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
 221     for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
 222         cap->mut_lists[g] = NULL;
 223     }
 224
 225     cap->free_tvar_watch_queues = END_STM_WATCH_QUEUE;
 226     cap->free_invariant_check_queues = END_INVARIANT_CHECK_QUEUE;
 227     cap->free_trec_chunks = END_STM_CHUNK_LIST;
 228     cap->free_trec_headers = NO_TREC;
 229     cap->transaction_tokens = 0;
 230     cap->context_switch = 0;
 231 }
 232
 233 /* ---------------------------------------------------------------------------
 234  * Function:  initCapabilities()
 235  *
 236  * Purpose:   set up the Capability handling. For the THREADED_RTS build,
 237  *            we keep a table of them, the size of which is
 238  *            controlled by the user via the RTS flag -N.
 239  *
 240  * ------------------------------------------------------------------------- */
 241 void
 242 initCapabilities( void )
 243 {
 244 #if defined(THREADED_RTS)
 245     nat i;
 246
 247 #ifndef REG_Base
 248     // We can't support multiple CPUs if BaseReg is not a register
 249     if (RtsFlags.ParFlags.nNodes > 1) {
 250         errorBelch("warning: multiple CPUs not supported in this build, reverting to 1");
 251         RtsFlags.ParFlags.nNodes = 1;
 252     }
 253 #endif
 254
 255     n_capabilities = RtsFlags.ParFlags.nNodes;
 256
 257     if (n_capabilities == 1) {
 258         capabilities = &MainCapability;
 259         // THREADED_RTS must work on builds that don't have a mutable
 260         // BaseReg (eg. unregisterised), so in this case
 261         // capabilities[0] must coincide with &MainCapability.
 262     } else {
 263         capabilities = stgMallocBytes(n_capabilities * sizeof(Capability),
 264                                       "initCapabilities");
 265     }
 266
 267     for (i = 0; i < n_capabilities; i++) {
 268         initCapability(&capabilities[i], i);
 269     }
 270
 271     debugTrace(DEBUG_sched, "allocated %d capabilities", n_capabilities);
 272
 273 #else /* !THREADED_RTS */
 274
 275     n_capabilities = 1;
 276     capabilities = &MainCapability;
 277     initCapability(&MainCapability, 0);
 278
 279 #endif
 280
 281     // There are no free capabilities to begin with.  We will start
 282     // a worker Task to each Capability, which will quickly put the
 283     // Capability on the free list when it finds nothing to do.
 284     last_free_capability = &capabilities[0];
 285 }
 286
 287 /* ----------------------------------------------------------------------------
 288  * setContextSwitches: cause all capabilities to context switch as
 289  * soon as possible.
 290  * ------------------------------------------------------------------------- */
 291
 292 void setContextSwitches(void)
 293 {
 294   nat i;
 295   for (i=0; i < n_capabilities; i++) {
 296     capabilities[i].context_switch = 1;
 297   }
 298 }
 299
 300 /* ----------------------------------------------------------------------------
 301  * Give a Capability to a Task.  The task must currently be sleeping
 302  * on its condition variable.
 303  *
 304  * Requires cap->lock (modifies cap->running_task).
 305  *
 306  * When migrating a Task, the migrater must take task->lock before
 307  * modifying task->cap, to synchronise with the waking up Task.
 308  * Additionally, the migrater should own the Capability (when
 309  * migrating the run queue), or cap->lock (when migrating
 310  * returning_workers).
 311  *
 312  * ------------------------------------------------------------------------- */
 313
 314 #if defined(THREADED_RTS)
 315 STATIC_INLINE void
 316 giveCapabilityToTask (Capability *cap USED_IF_DEBUG, Task *task)
 317 {
 318     ASSERT_LOCK_HELD(&cap->lock);
 319     ASSERT(task->cap == cap);
 320     trace(TRACE_sched | DEBUG_sched,
 321           "passing capability %d to %s %p",
 322           cap->no, task->tso ? "bound task" : "worker",
 323           (void *)task->id);
 324     ACQUIRE_LOCK(&task->lock);
 325     task->wakeup = rtsTrue;
 326     // the wakeup flag is needed because signalCondition() doesn't
 327     // flag the condition if the thread is already runniing, but we want
 328     // it to be sticky.
