ghc/rts/Capability.c

   1 /* ---------------------------------------------------------------------------
   2  *
   3  * (c) The GHC Team, 2003-2005
   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 SMP build will there be multiple capabilities, for
  14  * the threaded RTS and other non-threaded builds, there is only
  15  * one global capability, namely 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
  29 #if !defined(SMP)
  30 Capability MainCapability;     // for non-SMP, we have one global capability
  31 #endif
  32
  33 nat n_capabilities;
  34 Capability *capabilities = NULL;
  35
  36 // Holds the Capability which last became free.  This is used so that
  37 // an in-call has a chance of quickly finding a free Capability.
  38 // Maintaining a global free list of Capabilities would require global
  39 // locking, so we don't do that.
  40 Capability *last_free_capability;
  41
  42 #ifdef SMP
  43 #define UNUSED_IF_NOT_SMP
  44 #else
  45 #define UNUSED_IF_NOT_SMP STG_UNUSED
  46 #endif
  47
  48 #ifdef RTS_USER_SIGNALS
  49 #define UNUSED_IF_NOT_THREADS
  50 #else
  51 #define UNUSED_IF_NOT_THREADS STG_UNUSED
  52 #endif
  53
  54
  55 STATIC_INLINE rtsBool
  56 globalWorkToDo (void)
  57 {
  58     return blackholes_need_checking
  59         || interrupted
  60 #if defined(RTS_USER_SIGNALS)
  61         || signals_pending()
  62 #endif
  63         ;
  64 }
  65
  66 #if defined(THREADED_RTS)
  67 STATIC_INLINE rtsBool
  68 anyWorkForMe( Capability *cap, Task *task )
  69 {
  70     // If the run queue is not empty, then we only wake up the guy who
  71     // can run the thread at the head, even if there is some other
  72     // reason for this task to run (eg. interrupted=rtsTrue).
  73     if (!emptyRunQueue(cap)) {
  74         if (cap->run_queue_hd->bound == NULL) {
  75             return (task->tso == NULL);
  76         } else {
  77             return (cap->run_queue_hd->bound == task);
  78         }
  79     } else if (task->tso == NULL && !emptySparkPoolCap(cap)) {
  80         return rtsTrue;
  81     }
  82     return globalWorkToDo();
  83 }
  84 #endif
  85
  86 /* -----------------------------------------------------------------------------
  87  * Manage the returning_tasks lists.
  88  *
  89  * These functions require cap->lock
  90  * -------------------------------------------------------------------------- */
  91
  92 #if defined(THREADED_RTS)
  93 STATIC_INLINE void
  94 newReturningTask (Capability *cap, Task *task)
  95 {
  96     ASSERT_LOCK_HELD(&cap->lock);
  97     ASSERT(task->return_link == NULL);
  98     if (cap->returning_tasks_hd) {
  99         ASSERT(cap->returning_tasks_tl->return_link == NULL);
 100         cap->returning_tasks_tl->return_link = task;
 101     } else {
 102         cap->returning_tasks_hd = task;
 103     }
 104     cap->returning_tasks_tl = task;
 105 }
 106
 107 STATIC_INLINE Task *
 108 popReturningTask (Capability *cap)
 109 {
 110     ASSERT_LOCK_HELD(&cap->lock);
 111     Task *task;
 112     task = cap->returning_tasks_hd;
 113     ASSERT(task);
 114     cap->returning_tasks_hd = task->return_link;
 115     if (!cap->returning_tasks_hd) {
 116         cap->returning_tasks_tl = NULL;
 117     }
 118     task->return_link = NULL;
 119     return task;
 120 }
 121 #endif
 122
 123 /* ----------------------------------------------------------------------------
 124  * Initialisation
 125  *
 126  * The Capability is initially marked not free.
