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
  83       // Post event for running a spark from capability's own pool.
  84       postEvent(cap, EVENT_RUN_SPARK, cap->r.rCurrentTSO->id, 0);
  85
  86       return spark;
  87   }
  88
  89   if (n_capabilities == 1) { return NULL; } // makes no sense...
  90
  91   debugTrace(DEBUG_sched,
  92              "cap %d: Trying to steal work from other capabilities",
  93              cap->no);
  94
  95   do {
  96       retry = rtsFalse;
  97
  98       /* visit cap.s 0..n-1 in sequence until a theft succeeds. We could
  99       start at a random place instead of 0 as well.  */
 100       for ( i=0 ; i < n_capabilities ; i++ ) {
 101           robbed = &capabilities[i];
 102           if (cap == robbed)  // ourselves...
 103               continue;
 104
 105           if (emptySparkPoolCap(robbed)) // nothing to steal here
 106               continue;
 107
 108           spark = tryStealSpark(robbed);
 109           if (spark == NULL && !emptySparkPoolCap(robbed)) {
 110               // we conflicted with another thread while trying to steal;
 111               // try again later.
 112               retry = rtsTrue;
 113           }
 114
 115           if (spark != NULL) {
 116               debugTrace(DEBUG_sched,
 117                  "cap %d: Stole a spark from capability %d",
 118                          cap->no, robbed->no);
 119               cap->sparks_converted++;
 120
 121               postEvent(cap, EVENT_STEAL_SPARK,
 122                         cap->r.rCurrentTSO->id, robbed->no);
 123
 124
 125               return spark;
 126           }
 127           // otherwise: no success, try next one
 128       }
 129   } while (retry);
 130
 131   debugTrace(DEBUG_sched, "No sparks stolen");
 132   return NULL;
 133 }
 134
 135 // Returns True if any spark pool is non-empty at this moment in time
 136 // The result is only valid for an instant, of course, so in a sense
 137 // is immediately invalid, and should not be relied upon for
 138 // correctness.
 139 rtsBool
 140 anySparks (void)
 141 {
 142     nat i;
 143
 144     for (i=0; i < n_capabilities; i++) {
 145         if (!emptySparkPoolCap(&capabilities[i])) {
 146             return rtsTrue;
 147         }
 148     }
 149     return rtsFalse;
 150 }
 151 #endif
 152
 153 /* -----------------------------------------------------------------------------
 154  * Manage the returning_tasks lists.
 155  *
 156  * These functions require cap->lock
 157  * -------------------------------------------------------------------------- */
 158
 159 #if defined(THREADED_RTS)
 160 STATIC_INLINE void
 161 newReturningTask (Capability *cap, Task *task)
 162 {
 163     ASSERT_LOCK_HELD(&cap->lock);
 164     ASSERT(task->return_link == NULL);
 165     if (cap->returning_tasks_hd) {
 166         ASSERT(cap->returning_tasks_tl->return_link == NULL);
 167         cap->returning_tasks_tl->return_link = task;
 168     } else {
 169         cap->returning_tasks_hd = task;
 170     }
 171     cap->returning_tasks_tl = task;
 172 }
 173
 174 STATIC_INLINE Task *
 175 popReturningTask (Capability *cap)
 176 {
 177     ASSERT_LOCK_HELD(&cap->lock);
 178     Task *task;
 179     task = cap->returning_tasks_hd;
 180     ASSERT(task);
 181     cap->returning_tasks_hd = task->return_link;
 182     if (!cap->returning_tasks_hd) {
 183         cap->returning_tasks_tl = NULL;
 184     }
 185     task->return_link = NULL;
 186     return task;
 187 }
 188 #endif
 189
 190 /* ----------------------------------------------------------------------------
 191  * Initialisation
 192  *
 193  * The Capability is initially marked not free.
