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     capabilities[i].context_switch = 1;
 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     trace(TRACE_sched | DEBUG_sched,
 324           "passing capability %d to %s %p",
 325           cap->no, task->tso ? "bound task" : "worker",
 326           (void *)task->id);
 327     ACQUIRE_LOCK(&task->lock);
 328     task->wakeup = rtsTrue;
 329     // the wakeup flag is needed because signalCondition() doesn't
 330     // flag the condition if the thread is already runniing, but we want
 331     // it to be sticky.
 332     signalCondition(&task->cond);
 333     RELEASE_LOCK(&task->lock);
 334 }
 335 #endif
 336
 337 /* ----------------------------------------------------------------------------
 338  * Function:  releaseCapability(Capability*)
 339  *
 340  * Purpose:   Letting go of a capability. Causes a
 341  *            'returning worker' thread or a 'waiting worker'
 342  *            to wake up, in that order.
 343  * ------------------------------------------------------------------------- */
 344
 345 #if defined(THREADED_RTS)
 346 void
 347 releaseCapability_ (Capability* cap,
 348                     rtsBool always_wakeup)
 349 {
 350     Task *task;
 351
 352     task = cap->running_task;
 353
 354     ASSERT_PARTIAL_CAPABILITY_INVARIANTS(cap,task);
 355
 356     cap->running_task = NULL;
 357
 358     // Check to see whether a worker thread can be given
 359     // the go-ahead to return the result of an external call..
 360     if (cap->returning_tasks_hd != NULL) {
 361         giveCapabilityToTask(cap,cap->returning_tasks_hd);
 362         // The Task pops itself from the queue (see waitForReturnCapability())
 363         return;
 364     }
 365
 366     if (waiting_for_gc == PENDING_GC_SEQ) {
 367       last_free_capability = cap; // needed?
 368       trace(TRACE_sched | DEBUG_sched,
 369             "GC pending, set capability %d free", cap->no);
 370       return;
 371     }
 372
 373
 374     // If the next thread on the run queue is a bound thread,
 375     // give this Capability to the appropriate Task.
 376     if (!emptyRunQueue(cap) && cap->run_queue_hd->bound) {
 377         // Make sure we're not about to try to wake ourselves up
 378         ASSERT(task != cap->run_queue_hd->bound);
 379         task = cap->run_queue_hd->bound;
 380         giveCapabilityToTask(cap,task);
 381         return;
 382     }
 383
 384     if (!cap->spare_workers) {
 385         // Create a worker thread if we don't have one.  If the system
 386         // is interrupted, we only create a worker task if there
 387         // are threads that need to be completed.  If the system is
 388         // shutting down, we never create a new worker.
 389         if (sched_state < SCHED_SHUTTING_DOWN || !emptyRunQueue(cap)) {
 390             debugTrace(DEBUG_sched,
 391                        "starting new worker on capability %d", cap->no);
 392             startWorkerTask(cap, workerStart);
 393             return;
 394         }
 395     }
 396
 397     // If we have an unbound thread on the run queue, or if there's
 398     // anything else to do, give the Capability to a worker thread.
 399     if (always_wakeup ||
 400         !emptyRunQueue(cap) || !emptyWakeupQueue(cap) ||
 401         !emptySparkPoolCap(cap) || globalWorkToDo()) {
 402         if (cap->spare_workers) {
 403             giveCapabilityToTask(cap,cap->spare_workers);
 404             // The worker Task pops itself from the queue;
 405             return;
 406         }
 407     }
 408
 409     last_free_capability = cap;
 410     trace(TRACE_sched | DEBUG_sched, "freeing capability %d", cap->no);
 411 }
 412
 413 void
 414 releaseCapability (Capability* cap USED_IF_THREADS)
 415 {
 416     ACQUIRE_LOCK(&cap->lock);
 417     releaseCapability_(cap, rtsFalse);
 418     RELEASE_LOCK(&cap->lock);
 419 }
 420
 421 void
 422 releaseAndWakeupCapability (Capability* cap USED_IF_THREADS)
 423 {
 424     ACQUIRE_LOCK(&cap->lock);
 425     releaseCapability_(cap, rtsTrue);
 426     RELEASE_LOCK(&cap->lock);
 427 }
 428
 429 static void
 430 releaseCapabilityAndQueueWorker (Capability* cap USED_IF_THREADS)
 431 {
 432     Task *task;
 433
 434     ACQUIRE_LOCK(&cap->lock);
 435
 436     task = cap->running_task;
 437
 438     // If the current task is a worker, save it on the spare_workers
 439     // list of this Capability.  A worker can mark itself as stopped,
 440     // in which case it is not replaced on the spare_worker queue.