 329     signalCondition(&task->cond);
 330     RELEASE_LOCK(&task->lock);
 331 }
 332 #endif
 333
 334 /* ----------------------------------------------------------------------------
 335  * Function:  releaseCapability(Capability*)
 336  *
 337  * Purpose:   Letting go of a capability. Causes a
 338  *            'returning worker' thread or a 'waiting worker'
 339  *            to wake up, in that order.
 340  * ------------------------------------------------------------------------- */
 341
 342 #if defined(THREADED_RTS)
 343 void
 344 releaseCapability_ (Capability* cap,
 345                     rtsBool always_wakeup)
 346 {
 347     Task *task;
 348
 349     task = cap->running_task;
 350
 351     ASSERT_PARTIAL_CAPABILITY_INVARIANTS(cap,task);
 352
 353     cap->running_task = NULL;
 354
 355     // Check to see whether a worker thread can be given
 356     // the go-ahead to return the result of an external call..
 357     if (cap->returning_tasks_hd != NULL) {
 358         giveCapabilityToTask(cap,cap->returning_tasks_hd);
 359         // The Task pops itself from the queue (see waitForReturnCapability())
 360         return;
 361     }
 362
 363     if (waiting_for_gc == PENDING_GC_SEQ) {
 364       last_free_capability = cap; // needed?
 365       trace(TRACE_sched | DEBUG_sched,
 366             "GC pending, set capability %d free", cap->no);
 367       return;
 368     }
 369
 370
 371     // If the next thread on the run queue is a bound thread,
 372     // give this Capability to the appropriate Task.
 373     if (!emptyRunQueue(cap) && cap->run_queue_hd->bound) {
 374         // Make sure we're not about to try to wake ourselves up
 375         ASSERT(task != cap->run_queue_hd->bound);
 376         task = cap->run_queue_hd->bound;
 377         giveCapabilityToTask(cap,task);
 378         return;
 379     }
 380
 381     if (!cap->spare_workers) {
 382         // Create a worker thread if we don't have one.  If the system
 383         // is interrupted, we only create a worker task if there
 384         // are threads that need to be completed.  If the system is
 385         // shutting down, we never create a new worker.
 386         if (sched_state < SCHED_SHUTTING_DOWN || !emptyRunQueue(cap)) {
 387             debugTrace(DEBUG_sched,
 388                        "starting new worker on capability %d", cap->no);
 389             startWorkerTask(cap, workerStart);
 390             return;
 391         }
 392     }
 393
 394     // If we have an unbound thread on the run queue, or if there's
 395     // anything else to do, give the Capability to a worker thread.
 396     if (always_wakeup ||
 397         !emptyRunQueue(cap) || !emptyWakeupQueue(cap) ||
 398         !emptySparkPoolCap(cap) || globalWorkToDo()) {
 399         if (cap->spare_workers) {
 400             giveCapabilityToTask(cap,cap->spare_workers);
 401             // The worker Task pops itself from the queue;
 402             return;
 403         }
 404     }
 405
 406     last_free_capability = cap;
 407     trace(TRACE_sched | DEBUG_sched, "freeing capability %d", cap->no);
 408 }
 409
 410 void
 411 releaseCapability (Capability* cap USED_IF_THREADS)
 412 {
 413     ACQUIRE_LOCK(&cap->lock);
 414     releaseCapability_(cap, rtsFalse);
 415     RELEASE_LOCK(&cap->lock);
 416 }
 417
 418 void
 419 releaseAndWakeupCapability (Capability* cap USED_IF_THREADS)
 420 {
 421     ACQUIRE_LOCK(&cap->lock);
 422     releaseCapability_(cap, rtsTrue);
 423     RELEASE_LOCK(&cap->lock);
 424 }
 425
 426 static void
 427 releaseCapabilityAndQueueWorker (Capability* cap USED_IF_THREADS)
 428 {
 429     Task *task;
 430
 431     ACQUIRE_LOCK(&cap->lock);
 432
 433     task = cap->running_task;
 434
 435     // If the current task is a worker, save it on the spare_workers
 436     // list of this Capability.  A worker can mark itself as stopped,
 437     // in which case it is not replaced on the spare_worker queue.