 194  * ------------------------------------------------------------------------- */
 195
 196 static void
 197 initCapability( Capability *cap, nat i )
 198 {
 199     nat g;
 200
 201     cap->no = i;
 202     cap->in_haskell        = rtsFalse;
 203     cap->in_gc             = rtsFalse;
 204
 205     cap->run_queue_hd      = END_TSO_QUEUE;
 206     cap->run_queue_tl      = END_TSO_QUEUE;
 207
 208 #if defined(THREADED_RTS)
 209     initMutex(&cap->lock);
 210     cap->running_task      = NULL; // indicates cap is free
 211     cap->spare_workers     = NULL;
 212     cap->suspended_ccalling_tasks = NULL;
 213     cap->returning_tasks_hd = NULL;
 214     cap->returning_tasks_tl = NULL;
 215     cap->wakeup_queue_hd    = END_TSO_QUEUE;
 216     cap->wakeup_queue_tl    = END_TSO_QUEUE;
 217     cap->sparks_created     = 0;
 218     cap->sparks_converted   = 0;
 219     cap->sparks_pruned      = 0;
 220 #endif
 221
 222     cap->f.stgEagerBlackholeInfo = (W_)&__stg_EAGER_BLACKHOLE_info;
 223     cap->f.stgGCEnter1     = (F_)__stg_gc_enter_1;
 224     cap->f.stgGCFun        = (F_)__stg_gc_fun;
 225
 226     cap->mut_lists  = stgMallocBytes(sizeof(bdescr *) *
 227                                      RtsFlags.GcFlags.generations,
 228                                      "initCapability");
 229     cap->saved_mut_lists = stgMallocBytes(sizeof(bdescr *) *
 230                                           RtsFlags.GcFlags.generations,
 231                                           "initCapability");
 232
 233     for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
 234         cap->mut_lists[g] = NULL;
 235     }
 236
 237     cap->free_tvar_watch_queues = END_STM_WATCH_QUEUE;
 238     cap->free_invariant_check_queues = END_INVARIANT_CHECK_QUEUE;
 239     cap->free_trec_chunks = END_STM_CHUNK_LIST;
 240     cap->free_trec_headers = NO_TREC;
 241     cap->transaction_tokens = 0;
 242     cap->context_switch = 0;
 243 }
 244
 245 /* ---------------------------------------------------------------------------
 246  * Function:  initCapabilities()
 247  *
 248  * Purpose:   set up the Capability handling. For the THREADED_RTS build,
 249  *            we keep a table of them, the size of which is
 250  *            controlled by the user via the RTS flag -N.
 251  *
 252  * ------------------------------------------------------------------------- */
 253 void
 254 initCapabilities( void )
 255 {
 256 #if defined(THREADED_RTS)
 257     nat i;
 258
 259 #ifndef REG_Base
 260     // We can't support multiple CPUs if BaseReg is not a register
 261     if (RtsFlags.ParFlags.nNodes > 1) {
 262         errorBelch("warning: multiple CPUs not supported in this build, reverting to 1");
 263         RtsFlags.ParFlags.nNodes = 1;
 264     }
 265 #endif
 266
 267     n_capabilities = RtsFlags.ParFlags.nNodes;
 268
 269     if (n_capabilities == 1) {
 270         capabilities = &MainCapability;
 271         // THREADED_RTS must work on builds that don't have a mutable
 272         // BaseReg (eg. unregisterised), so in this case
 273         // capabilities[0] must coincide with &MainCapability.
 274     } else {
 275         capabilities = stgMallocBytes(n_capabilities * sizeof(Capability),
 276                                       "initCapabilities");
 277     }
 278
 279     for (i = 0; i < n_capabilities; i++) {
 280         initCapability(&capabilities[i], i);
 281     }
 282
 283     debugTrace(DEBUG_sched, "allocated %d capabilities", n_capabilities);
 284
 285 #else /* !THREADED_RTS */
 286
 287     n_capabilities = 1;
 288     capabilities = &MainCapability;
 289     initCapability(&MainCapability, 0);
 290
 291 #endif
 292
 293     // There are no free capabilities to begin with.  We will start
 294     // a worker Task to each Capability, which will quickly put the
 295     // Capability on the free list when it finds nothing to do.
 296     last_free_capability = &capabilities[0];
 297 }
 298
 299 /* ----------------------------------------------------------------------------
 300  * setContextSwitches: cause all capabilities to context switch as
 301  * soon as possible.
 302  * ------------------------------------------------------------------------- */
 303
 304 void setContextSwitches(void)
 305 {
 306     nat i;
 307     for (i=0; i < n_capabilities; i++) {
 308         contextSwitchCapability(&capabilities[i]);
 309     }
 310 }
 311
 312 /* ----------------------------------------------------------------------------
 313  * Give a Capability to a Task.  The task must currently be sleeping
 314  * on its condition variable.
 315  *
 316  * Requires cap->lock (modifies cap->running_task).
