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
  30 // one global capability, this is the Capability for non-threaded
  31 // builds, and for +RTS -N1
  32 Capability MainCapability;
  33
  34 nat n_capabilities;
  35 Capability *capabilities = NULL;
  36
  37 // Holds the Capability which last became free.  This is used so that
  38 // an in-call has a chance of quickly finding a free Capability.
  39 // Maintaining a global free list of Capabilities would require global
  40 // locking, so we don't do that.
  41 Capability *last_free_capability;
  42
  43 /* GC indicator, in scope for the scheduler, init'ed to false */
  44 volatile StgWord waiting_for_gc = 0;
  45
  46 #if defined(THREADED_RTS)
  47 STATIC_INLINE rtsBool
  48 globalWorkToDo (void)
  49 {
  50     return blackholes_need_checking
  51         || sched_state >= SCHED_INTERRUPTING
  52         ;
  53 }
  54 #endif
  55
  56 #if defined(THREADED_RTS)
  57 rtsBool
  58 stealWork (Capability *cap)
  59 {
  60   /* use the normal Sparks.h interface (internally modified to enable
  61      concurrent stealing)
  62      and immediately turn the spark into a thread when successful
  63   */
  64   Capability *robbed;
  65   StgClosurePtr spark;
  66   rtsBool success = rtsFalse;
  67   rtsBool retry;
  68   nat i = 0;
  69
  70   debugTrace(DEBUG_sched,
  71              "cap %d: Trying to steal work from other capabilities",
  72              cap->no);
  73
  74   if (n_capabilities == 1) { return rtsFalse; } // makes no sense...
  75
  76   do {
  77       retry = rtsFalse;
  78
  79       /* visit cap.s 0..n-1 in sequence until a theft succeeds. We could
  80       start at a random place instead of 0 as well.  */
  81       for ( i=0 ; i < n_capabilities ; i++ ) {
  82           robbed = &capabilities[i];
  83           if (cap == robbed)  // ourselves...
  84               continue;
  85
  86           if (emptySparkPoolCap(robbed)) // nothing to steal here
  87               continue;
  88
  89           spark = tryStealSpark(robbed->sparks);
  90           if (spark == NULL && !emptySparkPoolCap(robbed)) {
  91               // we conflicted with another thread while trying to steal;
  92               // try again later.
  93               retry = rtsTrue;
  94           }
  95
  96           if (spark != NULL) {
  97               debugTrace(DEBUG_sched,
  98                  "cap %d: Stole a spark from capability %d",
  99                          cap->no, robbed->no);
 100
 101               createSparkThread(cap,spark);
 102               return rtsTrue;
 103           }
 104           // otherwise: no success, try next one
 105       }
 106   } while (retry);
 107
 108   debugTrace(DEBUG_sched, "No sparks stolen");
 109   return rtsFalse;
 110 }
 111 #endif
 112
 113 /* -----------------------------------------------------------------------------
 114  * Manage the returning_tasks lists.
 115  *
 116  * These functions require cap->lock
 117  * -------------------------------------------------------------------------- */
 118
 119 #if defined(THREADED_RTS)
 120 STATIC_INLINE void
 121 newReturningTask (Capability *cap, Task *task)
 122 {
 123     ASSERT_LOCK_HELD(&cap->lock);
 124     ASSERT(task->return_link == NULL);
 125     if (cap->returning_tasks_hd) {
 126         ASSERT(cap->returning_tasks_tl->return_link == NULL);
 127         cap->returning_tasks_tl->return_link = task;
 128     } else {
 129         cap->returning_tasks_hd = task;
 130     }
 131     cap->returning_tasks_tl = task;
 132 }
 133
 134 STATIC_INLINE Task *
 135 popReturningTask (Capability *cap)
 136 {
 137     ASSERT_LOCK_HELD(&cap->lock);
 138     Task *task;
 139     task = cap->returning_tasks_hd;
 140     ASSERT(task);
 141     cap->returning_tasks_hd = task->return_link;
 142     if (!cap->returning_tasks_hd) {
 143         cap->returning_tasks_tl = NULL;
 144     }
 145     task->return_link = NULL;
 146     return task;
 147 }
 148 #endif
 149
 150 /* ----------------------------------------------------------------------------
 151  * Initialisation
 152  *
 153  * The Capability is initially marked not free.