 438     // This happens when the system is shutting down (see
 439     // Schedule.c:workerStart()).
 440     // Also, be careful to check that this task hasn't just exited
 441     // Haskell to do a foreign call (task->suspended_tso).
 442     if (!isBoundTask(task) && !task->stopped && !task->suspended_tso) {
 443         task->next = cap->spare_workers;
 444         cap->spare_workers = task;
 445     }
 446     // Bound tasks just float around attached to their TSOs.
 447
 448     releaseCapability_(cap,rtsFalse);
 449
 450     RELEASE_LOCK(&cap->lock);
 451 }
 452 #endif
 453
 454 /* ----------------------------------------------------------------------------
 455  * waitForReturnCapability( Task *task )
 456  *
 457  * Purpose:  when an OS thread returns from an external call,
 458  * it calls waitForReturnCapability() (via Schedule.resumeThread())
 459  * to wait for permission to enter the RTS & communicate the
 460  * result of the external call back to the Haskell thread that
 461  * made it.
 462  *
 463  * ------------------------------------------------------------------------- */
 464 void
 465 waitForReturnCapability (Capability **pCap, Task *task)
 466 {
 467 #if !defined(THREADED_RTS)
 468
 469     MainCapability.running_task = task;
 470     task->cap = &MainCapability;
 471     *pCap = &MainCapability;
 472
 473 #else
 474     Capability *cap = *pCap;
 475
 476     if (cap == NULL) {
 477         // Try last_free_capability first
 478         cap = last_free_capability;
 479         if (!cap->running_task) {
 480             nat i;
 481             // otherwise, search for a free capability
 482             for (i = 0; i < n_capabilities; i++) {
 483                 cap = &capabilities[i];
 484                 if (!cap->running_task) {
 485                     break;
 486                 }
 487             }
 488             // Can't find a free one, use last_free_capability.
 489             cap = last_free_capability;
 490         }
 491
 492         // record the Capability as the one this Task is now assocated with.
 493         task->cap = cap;
 494
 495     } else {
 496         ASSERT(task->cap == cap);
 497     }
 498
 499     ACQUIRE_LOCK(&cap->lock);
 500
 501     debugTrace(DEBUG_sched, "returning; I want capability %d", cap->no);
 502
 503     if (!cap->running_task) {
 504         // It's free; just grab it
 505         cap->running_task = task;
 506         RELEASE_LOCK(&cap->lock);
 507     } else {
 508         newReturningTask(cap,task);
 509         RELEASE_LOCK(&cap->lock);
 510
 511         for (;;) {
 512             ACQUIRE_LOCK(&task->lock);
 513             // task->lock held, cap->lock not held
 514             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 515             cap = task->cap;
 516             task->wakeup = rtsFalse;
 517             RELEASE_LOCK(&task->lock);
 518
 519             // now check whether we should wake up...
 520             ACQUIRE_LOCK(&cap->lock);
 521             if (cap->running_task == NULL) {
 522                 if (cap->returning_tasks_hd != task) {
 523                     giveCapabilityToTask(cap,cap->returning_tasks_hd);
 524                     RELEASE_LOCK(&cap->lock);
 525                     continue;
 526                 }
 527                 cap->running_task = task;
 528                 popReturningTask(cap);
 529                 RELEASE_LOCK(&cap->lock);
 530                 break;
 531             }
 532             RELEASE_LOCK(&cap->lock);
 533         }
 534
 535     }
 536
 537     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 538
 539     trace(TRACE_sched | DEBUG_sched, "resuming capability %d", cap->no);
 540
 541     *pCap = cap;
 542 #endif
 543 }
 544
 545 #if defined(THREADED_RTS)
 546 /* ----------------------------------------------------------------------------
 547  * yieldCapability
 548  * ------------------------------------------------------------------------- */
 549
 550 void
 551 yieldCapability (Capability** pCap, Task *task)
 552 {
 553     Capability *cap = *pCap;
 554
 555     if (waiting_for_gc == PENDING_GC_PAR) {
 556         debugTrace(DEBUG_sched, "capability %d: becoming a GC thread", cap->no);
 557         gcWorkerThread(cap);
 558         return;
 559     }
 560
 561         debugTrace(DEBUG_sched, "giving up capability %d", cap->no);
 562
 563         // We must now release the capability and wait to be woken up
 564         // again.