 317  *
 318  * When migrating a Task, the migrater must take task->lock before
 319  * modifying task->cap, to synchronise with the waking up Task.
 320  * Additionally, the migrater should own the Capability (when
 321  * migrating the run queue), or cap->lock (when migrating
 322  * returning_workers).
 323  *
 324  * ------------------------------------------------------------------------- */
 325
 326 #if defined(THREADED_RTS)
 327 STATIC_INLINE void
 328 giveCapabilityToTask (Capability *cap USED_IF_DEBUG, Task *task)
 329 {
 330     ASSERT_LOCK_HELD(&cap->lock);
 331     ASSERT(task->cap == cap);
 332     debugTrace(DEBUG_sched, "passing capability %d to %s %p",
 333                cap->no, task->tso ? "bound task" : "worker",
 334                (void *)task->id);
 335     ACQUIRE_LOCK(&task->lock);
 336     task->wakeup = rtsTrue;
 337     // the wakeup flag is needed because signalCondition() doesn't
 338     // flag the condition if the thread is already runniing, but we want
 339     // it to be sticky.
 340     signalCondition(&task->cond);
 341     RELEASE_LOCK(&task->lock);
 342 }
 343 #endif
 344
 345 /* ----------------------------------------------------------------------------
 346  * Function:  releaseCapability(Capability*)
 347  *
 348  * Purpose:   Letting go of a capability. Causes a
 349  *            'returning worker' thread or a 'waiting worker'
 350  *            to wake up, in that order.
 351  * ------------------------------------------------------------------------- */
 352
 353 #if defined(THREADED_RTS)
 354 void
 355 releaseCapability_ (Capability* cap,
 356                     rtsBool always_wakeup)
 357 {
 358     Task *task;
 359
 360     task = cap->running_task;
 361
 362     ASSERT_PARTIAL_CAPABILITY_INVARIANTS(cap,task);
 363
 364     cap->running_task = NULL;
 365
 366     // Check to see whether a worker thread can be given
 367     // the go-ahead to return the result of an external call..
 368     if (cap->returning_tasks_hd != NULL) {
 369         giveCapabilityToTask(cap,cap->returning_tasks_hd);
 370         // The Task pops itself from the queue (see waitForReturnCapability())
 371         return;
 372     }
 373
 374     if (waiting_for_gc == PENDING_GC_SEQ) {
 375       last_free_capability = cap; // needed?
 376       debugTrace(DEBUG_sched, "GC pending, set capability %d free", cap->no);
 377       return;
 378     }
 379
 380
 381     // If the next thread on the run queue is a bound thread,
 382     // give this Capability to the appropriate Task.
 383     if (!emptyRunQueue(cap) && cap->run_queue_hd->bound) {
 384         // Make sure we're not about to try to wake ourselves up
 385         ASSERT(task != cap->run_queue_hd->bound);
 386         task = cap->run_queue_hd->bound;
 387         giveCapabilityToTask(cap,task);
 388         return;
 389     }
 390
 391     if (!cap->spare_workers) {
 392         // Create a worker thread if we don't have one.  If the system
 393         // is interrupted, we only create a worker task if there
 394         // are threads that need to be completed.  If the system is
 395         // shutting down, we never create a new worker.
 396         if (sched_state < SCHED_SHUTTING_DOWN || !emptyRunQueue(cap)) {
 397             debugTrace(DEBUG_sched,
 398                        "starting new worker on capability %d", cap->no);
 399             startWorkerTask(cap, workerStart);
 400             return;
 401         }
 402     }
 403
 404     // If we have an unbound thread on the run queue, or if there's
 405     // anything else to do, give the Capability to a worker thread.
