
 463  *
 464  * ------------------------------------------------------------------------- */
 465 void
 466 waitForReturnCapability (Capability **pCap, Task *task)
 467 {
 468 #if !defined(THREADED_RTS)
 469
 470     MainCapability.running_task = task;
 471     task->cap = &MainCapability;
 472     *pCap = &MainCapability;
 473
 474 #else
 475     Capability *cap = *pCap;
 476
 477     if (cap == NULL) {
 478         // Try last_free_capability first
 479         cap = last_free_capability;
 480         if (!cap->running_task) {
 481             nat i;
 482             // otherwise, search for a free capability
 483             cap = NULL;
 484             for (i = 0; i < n_capabilities; i++) {
 485                 if (!capabilities[i].running_task) {
 486                     cap = &capabilities[i];
 487                     break;
 488                 }
 489             }
 490             if (cap == NULL) {
 491                 // Can't find a free one, use last_free_capability.
 492                 cap = last_free_capability;
 493             }
 494         }
 495
 496         // record the Capability as the one this Task is now assocated with.
 497         task->cap = cap;
 498
 499     } else {
 500         ASSERT(task->cap == cap);
 501     }
 502
 503     ACQUIRE_LOCK(&cap->lock);
 504
 505     debugTrace(DEBUG_sched, "returning; I want capability %d", cap->no);
 506
 507     if (!cap->running_task) {
 508         // It's free; just grab it
 509         cap->running_task = task;
 510         RELEASE_LOCK(&cap->lock);
 511     } else {
 512         newReturningTask(cap,task);
 513         RELEASE_LOCK(&cap->lock);
 514
 515         for (;;) {
 516             ACQUIRE_LOCK(&task->lock);
 517             // task->lock held, cap->lock not held
 518             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 519             cap = task->cap;
 520             task->wakeup = rtsFalse;
 521             RELEASE_LOCK(&task->lock);
 522
 523             // now check whether we should wake up...
 524             ACQUIRE_LOCK(&cap->lock);
 525             if (cap->running_task == NULL) {
 526                 if (cap->returning_tasks_hd != task) {
 527                     giveCapabilityToTask(cap,cap->returning_tasks_hd);
 528                     RELEASE_LOCK(&cap->lock);
 529                     continue;
 530                 }
 531                 cap->running_task = task;
 532                 popReturningTask(cap);
 533                 RELEASE_LOCK(&cap->lock);
 534                 break;
 535             }
 536             RELEASE_LOCK(&cap->lock);
 537         }
 538
 539     }
 540
 541     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 542
 543     debugTrace(DEBUG_sched, "resuming capability %d", cap->no);
 544
 545     *pCap = cap;
 546 #endif
 547 }
 548
 549 #if defined(THREADED_RTS)
 550 /* ----------------------------------------------------------------------------
 551  * yieldCapability
 552  * ------------------------------------------------------------------------- */
 553
 554 void
 555 yieldCapability (Capability** pCap, Task *task)
 556 {
 557     Capability *cap = *pCap;
 558
 559     if (waiting_for_gc == PENDING_GC_PAR) {
 560         debugTrace(DEBUG_sched, "capability %d: becoming a GC thread", cap->no);
 561         postEvent(cap, EVENT_GC_START, 0, 0);
 562         gcWorkerThread(cap);
 563         postEvent(cap, EVENT_GC_END, 0, 0);
 564         return;
 565     }
 566
 567         debugTrace(DEBUG_sched, "giving up capability %d", cap->no);
 568
 569         // We must now release the capability and wait to be woken up
 570         // again.