 565         task->wakeup = rtsFalse;
 566         releaseCapabilityAndQueueWorker(cap);
 567
 568         for (;;) {
 569             ACQUIRE_LOCK(&task->lock);
 570             // task->lock held, cap->lock not held
 571             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 572             cap = task->cap;
 573             task->wakeup = rtsFalse;
 574             RELEASE_LOCK(&task->lock);
 575
 576             debugTrace(DEBUG_sched, "woken up on capability %d", cap->no);
 577
 578             ACQUIRE_LOCK(&cap->lock);
 579             if (cap->running_task != NULL) {
 580                 debugTrace(DEBUG_sched,
 581                            "capability %d is owned by another task", cap->no);
 582                 RELEASE_LOCK(&cap->lock);
 583                 continue;
 584             }
 585
 586             if (task->tso == NULL) {
 587                 ASSERT(cap->spare_workers != NULL);
 588                 // if we're not at the front of the queue, release it
 589                 // again.  This is unlikely to happen.
 590                 if (cap->spare_workers != task) {
 591                     giveCapabilityToTask(cap,cap->spare_workers);
 592                     RELEASE_LOCK(&cap->lock);
 593                     continue;
 594                 }
 595                 cap->spare_workers = task->next;
 596                 task->next = NULL;
 597             }
 598             cap->running_task = task;
 599             RELEASE_LOCK(&cap->lock);
 600             break;
 601         }
 602
 603         trace(TRACE_sched | DEBUG_sched, "resuming capability %d", cap->no);
 604         ASSERT(cap->running_task == task);
 605
 606     *pCap = cap;
 607
 608     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 609
 610     return;
 611 }
 612
 613 /* ----------------------------------------------------------------------------
 614  * Wake up a thread on a Capability.
 615  *
 616  * This is used when the current Task is running on a Capability and
 617  * wishes to wake up a thread on a different Capability.
 618  * ------------------------------------------------------------------------- */
 619
 620 void
 621 wakeupThreadOnCapability (Capability *my_cap,
 622                           Capability *other_cap,
 623                           StgTSO *tso)
 624 {
 625     ACQUIRE_LOCK(&other_cap->lock);
 626
 627     // ASSUMES: cap->lock is held (asserted in wakeupThreadOnCapability)
 628     if (tso->bound) {
 629         ASSERT(tso->bound->cap == tso->cap);
 630         tso->bound->cap = other_cap;
 631     }
 632     tso->cap = other_cap;
 633
 634     ASSERT(tso->bound ? tso->bound->cap == other_cap : 1);
 635
 636     if (other_cap->running_task == NULL) {
 637         // nobody is running this Capability, we can add our thread
 638         // directly onto the run queue and start up a Task to run it.
 639
 640         other_cap->running_task = myTask();
 641             // precond for releaseCapability_() and appendToRunQueue()
 642
 643         appendToRunQueue(other_cap,tso);
 644
 645         trace(TRACE_sched, "resuming capability %d", other_cap->no);
 646         releaseCapability_(other_cap,rtsFalse);
 647     } else {
 648         appendToWakeupQueue(my_cap,other_cap,tso);
 649         other_cap->context_switch = 1;
 650         // someone is running on this Capability, so it cannot be
 651         // freed without first checking the wakeup queue (see
 652         // releaseCapability_).
 653     }
 654
 655     RELEASE_LOCK(&other_cap->lock);
 656 }
 657
 658 /* ----------------------------------------------------------------------------
 659  * prodCapability
 660  *
 661  * If a Capability is currently idle, wake up a Task on it.  Used to
 662  * get every Capability into the GC.