 406     if (always_wakeup ||
 407         !emptyRunQueue(cap) || !emptyWakeupQueue(cap) ||
 408         !emptySparkPoolCap(cap) || globalWorkToDo()) {
 409         if (cap->spare_workers) {
 410             giveCapabilityToTask(cap,cap->spare_workers);
 411             // The worker Task pops itself from the queue;
 412             return;
 413         }
 414     }
 415
 416     last_free_capability = cap;
 417     debugTrace(DEBUG_sched, "freeing capability %d", cap->no);
 418 }
 419
 420 void
 421 releaseCapability (Capability* cap USED_IF_THREADS)
 422 {
 423     ACQUIRE_LOCK(&cap->lock);
 424     releaseCapability_(cap, rtsFalse);
 425     RELEASE_LOCK(&cap->lock);
 426 }
 427
 428 void
 429 releaseAndWakeupCapability (Capability* cap USED_IF_THREADS)
 430 {
 431     ACQUIRE_LOCK(&cap->lock);
 432     releaseCapability_(cap, rtsTrue);
 433     RELEASE_LOCK(&cap->lock);
 434 }
 435
 436 static void
 437 releaseCapabilityAndQueueWorker (Capability* cap USED_IF_THREADS)
 438 {
 439     Task *task;
 440
 441     ACQUIRE_LOCK(&cap->lock);
 442
 443     task = cap->running_task;
 444
 445     // If the current task is a worker, save it on the spare_workers
 446     // list of this Capability.  A worker can mark itself as stopped,
 447     // in which case it is not replaced on the spare_worker queue.
 448     // This happens when the system is shutting down (see
 449     // Schedule.c:workerStart()).
 450     // Also, be careful to check that this task hasn't just exited
 451     // Haskell to do a foreign call (task->suspended_tso).
 452     if (!isBoundTask(task) && !task->stopped && !task->suspended_tso) {
 453         task->next = cap->spare_workers;
 454         cap->spare_workers = task;
 455     }
 456     // Bound tasks just float around attached to their TSOs.
 457
 458     releaseCapability_(cap,rtsFalse);
 459
 460     RELEASE_LOCK(&cap->lock);
 461 }
 462 #endif
 463
 464 /* ----------------------------------------------------------------------------
 465  * waitForReturnCapability( Task *task )
 466  *
 467  * Purpose:  when an OS thread returns from an external call,
 468  * it calls waitForReturnCapability() (via Schedule.resumeThread())
 469  * to wait for permission to enter the RTS & communicate the
 470  * result of the external call back to the Haskell thread that
 471  * made it.
 472  *
 473  * ------------------------------------------------------------------------- */
 474 void
 475 waitForReturnCapability (Capability **pCap, Task *task)
 476 {
 477 #if !defined(THREADED_RTS)
 478
 479     MainCapability.running_task = task;
 480     task->cap = &MainCapability;
 481     *pCap = &MainCapability;
 482
 483 #else
 484     Capability *cap = *pCap;
 485
 486     if (cap == NULL) {
 487         // Try last_free_capability first
 488         cap = last_free_capability;
 489         if (!cap->running_task) {
 490             nat i;
 491             // otherwise, search for a free capability
 492             cap = NULL;
 493             for (i = 0; i < n_capabilities; i++) {
 494                 if (!capabilities[i].running_task) {
 495                     cap = &capabilities[i];
 496                     break;
 497                 }
 498             }
 499             if (cap == NULL) {
 500                 // Can't find a free one, use last_free_capability.
 501                 cap = last_free_capability;
 502             }
 503         }
 504
 505         // record the Capability as the one this Task is now assocated with.
 506         task->cap = cap;
 507
 508     } else {
 509         ASSERT(task->cap == cap);
 510     }
 511
 512     ACQUIRE_LOCK(&cap->lock);
 513
 514     debugTrace(DEBUG_sched, "returning; I want capability %d", cap->no);
 515
 516     if (!cap->running_task) {
 517         // It's free; just grab it
 518         cap->running_task = task;
 519         RELEASE_LOCK(&cap->lock);
 520     } else {
 521         newReturningTask(cap,task);
 522         RELEASE_LOCK(&cap->lock);
 523
 524         for (;;) {
 525             ACQUIRE_LOCK(&task->lock);
 526             // task->lock held, cap->lock not held
 527             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 528             cap = task->cap;
 529             task->wakeup = rtsFalse;
 530             RELEASE_LOCK(&task->lock);
 531
 532             // now check whether we should wake up...