 441     // This happens when the system is shutting down (see
 442     // Schedule.c:workerStart()).
 443     // Also, be careful to check that this task hasn't just exited
 444     // Haskell to do a foreign call (task->suspended_tso).
 445     if (!isBoundTask(task) && !task->stopped && !task->suspended_tso) {
 446         task->next = cap->spare_workers;
 447         cap->spare_workers = task;
 448     }
 449     // Bound tasks just float around attached to their TSOs.
 450
 451     releaseCapability_(cap,rtsFalse);
 452
 453     RELEASE_LOCK(&cap->lock);
 454 }
 455 #endif
 456
 457 /* ----------------------------------------------------------------------------
 458  * waitForReturnCapability( Task *task )
 459  *
 460  * Purpose:  when an OS thread returns from an external call,
 461  * it calls waitForReturnCapability() (via Schedule.resumeThread())
 462  * to wait for permission to enter the RTS & communicate the
 463  * result of the external call back to the Haskell thread that
 464  * made it.
 465  *
 466  * ------------------------------------------------------------------------- */
 467 void
 468 waitForReturnCapability (Capability **pCap, Task *task)
 469 {
 470 #if !defined(THREADED_RTS)
 471
 472     MainCapability.running_task = task;
 473     task->cap = &MainCapability;
 474     *pCap = &MainCapability;
 475
 476 #else
 477     Capability *cap = *pCap;
 478
 479     if (cap == NULL) {
 480         // Try last_free_capability first
 481         cap = last_free_capability;
 482         if (!cap->running_task) {
 483             nat i;
 484             // otherwise, search for a free capability
 485             for (i = 0; i < n_capabilities; i++) {
 486                 cap = &capabilities[i];
 487                 if (!cap->running_task) {
 488                     break;
 489                 }
 490             }
 491             // Can't find a free one, use last_free_capability.
 492             cap = last_free_capability;
 493         }
 494
 495         // record the Capability as the one this Task is now assocated with.
 496         task->cap = cap;
 497
 498     } else {
 499         ASSERT(task->cap == cap);
 500     }
 501
 502     ACQUIRE_LOCK(&cap->lock);
 503
 504     debugTrace(DEBUG_sched, "returning; I want capability %d", cap->no);
 505
 506     if (!cap->running_task) {
 507         // It's free; just grab it
 508         cap->running_task = task;
 509         RELEASE_LOCK(&cap->lock);
 510     } else {
 511         newReturningTask(cap,task);
 512         RELEASE_LOCK(&cap->lock);
 513
 514         for (;;) {
 515             ACQUIRE_LOCK(&task->lock);
 516             // task->lock held, cap->lock not held
 517             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 518             cap = task->cap;
 519             task->wakeup = rtsFalse;
 520             RELEASE_LOCK(&task->lock);
 521
 522             // now check whether we should wake up...
 523             ACQUIRE_LOCK(&cap->lock);
 524             if (cap->running_task == NULL) {
 525                 if (cap->returning_tasks_hd != task) {
 526                     giveCapabilityToTask(cap,cap->returning_tasks_hd);
 527                     RELEASE_LOCK(&cap->lock);
 528                     continue;
 529                 }
 530                 cap->running_task = task;
 531                 popReturningTask(cap);
 532                 RELEASE_LOCK(&cap->lock);
 533                 break;
 534             }
 535             RELEASE_LOCK(&cap->lock);
 536         }
 537
 538     }
 539
 540     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 541
 542     trace(TRACE_sched | DEBUG_sched, "resuming capability %d", cap->no);
 543
 544     *pCap = cap;
 545 #endif
 546 }
 547
 548 #if defined(THREADED_RTS)
 549 /* ----------------------------------------------------------------------------
 550  * yieldCapability
 551  * ------------------------------------------------------------------------- */
 552
 553 void
 554 yieldCapability (Capability** pCap, Task *task)
 555 {
 556     Capability *cap = *pCap;
 557
 558     if (waiting_for_gc == PENDING_GC_PAR) {
 559         debugTrace(DEBUG_sched, "capability %d: becoming a GC thread", cap->no);
 560         gcWorkerThread(cap);
 561         return;
 562     }
 563
 564         debugTrace(DEBUG_sched, "giving up capability %d", cap->no);
 565
 566         // We must now release the capability and wait to be woken up
 567         // again.