 571         task->wakeup = rtsFalse;
 572         releaseCapabilityAndQueueWorker(cap);
 573
 574         for (;;) {
 575             ACQUIRE_LOCK(&task->lock);
 576             // task->lock held, cap->lock not held
 577             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 578             cap = task->cap;
 579             task->wakeup = rtsFalse;
 580             RELEASE_LOCK(&task->lock);
 581
 582             debugTrace(DEBUG_sched, "woken up on capability %d", cap->no);
 583
 584             ACQUIRE_LOCK(&cap->lock);
 585             if (cap->running_task != NULL) {
 586                 debugTrace(DEBUG_sched,
 587                            "capability %d is owned by another task", cap->no);
 588                 RELEASE_LOCK(&cap->lock);
 589                 continue;
 590             }
 591
 592             if (task->tso == NULL) {
 593                 ASSERT(cap->spare_workers != NULL);
 594                 // if we're not at the front of the queue, release it
 595                 // again.  This is unlikely to happen.
 596                 if (cap->spare_workers != task) {
 597                     giveCapabilityToTask(cap,cap->spare_workers);
 598                     RELEASE_LOCK(&cap->lock);
 599                     continue;
 600                 }
 601                 cap->spare_workers = task->next;
 602                 task->next = NULL;
 603             }
 604             cap->running_task = task;
 605             RELEASE_LOCK(&cap->lock);
 606             break;
 607         }
 608
 609         debugTrace(DEBUG_sched, "resuming capability %d", cap->no);
 610         ASSERT(cap->running_task == task);
 611
 612     *pCap = cap;
 613
 614     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 615
 616     return;
 617 }
 618
 619 /* ----------------------------------------------------------------------------
 620  * Wake up a thread on a Capability.
 621  *
 622  * This is used when the current Task is running on a Capability and
 623  * wishes to wake up a thread on a different Capability.
 624  * ------------------------------------------------------------------------- */
 625
 626 void
 627 wakeupThreadOnCapability (Capability *my_cap,
 628                           Capability *other_cap,
 629                           StgTSO *tso)
 630 {
 631     ACQUIRE_LOCK(&other_cap->lock);
 632
 633     // ASSUMES: cap->lock is held (asserted in wakeupThreadOnCapability)
 634     if (tso->bound) {
 635         ASSERT(tso->bound->cap == tso->cap);
 636         tso->bound->cap = other_cap;
 637     }
 638     tso->cap = other_cap;
 639
 640     ASSERT(tso->bound ? tso->bound->cap == other_cap : 1);
 641
 642     if (other_cap->running_task == NULL) {
 643         // nobody is running this Capability, we can add our thread
 644         // directly onto the run queue and start up a Task to run it.
 645
 646         other_cap->running_task = myTask();
 647             // precond for releaseCapability_() and appendToRunQueue()
 648
 649         appendToRunQueue(other_cap,tso);
 650
 651         releaseCapability_(other_cap,rtsFalse);
 652     } else {
 653         appendToWakeupQueue(my_cap,other_cap,tso);
 654         other_cap->context_switch = 1;
 655         // someone is running on this Capability, so it cannot be
 656         // freed without first checking the wakeup queue (see
 657         // releaseCapability_).
 658     }
 659
 660     RELEASE_LOCK(&other_cap->lock);
 661 }
 662
 663 /* ----------------------------------------------------------------------------
 664  * prodCapability
 665  *
 666  * If a Capability is currently idle, wake up a Task on it.  Used to
 667  * get every Capability into the GC.
 668  * ------------------------------------------------------------------------- */
 669
 670 void
 671 prodCapability (Capability *cap, Task *task)
 672 {
 673     ACQUIRE_LOCK(&cap->lock);
 674     if (!cap->running_task) {
 675         cap->running_task = task;
 676         releaseCapability_(cap,rtsTrue);
 677     }
 678     RELEASE_LOCK(&cap->lock);
 679 }
 680
 681 /* ----------------------------------------------------------------------------
 682  * shutdownCapability
 683  *
 684  * At shutdown time, we want to let everything exit as cleanly as
 685  * possible.  For each capability, we let its run queue drain, and
 686  * allow the workers to stop.