 533             ACQUIRE_LOCK(&cap->lock);
 534             if (cap->running_task == NULL) {
 535                 if (cap->returning_tasks_hd != task) {
 536                     giveCapabilityToTask(cap,cap->returning_tasks_hd);
 537                     RELEASE_LOCK(&cap->lock);
 538                     continue;
 539                 }
 540                 cap->running_task = task;
 541                 popReturningTask(cap);
 542                 RELEASE_LOCK(&cap->lock);
 543                 break;
 544             }
 545             RELEASE_LOCK(&cap->lock);
 546         }
 547
 548     }
 549
 550     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 551
 552     debugTrace(DEBUG_sched, "resuming capability %d", cap->no);
 553
 554     *pCap = cap;
 555 #endif
 556 }
 557
 558 #if defined(THREADED_RTS)
 559 /* ----------------------------------------------------------------------------
 560  * yieldCapability
 561  * ------------------------------------------------------------------------- */
 562
 563 void
 564 yieldCapability (Capability** pCap, Task *task)
 565 {
 566     Capability *cap = *pCap;
 567
 568     if (waiting_for_gc == PENDING_GC_PAR) {
 569         debugTrace(DEBUG_sched, "capability %d: becoming a GC thread", cap->no);
 570         postEvent(cap, EVENT_GC_START, 0, 0);
 571         gcWorkerThread(cap);
 572         postEvent(cap, EVENT_GC_END, 0, 0);
 573         return;
 574     }
 575
 576         debugTrace(DEBUG_sched, "giving up capability %d", cap->no);
 577
 578         // We must now release the capability and wait to be woken up
 579         // again.
 580         task->wakeup = rtsFalse;
 581         releaseCapabilityAndQueueWorker(cap);
 582
 583         for (;;) {
 584             ACQUIRE_LOCK(&task->lock);
 585             // task->lock held, cap->lock not held
 586             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 587             cap = task->cap;
 588             task->wakeup = rtsFalse;
 589             RELEASE_LOCK(&task->lock);
 590
 591             debugTrace(DEBUG_sched, "woken up on capability %d", cap->no);
 592
 593             ACQUIRE_LOCK(&cap->lock);
 594             if (cap->running_task != NULL) {
 595                 debugTrace(DEBUG_sched,
 596                            "capability %d is owned by another task", cap->no);
 597                 RELEASE_LOCK(&cap->lock);
 598                 continue;
 599             }
 600
 601             if (task->tso == NULL) {
 602                 ASSERT(cap->spare_workers != NULL);
 603                 // if we're not at the front of the queue, release it
 604                 // again.  This is unlikely to happen.
 605                 if (cap->spare_workers != task) {
 606                     giveCapabilityToTask(cap,cap->spare_workers);
 607                     RELEASE_LOCK(&cap->lock);
 608                     continue;
 609                 }
 610                 cap->spare_workers = task->next;
 611                 task->next = NULL;
 612             }
 613             cap->running_task = task;
 614             RELEASE_LOCK(&cap->lock);
 615             break;
 616         }
 617
 618         debugTrace(DEBUG_sched, "resuming capability %d", cap->no);
 619         ASSERT(cap->running_task == task);
 620
 621     *pCap = cap;
 622
 623     ASSERT_FULL_CAPABILITY_INVARIANTS(cap,task);
 624
 625     return;
 626 }
 627
 628 /* ----------------------------------------------------------------------------
 629  * Wake up a thread on a Capability.
 630  *
 631  * This is used when the current Task is running on a Capability and
 632  * wishes to wake up a thread on a different Capability.
 633  * ------------------------------------------------------------------------- */
 634
 635 void
 636 wakeupThreadOnCapability (Capability *my_cap,
 637                           Capability *other_cap,
 638                           StgTSO *tso)
 639 {
 640     ACQUIRE_LOCK(&other_cap->lock);
 641
 642     // ASSUMES: cap->lock is held (asserted in wakeupThreadOnCapability)
 643     if (tso->bound) {
 644         ASSERT(tso->bound->cap == tso->cap);
 645         tso->bound->cap = other_cap;
 646     }
 647     tso->cap = other_cap;
 648
 649     ASSERT(tso->bound ? tso->bound->cap == other_cap : 1);
 650
 651     if (other_cap->running_task == NULL) {
 652         // nobody is running this Capability, we can add our thread
 653         // directly onto the run queue and start up a Task to run it.
 654
 655         other_cap->running_task = myTask();
 656             // precond for releaseCapability_() and appendToRunQueue()
 657
 658         appendToRunQueue(other_cap,tso);
 659
 660         releaseCapability_(other_cap,rtsFalse);
 661     } else {
 662         appendToWakeupQueue(my_cap,other_cap,tso);
 663         other_cap->context_switch = 1;
 664         // someone is running on this Capability, so it cannot be
 665         // freed without first checking the wakeup queue (see
 666         // releaseCapability_).