 154  * ------------------------------------------------------------------------- */
 155
 156 static void
 157 initCapability( Capability *cap, nat i )
 158 {
 159     nat g;
 160
 161     cap->no = i;
 162     cap->in_haskell        = rtsFalse;
 163
 164     cap->run_queue_hd      = END_TSO_QUEUE;
 165     cap->run_queue_tl      = END_TSO_QUEUE;
 166
 167 #if defined(THREADED_RTS)
 168     initMutex(&cap->lock);
 169     cap->running_task      = NULL; // indicates cap is free
 170     cap->spare_workers     = NULL;
 171     cap->suspended_ccalling_tasks = NULL;
 172     cap->returning_tasks_hd = NULL;
 173     cap->returning_tasks_tl = NULL;
 174     cap->wakeup_queue_hd    = END_TSO_QUEUE;
 175     cap->wakeup_queue_tl    = END_TSO_QUEUE;
 176     cap->sparks_created     = 0;
 177     cap->sparks_converted   = 0;
 178     cap->sparks_pruned      = 0;
 179 #endif
 180
 181     cap->f.stgGCEnter1     = (F_)__stg_gc_enter_1;
 182     cap->f.stgGCFun        = (F_)__stg_gc_fun;
 183
 184     cap->mut_lists  = stgMallocBytes(sizeof(bdescr *) *
 185                                      RtsFlags.GcFlags.generations,
 186                                      "initCapability");
 187
 188     for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
 189         cap->mut_lists[g] = NULL;
 190     }
 191
 192     cap->free_tvar_watch_queues = END_STM_WATCH_QUEUE;
 193     cap->free_invariant_check_queues = END_INVARIANT_CHECK_QUEUE;
 194     cap->free_trec_chunks = END_STM_CHUNK_LIST;
 195     cap->free_trec_headers = NO_TREC;
 196     cap->transaction_tokens = 0;
 197     cap->context_switch = 0;
 198 }
 199
 200 /* ---------------------------------------------------------------------------
 201  * Function:  initCapabilities()
 202  *
 203  * Purpose:   set up the Capability handling. For the THREADED_RTS build,
 204  *            we keep a table of them, the size of which is
 205  *            controlled by the user via the RTS flag -N.
 206  *
 207  * ------------------------------------------------------------------------- */
 208 void
 209 initCapabilities( void )
 210 {
 211 #if defined(THREADED_RTS)
 212     nat i;
 213
 214 #ifndef REG_Base
 215     // We can't support multiple CPUs if BaseReg is not a register
 216     if (RtsFlags.ParFlags.nNodes > 1) {
 217         errorBelch("warning: multiple CPUs not supported in this build, reverting to 1");
 218         RtsFlags.ParFlags.nNodes = 1;
 219     }
 220 #endif
 221
 222     n_capabilities = RtsFlags.ParFlags.nNodes;
 223
 224     if (n_capabilities == 1) {
 225         capabilities = &MainCapability;
 226         // THREADED_RTS must work on builds that don't have a mutable
 227         // BaseReg (eg. unregisterised), so in this case
 228         // capabilities[0] must coincide with &MainCapability.
 229     } else {
 230         capabilities = stgMallocBytes(n_capabilities * sizeof(Capability),
 231                                       "initCapabilities");
 232     }
 233
 234     for (i = 0; i < n_capabilities; i++) {
 235         initCapability(&capabilities[i], i);
 236     }
 237
 238     debugTrace(DEBUG_sched, "allocated %d capabilities", n_capabilities);
 239
 240 #else /* !THREADED_RTS */
 241
 242     n_capabilities = 1;
 243     capabilities = &MainCapability;
 244     initCapability(&MainCapability, 0);
 245
 246 #endif
 247
 248     // There are no free capabilities to begin with.  We will start
 249     // a worker Task to each Capability, which will quickly put the
 250     // Capability on the free list when it finds nothing to do.
 251     last_free_capability = &capabilities[0];
 252 }
 253
 254 /* ----------------------------------------------------------------------------
 255  * setContextSwitches: cause all capabilities to context switch as
 256  * soon as possible.
 257  * ------------------------------------------------------------------------- */
 258
 259 void setContextSwitches(void)
 260 {
 261   nat i;
 262   for (i=0; i < n_capabilities; i++) {
 263     capabilities[i].context_switch = 1;
 264   }
 265 }
 266
 267 /* ----------------------------------------------------------------------------
 268  * Give a Capability to a Task.  The task must currently be sleeping
 269  * on its condition variable.
 270  *
 271  * Requires cap->lock (modifies cap->running_task).
 272  *
 273  * When migrating a Task, the migrater must take task->lock before
 274  * modifying task->cap, to synchronise with the waking up Task.