 687  *
 688  * This function should be called when interrupted and
 689  * shutting_down_scheduler = rtsTrue, thus any worker that wakes up
 690  * will exit the scheduler and call taskStop(), and any bound thread
 691  * that wakes up will return to its caller.  Runnable threads are
 692  * killed.
 693  *
 694  * ------------------------------------------------------------------------- */
 695
 696 void
 697 shutdownCapability (Capability *cap, Task *task, rtsBool safe)
 698 {
 699     nat i;
 700
 701     task->cap = cap;
 702
 703     // Loop indefinitely until all the workers have exited and there
 704     // are no Haskell threads left.  We used to bail out after 50
 705     // iterations of this loop, but that occasionally left a worker
 706     // running which caused problems later (the closeMutex() below
 707     // isn't safe, for one thing).
 708
 709     for (i = 0; /* i < 50 */; i++) {
 710         ASSERT(sched_state == SCHED_SHUTTING_DOWN);
 711
 712         debugTrace(DEBUG_sched,
 713                    "shutting down capability %d, attempt %d", cap->no, i);
 714         ACQUIRE_LOCK(&cap->lock);
 715         if (cap->running_task) {
 716             RELEASE_LOCK(&cap->lock);
 717             debugTrace(DEBUG_sched, "not owner, yielding");
 718             yieldThread();
 719             continue;
 720         }
 721         cap->running_task = task;
 722
 723         if (cap->spare_workers) {
 724             // Look for workers that have died without removing
 725             // themselves from the list; this could happen if the OS
 726             // summarily killed the thread, for example.  This
 727             // actually happens on Windows when the system is
 728             // terminating the program, and the RTS is running in a
 729             // DLL.
 730             Task *t, *prev;
 731             prev = NULL;
 732             for (t = cap->spare_workers; t != NULL; t = t->next) {
 733                 if (!osThreadIsAlive(t->id)) {
 734                     debugTrace(DEBUG_sched,
 735                                "worker thread %p has died unexpectedly", (void *)t->id);
 736                         if (!prev) {
 737                             cap->spare_workers = t->next;
 738                         } else {
 739                             prev->next = t->next;
 740                         }
 741                         prev = t;
 742                 }
 743             }
 744         }
 745
 746         if (!emptyRunQueue(cap) || cap->spare_workers) {
 747             debugTrace(DEBUG_sched,
 748                        "runnable threads or workers still alive, yielding");
 749             releaseCapability_(cap,rtsFalse); // this will wake up a worker
 750             RELEASE_LOCK(&cap->lock);
 751             yieldThread();
 752             continue;
 753         }
 754
 755         // If "safe", then busy-wait for any threads currently doing
 756         // foreign calls.  If we're about to unload this DLL, for
 757         // example, we need to be sure that there are no OS threads
 758         // that will try to return to code that has been unloaded.
 759         // We can be a bit more relaxed when this is a standalone
 760         // program that is about to terminate, and let safe=false.
 761         if (cap->suspended_ccalling_tasks && safe) {
 762             debugTrace(DEBUG_sched,
 763                        "thread(s) are involved in foreign calls, yielding");
 764             cap->running_task = NULL;
 765             RELEASE_LOCK(&cap->lock);
 766             yieldThread();
 767             continue;
 768         }
 769
 770         postEvent(cap, EVENT_SHUTDOWN, 0, 0);
 771         debugTrace(DEBUG_sched, "capability %d is stopped.", cap->no);
 772         RELEASE_LOCK(&cap->lock);
 773         break;
 774     }
 775     // we now have the Capability, its run queue and spare workers
 776     // list are both empty.
 777
 778     // ToDo: we can't drop this mutex, because there might still be
 779     // threads performing foreign calls that will eventually try to
 780     // return via resumeThread() and attempt to grab cap->lock.