 667     }
 668
 669     RELEASE_LOCK(&other_cap->lock);
 670 }
 671
 672 /* ----------------------------------------------------------------------------
 673  * prodCapability
 674  *
 675  * If a Capability is currently idle, wake up a Task on it.  Used to
 676  * get every Capability into the GC.
 677  * ------------------------------------------------------------------------- */
 678
 679 void
 680 prodCapability (Capability *cap, Task *task)
 681 {
 682     ACQUIRE_LOCK(&cap->lock);
 683     if (!cap->running_task) {
 684         cap->running_task = task;
 685         releaseCapability_(cap,rtsTrue);
 686     }
 687     RELEASE_LOCK(&cap->lock);
 688 }
 689
 690 /* ----------------------------------------------------------------------------
 691  * shutdownCapability
 692  *
 693  * At shutdown time, we want to let everything exit as cleanly as
 694  * possible.  For each capability, we let its run queue drain, and
 695  * allow the workers to stop.
 696  *
 697  * This function should be called when interrupted and
 698  * shutting_down_scheduler = rtsTrue, thus any worker that wakes up
 699  * will exit the scheduler and call taskStop(), and any bound thread
 700  * that wakes up will return to its caller.  Runnable threads are
 701  * killed.
 702  *
 703  * ------------------------------------------------------------------------- */
 704
 705 void
 706 shutdownCapability (Capability *cap, Task *task, rtsBool safe)
 707 {
 708     nat i;
 709
 710     task->cap = cap;
 711
 712     // Loop indefinitely until all the workers have exited and there
 713     // are no Haskell threads left.  We used to bail out after 50
 714     // iterations of this loop, but that occasionally left a worker
 715     // running which caused problems later (the closeMutex() below
 716     // isn't safe, for one thing).
 717
 718     for (i = 0; /* i < 50 */; i++) {
 719         ASSERT(sched_state == SCHED_SHUTTING_DOWN);
 720
 721         debugTrace(DEBUG_sched,
 722                    "shutting down capability %d, attempt %d", cap->no, i);
 723         ACQUIRE_LOCK(&cap->lock);
 724         if (cap->running_task) {
 725             RELEASE_LOCK(&cap->lock);
 726             debugTrace(DEBUG_sched, "not owner, yielding");
 727             yieldThread();
 728             continue;
 729         }
 730         cap->running_task = task;
 731
 732         if (cap->spare_workers) {
 733             // Look for workers that have died without removing
 734             // themselves from the list; this could happen if the OS
 735             // summarily killed the thread, for example.  This
 736             // actually happens on Windows when the system is
 737             // terminating the program, and the RTS is running in a
 738             // DLL.
 739             Task *t, *prev;
 740             prev = NULL;
 741             for (t = cap->spare_workers; t != NULL; t = t->next) {
 742                 if (!osThreadIsAlive(t->id)) {
 743                     debugTrace(DEBUG_sched,
 744                                "worker thread %p has died unexpectedly", (void *)t->id);
 745                         if (!prev) {
 746                             cap->spare_workers = t->next;
 747                         } else {
 748                             prev->next = t->next;
 749                         }
 750                         prev = t;
 751                 }
 752             }
 753         }
 754
 755         if (!emptyRunQueue(cap) || cap->spare_workers) {
 756             debugTrace(DEBUG_sched,
 757                        "runnable threads or workers still alive, yielding");
 758             releaseCapability_(cap,rtsFalse); // this will wake up a worker
 759             RELEASE_LOCK(&cap->lock);
 760             yieldThread();
 761             continue;
 762         }
 763
 764         // If "safe", then busy-wait for any threads currently doing
 765         // foreign calls.  If we're about to unload this DLL, for
 766         // example, we need to be sure that there are no OS threads
 767         // that will try to return to code that has been unloaded.
 768         // We can be a bit more relaxed when this is a standalone
 769         // program that is about to terminate, and let safe=false.
 770         if (cap->suspended_ccalling_tasks && safe) {
 771             debugTrace(DEBUG_sched,
 772                        "thread(s) are involved in foreign calls, yielding");
 773             cap->running_task = NULL;
 774             RELEASE_LOCK(&cap->lock);
 775             yieldThread();
 776             continue;
 777         }
 778
 779         postEvent(cap, EVENT_SHUTDOWN, 0, 0);
 780         debugTrace(DEBUG_sched, "capability %d is stopped.", cap->no);
 781         RELEASE_LOCK(&cap->lock);
 782         break;
 783     }
 784     // we now have the Capability, its run queue and spare workers
 785     // list are both empty.