 568         task->wakeup = rtsFalse;
 569         releaseCapabilityAndQueueWorker(cap);
 570
 571         for (;;) {
 572             ACQUIRE_LOCK(&task->lock);
 573             // task->lock held, cap->lock not held
 574             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 575             cap = task->cap;
 576             task->wakeup = rtsFalse;
 577             RELEASE_LOCK(&task->lock);
 578
 579             debugTrace(DEBUG_sched, "woken up on capability %d", cap->no);
 580
 581             ACQUIRE_LOCK(&cap->lock);
 582             if (cap->running_task != NULL) {
 583                 debugTrace(DEBUG_sched,
 584                            "capability %d is owned by another task", cap->no);
 585                 RELEASE_LOCK(&cap->lock);
 586                 continue;
 587             }
 588
 589             if (task->tso == NULL) {
 590                 ASSERT(cap->spare_workers != NULL);
 591                 // if we're not at the front of the queue, release it
 592                 // again.  This is unlikely to happen.
 593                 if (cap->spare_workers != task) {
 594                     giveCapabilityToTask(cap,cap->spare_workers);
 595                     RELEASE_LOCK(&cap->lock);
 596                     continue;
 597                 }
 598                 cap->spare_workers = task->next;
 599                 task->next = NULL;
 600             }
 601             cap->running_task = task;
 602             RELEASE_LOCK(&cap->lock);
 603             break;
 604         }
 605
 606         trace(TRACE_sched | DEBUG_sched, "resuming capability %d", cap->no);
 607         ASSERT(cap->running_task == task);
 608
 609     *pCap = cap;
 610
 611     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 612
 613     return;
 614 }
 615
 616 /* ----------------------------------------------------------------------------
 617  * Wake up a thread on a Capability.
 618  *
 619  * This is used when the current Task is running on a Capability and
 620  * wishes to wake up a thread on a different Capability.
 621  * ------------------------------------------------------------------------- */
 622
 623 void
 624 wakeupThreadOnCapability (Capability *my_cap,
 625                           Capability *other_cap,
 626                           StgTSO *tso)
 627 {
 628     ACQUIRE_LOCK(&other_cap->lock);
 629
 630     // ASSUMES: cap->lock is held (asserted in wakeupThreadOnCapability)
 631     if (tso->bound) {
 632         ASSERT(tso->bound->cap == tso->cap);
 633         tso->bound->cap = other_cap;
 634     }
 635     tso->cap = other_cap;
 636
 637     ASSERT(tso->bound ? tso->bound->cap == other_cap : 1);
 638
 639     if (other_cap->running_task == NULL) {
 640         // nobody is running this Capability, we can add our thread
 641         // directly onto the run queue and start up a Task to run it.
 642
 643         other_cap->running_task = myTask();
 644             // precond for releaseCapability_() and appendToRunQueue()
 645
 646         appendToRunQueue(other_cap,tso);
 647
 648         trace(TRACE_sched, "resuming capability %d", other_cap->no);
 649         releaseCapability_(other_cap,rtsFalse);
 650     } else {
 651         appendToWakeupQueue(my_cap,other_cap,tso);
 652         other_cap->context_switch = 1;
 653         // someone is running on this Capability, so it cannot be
 654         // freed without first checking the wakeup queue (see
 655         // releaseCapability_).
 656     }
 657
 658     RELEASE_LOCK(&other_cap->lock);
 659 }
 660
 661 /* ----------------------------------------------------------------------------
 662  * prodCapability
 663  *
 664  * If a Capability is currently idle, wake up a Task on it.  Used to
 665  * get every Capability into the GC.