 275  * Additionally, the migrater should own the Capability (when
 276  * migrating the run queue), or cap->lock (when migrating
 277  * returning_workers).
 278  *
 279  * ------------------------------------------------------------------------- */
 280
 281 #if defined(THREADED_RTS)
 282 STATIC_INLINE void
 283 giveCapabilityToTask (Capability *cap USED_IF_DEBUG, Task *task)
 284 {
 285     ASSERT_LOCK_HELD(&cap->lock);
 286     ASSERT(task->cap == cap);
 287     trace(TRACE_sched | DEBUG_sched,
 288           "passing capability %d to %s %p",
 289           cap->no, task->tso ? "bound task" : "worker",
 290           (void *)task->id);
 291     ACQUIRE_LOCK(&task->lock);
 292     task->wakeup = rtsTrue;
 293     // the wakeup flag is needed because signalCondition() doesn't
 294     // flag the condition if the thread is already runniing, but we want
 295     // it to be sticky.
 296     signalCondition(&task->cond);
 297     RELEASE_LOCK(&task->lock);
 298 }
 299 #endif
 300
 301 /* ----------------------------------------------------------------------------
 302  * Function:  releaseCapability(Capability*)
 303  *
 304  * Purpose:   Letting go of a capability. Causes a
 305  *            'returning worker' thread or a 'waiting worker'
 306  *            to wake up, in that order.
 307  * ------------------------------------------------------------------------- */
 308
 309 #if defined(THREADED_RTS)
 310 void
 311 releaseCapability_ (Capability* cap,
 312                     rtsBool always_wakeup)
 313 {
 314     Task *task;
 315
 316     task = cap->running_task;
 317
 318     ASSERT_PARTIAL_CAPABILITY_INVARIANTS(cap,task);
 319
 320     cap->running_task = NULL;
 321
 322     // Check to see whether a worker thread can be given
 323     // the go-ahead to return the result of an external call..
 324     if (cap->returning_tasks_hd != NULL) {
 325         giveCapabilityToTask(cap,cap->returning_tasks_hd);
 326         // The Task pops itself from the queue (see waitForReturnCapability())
 327         return;
 328     }
 329
 330     /* if waiting_for_gc was the reason to release the cap: thread
 331        comes from yieldCap->releaseAndQueueWorker. Unconditionally set
 332        cap. free and return (see default after the if-protected other
 333        special cases). Thread will wait on cond.var and re-acquire the
 334        same cap after GC (GC-triggering cap. calls releaseCap and
 335        enters the spare_workers case)
 336     */
 337     if (waiting_for_gc) {
 338       last_free_capability = cap; // needed?
 339       trace(TRACE_sched | DEBUG_sched,
 340             "GC pending, set capability %d free", cap->no);
 341       return;
 342     }
 343
 344
 345     // If the next thread on the run queue is a bound thread,
 346     // give this Capability to the appropriate Task.
 347     if (!emptyRunQueue(cap) && cap->run_queue_hd->bound) {
 348         // Make sure we're not about to try to wake ourselves up
 349         ASSERT(task != cap->run_queue_hd->bound);
 350         task = cap->run_queue_hd->bound;
 351         giveCapabilityToTask(cap,task);
 352         return;
 353     }
 354
 355     if (!cap->spare_workers) {
 356         // Create a worker thread if we don't have one.  If the system
 357         // is interrupted, we only create a worker task if there
 358         // are threads that need to be completed.  If the system is
 359         // shutting down, we never create a new worker.
 360         if (sched_state < SCHED_SHUTTING_DOWN || !emptyRunQueue(cap)) {
 361             debugTrace(DEBUG_sched,
 362                        "starting new worker on capability %d", cap->no);
 363             startWorkerTask(cap, workerStart);
 364             return;
 365         }
 366     }
 367
 368     // If we have an unbound thread on the run queue, or if there's
 369     // anything else to do, give the Capability to a worker thread.