 663  * ------------------------------------------------------------------------- */
 664
 665 void
 666 prodCapability (Capability *cap, Task *task)
 667 {
 668     ACQUIRE_LOCK(&cap->lock);
 669     if (!cap->running_task) {
 670         cap->running_task = task;
 671         releaseCapability_(cap,rtsTrue);
 672     }
 673     RELEASE_LOCK(&cap->lock);
 674 }
 675
 676 /* ----------------------------------------------------------------------------
 677  * shutdownCapability
 678  *
 679  * At shutdown time, we want to let everything exit as cleanly as
 680  * possible.  For each capability, we let its run queue drain, and
 681  * allow the workers to stop.
 682  *
 683  * This function should be called when interrupted and
 684  * shutting_down_scheduler = rtsTrue, thus any worker that wakes up
 685  * will exit the scheduler and call taskStop(), and any bound thread
 686  * that wakes up will return to its caller.  Runnable threads are
 687  * killed.
 688  *
 689  * ------------------------------------------------------------------------- */
 690
 691 void
 692 shutdownCapability (Capability *cap, Task *task, rtsBool safe)
 693 {
 694     nat i;
 695
 696     task->cap = cap;
 697
 698     // Loop indefinitely until all the workers have exited and there
 699     // are no Haskell threads left.  We used to bail out after 50
 700     // iterations of this loop, but that occasionally left a worker
 701     // running which caused problems later (the closeMutex() below
 702     // isn't safe, for one thing).
 703
 704     for (i = 0; /* i < 50 */; i++) {
 705         ASSERT(sched_state == SCHED_SHUTTING_DOWN);
 706
 707         debugTrace(DEBUG_sched,
 708                    "shutting down capability %d, attempt %d", cap->no, i);
 709         ACQUIRE_LOCK(&cap->lock);
 710         if (cap->running_task) {
 711             RELEASE_LOCK(&cap->lock);
 712             debugTrace(DEBUG_sched, "not owner, yielding");
 713             yieldThread();
 714             continue;
 715         }
 716         cap->running_task = task;
 717
 718         if (cap->spare_workers) {
 719             // Look for workers that have died without removing
 720             // themselves from the list; this could happen if the OS
 721             // summarily killed the thread, for example.  This
 722             // actually happens on Windows when the system is
 723             // terminating the program, and the RTS is running in a
 724             // DLL.
 725             Task *t, *prev;
 726             prev = NULL;
 727             for (t = cap->spare_workers; t != NULL; t = t->next) {
 728                 if (!osThreadIsAlive(t->id)) {
 729                     debugTrace(DEBUG_sched,
 730                                "worker thread %p has died unexpectedly", (void *)t->id);
 731                         if (!prev) {
 732                             cap->spare_workers = t->next;
 733                         } else {
 734                             prev->next = t->next;
 735                         }
 736                         prev = t;
 737                 }
 738             }
 739         }
 740
 741         if (!emptyRunQueue(cap) || cap->spare_workers) {
 742             debugTrace(DEBUG_sched,
 743                        "runnable threads or workers still alive, yielding");
 744             releaseCapability_(cap,rtsFalse); // this will wake up a worker
 745             RELEASE_LOCK(&cap->lock);
 746             yieldThread();
 747             continue;
 748         }
 749
 750         // If "safe", then busy-wait for any threads currently doing
 751         // foreign calls.  If we're about to unload this DLL, for
 752         // example, we need to be sure that there are no OS threads
 753         // that will try to return to code that has been unloaded.
 754         // We can be a bit more relaxed when this is a standalone
 755         // program that is about to terminate, and let safe=false.
 756         if (cap->suspended_ccalling_tasks && safe) {
 757             debugTrace(DEBUG_sched,
 758                        "thread(s) are involved in foreign calls, yielding");
 759             cap->running_task = NULL;
 760             RELEASE_LOCK(&cap->lock);
 761             yieldThread();
 762             continue;
 763         }
 764
 765         debugTrace(DEBUG_sched, "capability %d is stopped.", cap->no);
 766         RELEASE_LOCK(&cap->lock);
 767         break;
 768     }
 769     // we now have the Capability, its run queue and spare workers
 770     // list are both empty.