 666  * ------------------------------------------------------------------------- */
 667
 668 void
 669 prodCapability (Capability *cap, Task *task)
 670 {
 671     ACQUIRE_LOCK(&cap->lock);
 672     if (!cap->running_task) {
 673         cap->running_task = task;
 674         releaseCapability_(cap,rtsTrue);
 675     }
 676     RELEASE_LOCK(&cap->lock);
 677 }
 678
 679 /* ----------------------------------------------------------------------------
 680  * shutdownCapability
 681  *
 682  * At shutdown time, we want to let everything exit as cleanly as
 683  * possible.  For each capability, we let its run queue drain, and
 684  * allow the workers to stop.
 685  *
 686  * This function should be called when interrupted and
 687  * shutting_down_scheduler = rtsTrue, thus any worker that wakes up
 688  * will exit the scheduler and call taskStop(), and any bound thread
 689  * that wakes up will return to its caller.  Runnable threads are
 690  * killed.
 691  *
 692  * ------------------------------------------------------------------------- */
 693
 694 void
 695 shutdownCapability (Capability *cap, Task *task, rtsBool safe)
 696 {
 697     nat i;
 698
 699     task->cap = cap;
 700
 701     // Loop indefinitely until all the workers have exited and there
 702     // are no Haskell threads left.  We used to bail out after 50
 703     // iterations of this loop, but that occasionally left a worker
 704     // running which caused problems later (the closeMutex() below
 705     // isn't safe, for one thing).
 706
 707     for (i = 0; /* i < 50 */; i++) {
 708         ASSERT(sched_state == SCHED_SHUTTING_DOWN);
 709
 710         debugTrace(DEBUG_sched,
 711                    "shutting down capability %d, attempt %d", cap->no, i);
 712         ACQUIRE_LOCK(&cap->lock);
 713         if (cap->running_task) {
 714             RELEASE_LOCK(&cap->lock);
 715             debugTrace(DEBUG_sched, "not owner, yielding");
 716             yieldThread();
 717             continue;
 718         }
 719         cap->running_task = task;
 720
 721         if (cap->spare_workers) {
 722             // Look for workers that have died without removing
 723             // themselves from the list; this could happen if the OS
 724             // summarily killed the thread, for example.  This
 725             // actually happens on Windows when the system is
 726             // terminating the program, and the RTS is running in a
 727             // DLL.
 728             Task *t, *prev;
 729             prev = NULL;
 730             for (t = cap->spare_workers; t != NULL; t = t->next) {
 731                 if (!osThreadIsAlive(t->id)) {
 732                     debugTrace(DEBUG_sched,
 733                                "worker thread %p has died unexpectedly", (void *)t->id);
 734                         if (!prev) {
 735                             cap->spare_workers = t->next;
 736                         } else {
 737                             prev->next = t->next;
 738                         }
 739                         prev = t;
 740                 }
 741             }
 742         }
 743
 744         if (!emptyRunQueue(cap) || cap->spare_workers) {
 745             debugTrace(DEBUG_sched,
 746                        "runnable threads or workers still alive, yielding");
 747             releaseCapability_(cap,rtsFalse); // this will wake up a worker
 748             RELEASE_LOCK(&cap->lock);
 749             yieldThread();
 750             continue;
 751         }
 752
 753         // If "safe", then busy-wait for any threads currently doing
 754         // foreign calls.  If we're about to unload this DLL, for
 755         // example, we need to be sure that there are no OS threads
 756         // that will try to return to code that has been unloaded.
 757         // We can be a bit more relaxed when this is a standalone
 758         // program that is about to terminate, and let safe=false.
 759         if (cap->suspended_ccalling_tasks && safe) {
 760             debugTrace(DEBUG_sched,
 761                        "thread(s) are involved in foreign calls, yielding");
 762             cap->running_task = NULL;
 763             RELEASE_LOCK(&cap->lock);
 764             yieldThread();
 765             continue;
 766         }
 767
 768         debugTrace(DEBUG_sched, "capability %d is stopped.", cap->no);
 769         RELEASE_LOCK(&cap->lock);
 770         break;
 771     }
 772     // we now have the Capability, its run queue and spare workers
 773     // list are both empty.