 370     if (always_wakeup ||
 371         !emptyRunQueue(cap) || !emptyWakeupQueue(cap) ||
 372         !emptySparkPoolCap(cap) || globalWorkToDo()) {
 373         if (cap->spare_workers) {
 374             giveCapabilityToTask(cap,cap->spare_workers);
 375             // The worker Task pops itself from the queue;
 376             return;
 377         }
 378     }
 379
 380     last_free_capability = cap;
 381     trace(TRACE_sched | DEBUG_sched, "freeing capability %d", cap->no);
 382 }
 383
 384 void
 385 releaseCapability (Capability* cap USED_IF_THREADS)
 386 {
 387     ACQUIRE_LOCK(&cap->lock);
 388     releaseCapability_(cap, rtsFalse);
 389     RELEASE_LOCK(&cap->lock);
 390 }
 391
 392 void
 393 releaseAndWakeupCapability (Capability* cap USED_IF_THREADS)
 394 {
 395     ACQUIRE_LOCK(&cap->lock);
 396     releaseCapability_(cap, rtsTrue);
 397     RELEASE_LOCK(&cap->lock);
 398 }
 399
 400 static void
 401 releaseCapabilityAndQueueWorker (Capability* cap USED_IF_THREADS)
 402 {
 403     Task *task;
 404
 405     ACQUIRE_LOCK(&cap->lock);
 406
 407     task = cap->running_task;
 408
 409     // If the current task is a worker, save it on the spare_workers
 410     // list of this Capability.  A worker can mark itself as stopped,
 411     // in which case it is not replaced on the spare_worker queue.
 412     // This happens when the system is shutting down (see
 413     // Schedule.c:workerStart()).
 414     // Also, be careful to check that this task hasn't just exited
 415     // Haskell to do a foreign call (task->suspended_tso).
 416     if (!isBoundTask(task) && !task->stopped && !task->suspended_tso) {
 417         task->next = cap->spare_workers;
 418         cap->spare_workers = task;
 419     }
 420     // Bound tasks just float around attached to their TSOs.
 421
 422     releaseCapability_(cap,rtsFalse);
 423
 424     RELEASE_LOCK(&cap->lock);
 425 }
 426 #endif
 427
 428 /* ----------------------------------------------------------------------------
 429  * waitForReturnCapability( Task *task )
 430  *
 431  * Purpose:  when an OS thread returns from an external call,
 432  * it calls waitForReturnCapability() (via Schedule.resumeThread())
 433  * to wait for permission to enter the RTS & communicate the
 434  * result of the external call back to the Haskell thread that
 435  * made it.
 436  *
 437  * ------------------------------------------------------------------------- */
 438 void
 439 waitForReturnCapability (Capability **pCap, Task *task)
 440 {
 441 #if !defined(THREADED_RTS)
 442
 443     MainCapability.running_task = task;
 444     task->cap = &MainCapability;
 445     *pCap = &MainCapability;
 446
 447 #else
 448     Capability *cap = *pCap;
 449
 450     if (cap == NULL) {
 451         // Try last_free_capability first
 452         cap = last_free_capability;
 453         if (!cap->running_task) {
 454             nat i;
 455             // otherwise, search for a free capability
 456             for (i = 0; i < n_capabilities; i++) {
 457                 cap = &capabilities[i];
 458                 if (!cap->running_task) {
 459                     break;
 460                 }
 461             }
 462             // Can't find a free one, use last_free_capability.
 463             cap = last_free_capability;
 464         }
 465
 466         // record the Capability as the one this Task is now assocated with.
 467         task->cap = cap;
 468
 469     } else {
 470         ASSERT(task->cap == cap);
 471     }
 472
 473     ACQUIRE_LOCK(&cap->lock);
 474
 475     debugTrace(DEBUG_sched, "returning; I want capability %d", cap->no);
 476
 477     if (!cap->running_task) {
 478         // It's free; just grab it
 479         cap->running_task = task;
 480         RELEASE_LOCK(&cap->lock);
 481     } else {
 482         newReturningTask(cap,task);
 483         RELEASE_LOCK(&cap->lock);
 484
 485         for (;;) {
 486             ACQUIRE_LOCK(&task->lock);
 487             // task->lock held, cap->lock not held
 488             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 489             cap = task->cap;
 490             task->wakeup = rtsFalse;
 491             RELEASE_LOCK(&task->lock);
 492
 493             // now check whether we should wake up...
 494             ACQUIRE_LOCK(&cap->lock);
 495             if (cap->running_task == NULL) {
 496                 if (cap->returning_tasks_hd != task) {
 497                     giveCapabilityToTask(cap,cap->returning_tasks_hd);
 498                     RELEASE_LOCK(&cap->lock);
 499                     continue;
 500                 }
 501                 cap->running_task = task;
 502                 popReturningTask(cap);
 503                 RELEASE_LOCK(&cap->lock);
 504                 break;
 505             }
 506             RELEASE_LOCK(&cap->lock);
 507         }
 508
 509     }
 510
 511     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 512
 513     trace(TRACE_sched | DEBUG_sched, "resuming capability %d", cap->no);
 514
 515     *pCap = cap;
 516 #endif
 517 }
 518
 519 #if defined(THREADED_RTS)
 520 /* ----------------------------------------------------------------------------
 521  * yieldCapability
 522  * ------------------------------------------------------------------------- */
 523
 524 void
 525 yieldCapability (Capability** pCap, Task *task)
 526 {
 527     Capability *cap = *pCap;
 528
 529         debugTrace(DEBUG_sched, "giving up capability %d", cap->no);
 530
 531         // We must now release the capability and wait to be woken up
 532         // again.