 127  * ------------------------------------------------------------------------- */
 128
 129 static void
 130 initCapability( Capability *cap, nat i )
 131 {
 132     nat g;
 133
 134     cap->no = i;
 135     cap->in_haskell        = rtsFalse;
 136
 137     cap->run_queue_hd      = END_TSO_QUEUE;
 138     cap->run_queue_tl      = END_TSO_QUEUE;
 139
 140 #if defined(THREADED_RTS)
 141     initMutex(&cap->lock);
 142     cap->running_task      = NULL; // indicates cap is free
 143     cap->spare_workers     = NULL;
 144     cap->suspended_ccalling_tasks = NULL;
 145     cap->returning_tasks_hd = NULL;
 146     cap->returning_tasks_tl = NULL;
 147 #endif
 148
 149     cap->f.stgGCEnter1     = (F_)__stg_gc_enter_1;
 150     cap->f.stgGCFun        = (F_)__stg_gc_fun;
 151
 152     cap->mut_lists  = stgMallocBytes(sizeof(bdescr *) *
 153                                      RtsFlags.GcFlags.generations,
 154                                      "initCapability");
 155
 156     for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
 157         cap->mut_lists[g] = NULL;
 158     }
 159
 160     cap->free_tvar_wait_queues = END_STM_WAIT_QUEUE;
 161     cap->free_trec_chunks = END_STM_CHUNK_LIST;
 162     cap->free_trec_headers = NO_TREC;
 163     cap->transaction_tokens = 0;
 164 }
 165
 166 /* ---------------------------------------------------------------------------
 167  * Function:  initCapabilities()
 168  *
 169  * Purpose:   set up the Capability handling. For the SMP build,
 170  *            we keep a table of them, the size of which is
 171  *            controlled by the user via the RTS flag -N.
 172  *
 173  * ------------------------------------------------------------------------- */
 174 void
 175 initCapabilities( void )
 176 {
 177 #if defined(SMP)
 178     nat i,n;
 179
 180     n_capabilities = n = RtsFlags.ParFlags.nNodes;
 181     capabilities = stgMallocBytes(n * sizeof(Capability), "initCapabilities");
 182
 183     for (i = 0; i < n; i++) {
 184         initCapability(&capabilities[i], i);
 185     }
 186
 187     IF_DEBUG(scheduler, sched_belch("allocated %d capabilities", n));
 188 #else
 189     n_capabilities = 1;
 190     capabilities = &MainCapability;
 191     initCapability(&MainCapability, 0);
 192 #endif
 193
 194     // There are no free capabilities to begin with.  We will start
 195     // a worker Task to each Capability, which will quickly put the
 196     // Capability on the free list when it finds nothing to do.
 197     last_free_capability = &capabilities[0];
 198 }
 199
 200 /* ----------------------------------------------------------------------------
 201  * Give a Capability to a Task.  The task must currently be sleeping
 202  * on its condition variable.
 203  *
 204  * Requires cap->lock (modifies cap->running_task).
 205  *
 206  * When migrating a Task, the migrater must take task->lock before
 207  * modifying task->cap, to synchronise with the waking up Task.
 208  * Additionally, the migrater should own the Capability (when
 209  * migrating the run queue), or cap->lock (when migrating
 210  * returning_workers).
 211  *
 212  * ------------------------------------------------------------------------- */
 213
 214 #if defined(THREADED_RTS)
 215 STATIC_INLINE void
 216 giveCapabilityToTask (Capability *cap, Task *task)
 217 {
 218     ASSERT_LOCK_HELD(&cap->lock);
 219     ASSERT(task->cap == cap);
 220     // We are not modifying task->cap, so we do not need to take task->lock.
 221     IF_DEBUG(scheduler,
 222              sched_belch("passing capability %d to %s %p",
 223                          cap->no, task->tso ? "bound task" : "worker",
 224                          (void *)task->id));
 225     ACQUIRE_LOCK(&task->lock);
 226     task->wakeup = rtsTrue;
 227     // the wakeup flag is needed because signalCondition() doesn't
 228     // flag the condition if the thread is already runniing, but we want
 229     // it to be sticky.