 771
 772     // ToDo: we can't drop this mutex, because there might still be
 773     // threads performing foreign calls that will eventually try to
 774     // return via resumeThread() and attempt to grab cap->lock.
 775     // closeMutex(&cap->lock);
 776 }
 777
 778 /* ----------------------------------------------------------------------------
 779  * tryGrabCapability
 780  *
 781  * Attempt to gain control of a Capability if it is free.
 782  *
 783  * ------------------------------------------------------------------------- */
 784
 785 rtsBool
 786 tryGrabCapability (Capability *cap, Task *task)
 787 {
 788     if (cap->running_task != NULL) return rtsFalse;
 789     ACQUIRE_LOCK(&cap->lock);
 790     if (cap->running_task != NULL) {
 791         RELEASE_LOCK(&cap->lock);
 792         return rtsFalse;
 793     }
 794     task->cap = cap;
 795     cap->running_task = task;
 796     RELEASE_LOCK(&cap->lock);
 797     return rtsTrue;
 798 }
 799
 800
 801 #endif /* THREADED_RTS */
 802
 803 static void
 804 freeCapability (Capability *cap)
 805 {
 806     stgFree(cap->mut_lists);
 807 #if defined(THREADED_RTS) || defined(PARALLEL_HASKELL)
 808     freeSparkPool(cap->sparks);
 809 #endif
 810 }
 811
 812 void
 813 freeCapabilities (void)
 814 {
 815 #if defined(THREADED_RTS)
 816     nat i;
 817     for (i=0; i < n_capabilities; i++) {
 818         freeCapability(&capabilities[i]);
 819     }
 820 #else
 821     freeCapability(&MainCapability);
 822 #endif
 823 }
 824
 825 /* ---------------------------------------------------------------------------
 826    Mark everything directly reachable from the Capabilities.  When
 827    using multiple GC threads, each GC thread marks all Capabilities
 828    for which (c `mod` n == 0), for Capability c and thread n.
 829    ------------------------------------------------------------------------ */
 830
 831 void
 832 markSomeCapabilities (evac_fn evac, void *user, nat i0, nat delta,
 833                       rtsBool prune_sparks USED_IF_THREADS)
 834 {
 835     nat i;
 836     Capability *cap;
 837     Task *task;
 838
 839     // Each GC thread is responsible for following roots from the
 840     // Capability of the same number.  There will usually be the same
 841     // or fewer Capabilities as GC threads, but just in case there
 842     // are more, we mark every Capability whose number is the GC
 843     // thread's index plus a multiple of the number of GC threads.
 844     for (i = i0; i < n_capabilities; i += delta) {
 845         cap = &capabilities[i];
 846         evac(user, (StgClosure **)(void *)&cap->run_queue_hd);
 847         evac(user, (StgClosure **)(void *)&cap->run_queue_tl);
 848 #if defined(THREADED_RTS)
 849         evac(user, (StgClosure **)(void *)&cap->wakeup_queue_hd);
 850         evac(user, (StgClosure **)(void *)&cap->wakeup_queue_tl);
 851 #endif
 852         for (task = cap->suspended_ccalling_tasks; task != NULL;
 853              task=task->next) {
 854             debugTrace(DEBUG_sched,
 855                        "evac'ing suspended TSO %lu", (unsigned long)task->suspended_tso->id);
 856             evac(user, (StgClosure **)(void *)&task->suspended_tso);
 857         }
 858
 859 #if defined(THREADED_RTS)
 860         if (prune_sparks) {
 861             pruneSparkQueue (evac, user, cap);
 862         } else {
 863             traverseSparkQueue (evac, user, cap);
 864         }
 865 #endif
 866     }
 867
 868 #if !defined(THREADED_RTS)
 869     evac(user, (StgClosure **)(void *)&blocked_queue_hd);
 870     evac(user, (StgClosure **)(void *)&blocked_queue_tl);
 871     evac(user, (StgClosure **)(void *)&sleeping_queue);
 872 #endif
 873 }
 874
 875 void
 876 markCapabilities (evac_fn evac, void *user)
 877 {
 878     markSomeCapabilities(evac, user, 0, 1, rtsFalse);
 879 }