 533         task->wakeup = rtsFalse;
 534         releaseCapabilityAndQueueWorker(cap);
 535
 536         for (;;) {
 537             ACQUIRE_LOCK(&task->lock);
 538             // task->lock held, cap->lock not held
 539             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 540             cap = task->cap;
 541             task->wakeup = rtsFalse;
 542             RELEASE_LOCK(&task->lock);
 543
 544             debugTrace(DEBUG_sched, "woken up on capability %d", cap->no);
 545
 546             ACQUIRE_LOCK(&cap->lock);
 547             if (cap->running_task != NULL) {
 548                 debugTrace(DEBUG_sched,
 549                            "capability %d is owned by another task", cap->no);
 550                 RELEASE_LOCK(&cap->lock);
 551                 continue;
 552             }
 553
 554             if (task->tso == NULL) {
 555                 ASSERT(cap->spare_workers != NULL);
 556                 // if we're not at the front of the queue, release it
 557                 // again.  This is unlikely to happen.
 558                 if (cap->spare_workers != task) {
 559                     giveCapabilityToTask(cap,cap->spare_workers);
 560                     RELEASE_LOCK(&cap->lock);
 561                     continue;
 562                 }
 563                 cap->spare_workers = task->next;
 564                 task->next = NULL;
 565             }
 566             cap->running_task = task;
 567             RELEASE_LOCK(&cap->lock);
 568             break;
 569         }
 570
 571         trace(TRACE_sched | DEBUG_sched, "resuming capability %d", cap->no);
 572         ASSERT(cap->running_task == task);
 573
 574     *pCap = cap;
 575
 576     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 577
 578     return;
 579 }
 580
 581 /* ----------------------------------------------------------------------------
 582  * Wake up a thread on a Capability.
 583  *
 584  * This is used when the current Task is running on a Capability and
 585  * wishes to wake up a thread on a different Capability.
 586  * ------------------------------------------------------------------------- */
 587
 588 void
 589 wakeupThreadOnCapability (Capability *my_cap,
 590                           Capability *other_cap,
 591                           StgTSO *tso)
 592 {
 593     ACQUIRE_LOCK(&other_cap->lock);
 594
 595     // ASSUMES: cap->lock is held (asserted in wakeupThreadOnCapability)
 596     if (tso->bound) {
 597         ASSERT(tso->bound->cap == tso->cap);
 598         tso->bound->cap = other_cap;
 599     }
 600     tso->cap = other_cap;
 601
 602     ASSERT(tso->bound ? tso->bound->cap == other_cap : 1);
 603
 604     if (other_cap->running_task == NULL) {
 605         // nobody is running this Capability, we can add our thread
 606         // directly onto the run queue and start up a Task to run it.
 607
 608         other_cap->running_task = myTask();
 609             // precond for releaseCapability_() and appendToRunQueue()
 610
 611         appendToRunQueue(other_cap,tso);
 612
 613         trace(TRACE_sched, "resuming capability %d", other_cap->no);
 614         releaseCapability_(other_cap,rtsFalse);
 615     } else {
 616         appendToWakeupQueue(my_cap,other_cap,tso);
 617         other_cap->context_switch = 1;
 618         // someone is running on this Capability, so it cannot be
 619         // freed without first checking the wakeup queue (see
 620         // releaseCapability_).
 621     }
 622
 623     RELEASE_LOCK(&other_cap->lock);
 624 }
 625
 626 /* ----------------------------------------------------------------------------
 627  * prodCapabilities
 628  *
 629  * Used to indicate that the interrupted flag is now set, or some
 630  * other global condition that might require waking up a Task on each
 631  * Capability.