 774
 775     // ToDo: we can't drop this mutex, because there might still be
 776     // threads performing foreign calls that will eventually try to
 777     // return via resumeThread() and attempt to grab cap->lock.
 778     // closeMutex(&cap->lock);
 779 }
 780
 781 /* ----------------------------------------------------------------------------
 782  * tryGrabCapability
 783  *
 784  * Attempt to gain control of a Capability if it is free.
 785  *
 786  * ------------------------------------------------------------------------- */
 787
 788 rtsBool
 789 tryGrabCapability (Capability *cap, Task *task)
 790 {
 791     if (cap->running_task != NULL) return rtsFalse;
 792     ACQUIRE_LOCK(&cap->lock);
 793     if (cap->running_task != NULL) {
 794         RELEASE_LOCK(&cap->lock);
 795         return rtsFalse;
 796     }
 797     task->cap = cap;
 798     cap->running_task = task;
 799     RELEASE_LOCK(&cap->lock);
 800     return rtsTrue;
 801 }
 802
 803
 804 #endif /* THREADED_RTS */
 805
 806 static void
 807 freeCapability (Capability *cap)
 808 {
 809     stgFree(cap->mut_lists);
 810 #if defined(THREADED_RTS) || defined(PARALLEL_HASKELL)
 811     freeSparkPool(cap->sparks);
 812 #endif
 813 }
 814
 815 void
 816 freeCapabilities (void)
 817 {
 818 #if defined(THREADED_RTS)
 819     nat i;
 820     for (i=0; i < n_capabilities; i++) {
 821         freeCapability(&capabilities[i]);
 822     }
 823 #else
 824     freeCapability(&MainCapability);
 825 #endif
 826 }
 827
 828 /* ---------------------------------------------------------------------------
 829    Mark everything directly reachable from the Capabilities.  When
 830    using multiple GC threads, each GC thread marks all Capabilities
 831    for which (c `mod` n == 0), for Capability c and thread n.
 832    ------------------------------------------------------------------------ */
 833
 834 void
 835 markSomeCapabilities (evac_fn evac, void *user, nat i0, nat delta,
 836                       rtsBool prune_sparks USED_IF_THREADS)
 837 {
 838     nat i;
 839     Capability *cap;
 840     Task *task;
 841
 842     // Each GC thread is responsible for following roots from the
 843     // Capability of the same number.  There will usually be the same
 844     // or fewer Capabilities as GC threads, but just in case there
 845     // are more, we mark every Capability whose number is the GC
 846     // thread's index plus a multiple of the number of GC threads.
 847     for (i = i0; i < n_capabilities; i += delta) {
 848         cap = &capabilities[i];
 849         evac(user, (StgClosure **)(void *)&cap->run_queue_hd);
 850         evac(user, (StgClosure **)(void *)&cap->run_queue_tl);
 851 #if defined(THREADED_RTS)
 852         evac(user, (StgClosure **)(void *)&cap->wakeup_queue_hd);
 853         evac(user, (StgClosure **)(void *)&cap->wakeup_queue_tl);
 854 #endif
 855         for (task = cap->suspended_ccalling_tasks; task != NULL;
 856              task=task->next) {
 857             debugTrace(DEBUG_sched,
 858                        "evac'ing suspended TSO %lu", (unsigned long)task->suspended_tso->id);
 859             evac(user, (StgClosure **)(void *)&task->suspended_tso);
 860         }
 861
 862 #if defined(THREADED_RTS)
 863         if (prune_sparks) {
 864             pruneSparkQueue (evac, user, cap);
 865         } else {
 866             traverseSparkQueue (evac, user, cap);
 867         }
 868 #endif
 869     }
 870
 871 #if !defined(THREADED_RTS)
 872     evac(user, (StgClosure **)(void *)&blocked_queue_hd);
 873     evac(user, (StgClosure **)(void *)&blocked_queue_tl);
 874     evac(user, (StgClosure **)(void *)&sleeping_queue);
 875 #endif
 876 }
 877
 878 void
 879 markCapabilities (evac_fn evac, void *user)
 880 {
 881     markSomeCapabilities(evac, user, 0, 1, rtsFalse);
 882 }