 781     // closeMutex(&cap->lock);
 782 }
 783
 784 /* ----------------------------------------------------------------------------
 785  * tryGrabCapability
 786  *
 787  * Attempt to gain control of a Capability if it is free.
 788  *
 789  * ------------------------------------------------------------------------- */
 790
 791 rtsBool
 792 tryGrabCapability (Capability *cap, Task *task)
 793 {
 794     if (cap->running_task != NULL) return rtsFalse;
 795     ACQUIRE_LOCK(&cap->lock);
 796     if (cap->running_task != NULL) {
 797         RELEASE_LOCK(&cap->lock);
 798         return rtsFalse;
 799     }
 800     task->cap = cap;
 801     cap->running_task = task;
 802     RELEASE_LOCK(&cap->lock);
 803     return rtsTrue;
 804 }
 805
 806
 807 #endif /* THREADED_RTS */
 808
 809 static void
 810 freeCapability (Capability *cap)
 811 {
 812     stgFree(cap->mut_lists);
 813 #if defined(THREADED_RTS) || defined(PARALLEL_HASKELL)
 814     freeSparkPool(cap->sparks);
 815 #endif
 816 }
 817
 818 void
 819 freeCapabilities (void)
 820 {
 821 #if defined(THREADED_RTS)
 822     nat i;
 823     for (i=0; i < n_capabilities; i++) {
 824         freeCapability(&capabilities[i]);
 825     }
 826 #else
 827     freeCapability(&MainCapability);
 828 #endif
 829 }
 830
 831 /* ---------------------------------------------------------------------------
 832    Mark everything directly reachable from the Capabilities.  When
 833    using multiple GC threads, each GC thread marks all Capabilities
 834    for which (c `mod` n == 0), for Capability c and thread n.
 835    ------------------------------------------------------------------------ */
 836
 837 void
 838 markSomeCapabilities (evac_fn evac, void *user, nat i0, nat delta,
 839                       rtsBool prune_sparks USED_IF_THREADS)
 840 {
 841     nat i;
 842     Capability *cap;
 843     Task *task;
 844
 845     // Each GC thread is responsible for following roots from the
 846     // Capability of the same number.  There will usually be the same
 847     // or fewer Capabilities as GC threads, but just in case there
 848     // are more, we mark every Capability whose number is the GC
 849     // thread's index plus a multiple of the number of GC threads.
 850     for (i = i0; i < n_capabilities; i += delta) {
 851         cap = &capabilities[i];
 852         evac(user, (StgClosure **)(void *)&cap->run_queue_hd);
 853         evac(user, (StgClosure **)(void *)&cap->run_queue_tl);
 854 #if defined(THREADED_RTS)
 855         evac(user, (StgClosure **)(void *)&cap->wakeup_queue_hd);
 856         evac(user, (StgClosure **)(void *)&cap->wakeup_queue_tl);
 857 #endif
 858         for (task = cap->suspended_ccalling_tasks; task != NULL;
 859              task=task->next) {
 860             debugTrace(DEBUG_sched,
 861                        "evac'ing suspended TSO %lu", (unsigned long)task->suspended_tso->id);
 862             evac(user, (StgClosure **)(void *)&task->suspended_tso);
 863         }
 864
 865 #if defined(THREADED_RTS)
 866         if (prune_sparks) {
 867             pruneSparkQueue (evac, user, cap);
 868         } else {
 869             traverseSparkQueue (evac, user, cap);
 870         }
 871 #endif
 872     }
 873
 874 #if !defined(THREADED_RTS)
 875     evac(user, (StgClosure **)(void *)&blocked_queue_hd);
 876     evac(user, (StgClosure **)(void *)&blocked_queue_tl);
 877     evac(user, (StgClosure **)(void *)&sleeping_queue);
 878 #endif
 879 }
 880
 881 void
 882 markCapabilities (evac_fn evac, void *user)
 883 {
 884     markSomeCapabilities(evac, user, 0, 1, rtsFalse);
 885 }