 632  * ------------------------------------------------------------------------- */
 633
 634 static void
 635 prodCapabilities(rtsBool all)
 636 {
 637     nat i;
 638     Capability *cap;
 639     Task *task;
 640
 641     for (i=0; i < n_capabilities; i++) {
 642         cap = &capabilities[i];
 643         ACQUIRE_LOCK(&cap->lock);
 644         if (!cap->running_task) {
 645             if (cap->spare_workers) {
 646                 trace(TRACE_sched, "resuming capability %d", cap->no);
 647                 task = cap->spare_workers;
 648                 ASSERT(!task->stopped);
 649                 giveCapabilityToTask(cap,task);
 650                 if (!all) {
 651                     RELEASE_LOCK(&cap->lock);
 652                     return;
 653                 }
 654             }
 655         }
 656         RELEASE_LOCK(&cap->lock);
 657     }
 658     return;
 659 }
 660
 661 void
 662 prodAllCapabilities (void)
 663 {
 664     prodCapabilities(rtsTrue);
 665 }
 666
 667 /* ----------------------------------------------------------------------------
 668  * prodOneCapability
 669  *
 670  * Like prodAllCapabilities, but we only require a single Task to wake
 671  * up in order to service some global event, such as checking for
 672  * deadlock after some idle time has passed.
 673  * ------------------------------------------------------------------------- */
 674
 675 void
 676 prodOneCapability (void)
 677 {
 678     prodCapabilities(rtsFalse);
 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     ASSERT(sched_state == SCHED_SHUTTING_DOWN);
 702
 703     task->cap = cap;
 704
 705     // Loop indefinitely until all the workers have exited and there
 706     // are no Haskell threads left.  We used to bail out after 50
 707     // iterations of this loop, but that occasionally left a worker
 708     // running which caused problems later (the closeMutex() below
 709     // isn't safe, for one thing).
 710
 711     for (i = 0; /* i < 50 */; i++) {
 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         debugTrace(DEBUG_sched, "capability %d is stopped.", cap->no);
 771         freeCapability(cap);
 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 void
 810 freeCapability (Capability *cap) {
 811     stgFree(cap->mut_lists);
 812 #if defined(THREADED_RTS) || defined(PARALLEL_HASKELL)
 813     freeSparkPool(cap->sparks);
 814 #endif
 815 }
 816
 817 /* ---------------------------------------------------------------------------
 818    Mark everything directly reachable from the Capabilities.  When
 819    using multiple GC threads, each GC thread marks all Capabilities
 820    for which (c `mod` n == 0), for Capability c and thread n.
 821    ------------------------------------------------------------------------ */
 822
 823 void
 824 markSomeCapabilities (evac_fn evac, void *user, nat i0, nat delta,
 825                       rtsBool prune_sparks USED_IF_THREADS)
 826 {
 827     nat i;
 828     Capability *cap;
 829     Task *task;
 830
 831     // Each GC thread is responsible for following roots from the
 832     // Capability of the same number.  There will usually be the same
 833     // or fewer Capabilities as GC threads, but just in case there
 834     // are more, we mark every Capability whose number is the GC
 835     // thread's index plus a multiple of the number of GC threads.
 836     for (i = i0; i < n_capabilities; i += delta) {
 837         cap = &capabilities[i];
 838         evac(user, (StgClosure **)(void *)&cap->run_queue_hd);
 839         evac(user, (StgClosure **)(void *)&cap->run_queue_tl);
 840 #if defined(THREADED_RTS)
 841         evac(user, (StgClosure **)(void *)&cap->wakeup_queue_hd);
 842         evac(user, (StgClosure **)(void *)&cap->wakeup_queue_tl);
 843 #endif
 844         for (task = cap->suspended_ccalling_tasks; task != NULL;
 845              task=task->next) {
 846             debugTrace(DEBUG_sched,
 847                        "evac'ing suspended TSO %lu", (unsigned long)task->suspended_tso->id);
 848             evac(user, (StgClosure **)(void *)&task->suspended_tso);
 849         }
 850
 851 #if defined(THREADED_RTS)
 852         if (prune_sparks) {
 853             pruneSparkQueue (evac, user, cap);
 854         } else {
 855             traverseSparkQueue (evac, user, cap);
 856         }
 857 #endif
 858     }
 859
 860 #if !defined(THREADED_RTS)
 861     evac(user, (StgClosure **)(void *)&blocked_queue_hd);
 862     evac(user, (StgClosure **)(void *)&blocked_queue_tl);
 863     evac(user, (StgClosure **)(void *)&sleeping_queue);
 864 #endif
 865 }
 866
 867 void
 868 markCapabilities (evac_fn evac, void *user)
 869 {
 870     markSomeCapabilities(evac, user, 0, 1, rtsFalse);
 871 }