 786
 787     // ToDo: we can't drop this mutex, because there might still be
 788     // threads performing foreign calls that will eventually try to
 789     // return via resumeThread() and attempt to grab cap->lock.
 790     // closeMutex(&cap->lock);
 791 }
 792
 793 /* ----------------------------------------------------------------------------
 794  * tryGrabCapability
 795  *
 796  * Attempt to gain control of a Capability if it is free.
 797  *
 798  * ------------------------------------------------------------------------- */
 799
 800 rtsBool
 801 tryGrabCapability (Capability *cap, Task *task)
 802 {
 803     if (cap->running_task != NULL) return rtsFalse;
 804     ACQUIRE_LOCK(&cap->lock);
 805     if (cap->running_task != NULL) {
 806         RELEASE_LOCK(&cap->lock);
 807         return rtsFalse;
 808     }
 809     task->cap = cap;
 810     cap->running_task = task;
 811     RELEASE_LOCK(&cap->lock);
 812     return rtsTrue;
 813 }
 814
 815
 816 #endif /* THREADED_RTS */
 817
 818 static void
 819 freeCapability (Capability *cap)
 820 {
 821     stgFree(cap->mut_lists);
 822 #if defined(THREADED_RTS) || defined(PARALLEL_HASKELL)
 823     freeSparkPool(cap->sparks);
 824 #endif
 825 }
 826
 827 void
 828 freeCapabilities (void)
 829 {
 830 #if defined(THREADED_RTS)
 831     nat i;
 832     for (i=0; i < n_capabilities; i++) {
 833         freeCapability(&capabilities[i]);
 834     }
 835 #else
 836     freeCapability(&MainCapability);
 837 #endif
 838 }
 839
 840 /* ---------------------------------------------------------------------------
 841    Mark everything directly reachable from the Capabilities.  When
 842    using multiple GC threads, each GC thread marks all Capabilities
 843    for which (c `mod` n == 0), for Capability c and thread n.
 844    ------------------------------------------------------------------------ */
 845
 846 void
 847 markSomeCapabilities (evac_fn evac, void *user, nat i0, nat delta,
 848                       rtsBool prune_sparks USED_IF_THREADS)
 849 {
 850     nat i;
 851     Capability *cap;
 852     Task *task;
 853
 854     // Each GC thread is responsible for following roots from the
 855     // Capability of the same number.  There will usually be the same
 856     // or fewer Capabilities as GC threads, but just in case there
 857     // are more, we mark every Capability whose number is the GC
 858     // thread's index plus a multiple of the number of GC threads.
 859     for (i = i0; i < n_capabilities; i += delta) {
 860         cap = &capabilities[i];
 861         evac(user, (StgClosure **)(void *)&cap->run_queue_hd);
 862         evac(user, (StgClosure **)(void *)&cap->run_queue_tl);
 863 #if defined(THREADED_RTS)
 864         evac(user, (StgClosure **)(void *)&cap->wakeup_queue_hd);
 865         evac(user, (StgClosure **)(void *)&cap->wakeup_queue_tl);
 866 #endif
 867         for (task = cap->suspended_ccalling_tasks; task != NULL;
 868              task=task->next) {
 869             debugTrace(DEBUG_sched,
 870                        "evac'ing suspended TSO %lu", (unsigned long)task->suspended_tso->id);
 871             evac(user, (StgClosure **)(void *)&task->suspended_tso);
 872         }
 873
 874 #if defined(THREADED_RTS)
 875         if (prune_sparks) {
 876             pruneSparkQueue (evac, user, cap);
 877         } else {
 878             traverseSparkQueue (evac, user, cap);
 879         }
 880 #endif
 881     }
 882
 883 #if !defined(THREADED_RTS)
 884     evac(user, (StgClosure **)(void *)&blocked_queue_hd);
 885     evac(user, (StgClosure **)(void *)&blocked_queue_tl);
 886     evac(user, (StgClosure **)(void *)&sleeping_queue);
 887 #endif
 888 }
 889
 890 void
 891 markCapabilities (evac_fn evac, void *user)
 892 {
 893     markSomeCapabilities(evac, user, 0, 1, rtsFalse);
 894 }