 230     signalCondition(&task->cond);
 231     RELEASE_LOCK(&task->lock);
 232 }
 233 #endif
 234
 235 /* ----------------------------------------------------------------------------
 236  * Function:  releaseCapability(Capability*)
 237  *
 238  * Purpose:   Letting go of a capability. Causes a
 239  *            'returning worker' thread or a 'waiting worker'
 240  *            to wake up, in that order.
 241  * ------------------------------------------------------------------------- */
 242
 243 #if defined(THREADED_RTS)
 244 void
 245 releaseCapability_ (Capability* cap)
 246 {
 247     Task *task;
 248
 249     task = cap->running_task;
 250
 251     ASSERT_CAPABILITY_INVARIANTS(cap,task);
 252
 253     cap->running_task = NULL;
 254
 255     // Check to see whether a worker thread can be given
 256     // the go-ahead to return the result of an external call..
 257     if (cap->returning_tasks_hd != NULL) {
 258         giveCapabilityToTask(cap,cap->returning_tasks_hd);
 259         // The Task pops itself from the queue (see waitForReturnCapability())
 260         return;
 261     }
 262
 263     // If the next thread on the run queue is a bound thread,
 264     // give this Capability to the appropriate Task.
 265     if (!emptyRunQueue(cap) && cap->run_queue_hd->bound) {
 266         // Make sure we're not about to try to wake ourselves up
 267         ASSERT(task != cap->run_queue_hd->bound);
 268         task = cap->run_queue_hd->bound;
 269         giveCapabilityToTask(cap,task);
 270         return;
 271     }
 272
 273     // If we have an unbound thread on the run queue, or if there's
 274     // anything else to do, give the Capability to a worker thread.
 275     if (!emptyRunQueue(cap) || !emptySparkPoolCap(cap) || globalWorkToDo()) {
 276         if (cap->spare_workers) {
 277             giveCapabilityToTask(cap,cap->spare_workers);
 278             // The worker Task pops itself from the queue;
 279             return;
 280         }
 281
 282         // Create a worker thread if we don't have one.  If the system
 283         // is interrupted, we only create a worker task if there
 284         // are threads that need to be completed.  If the system is
 285         // shutting down, we never create a new worker.
 286         if (!shutting_down_scheduler) {
 287             IF_DEBUG(scheduler,
 288                      sched_belch("starting new worker on capability %d", cap->no));
 289             startWorkerTask(cap, workerStart);
 290             return;
 291         }
 292     }
 293
 294     last_free_capability = cap;
 295     IF_DEBUG(scheduler, sched_belch("freeing capability %d", cap->no));
 296 }
 297
 298 void
 299 releaseCapability (Capability* cap UNUSED_IF_NOT_THREADS)
 300 {
 301     ACQUIRE_LOCK(&cap->lock);
 302     releaseCapability_(cap);
 303     RELEASE_LOCK(&cap->lock);
 304 }
 305
 306 static void
 307 releaseCapabilityAndQueueWorker (Capability* cap UNUSED_IF_NOT_THREADS)
 308 {
 309     Task *task;
 310
 311     ACQUIRE_LOCK(&cap->lock);
 312
 313     task = cap->running_task;
 314
 315     // If the current task is a worker, save it on the spare_workers
 316     // list of this Capability.  A worker can mark itself as stopped,
 317     // in which case it is not replaced on the spare_worker queue.
 318     // This happens when the system is shutting down (see
 319     // Schedule.c:workerStart()).
 320     // Also, be careful to check that this task hasn't just exited
 321     // Haskell to do a foreign call (task->suspended_tso).
 322     if (!isBoundTask(task) && !task->stopped && !task->suspended_tso) {
 323         task->next = cap->spare_workers;
 324         cap->spare_workers = task;
 325     }
 326     // Bound tasks just float around attached to their TSOs.
 327
 328     releaseCapability_(cap);
 329
 330     RELEASE_LOCK(&cap->lock);
 331 }
 332 #endif
 333
 334 /* ----------------------------------------------------------------------------
 335  * waitForReturnCapability( Task *task )
 336  *
 337  * Purpose:  when an OS thread returns from an external call,
 338  * it calls waitForReturnCapability() (via Schedule.resumeThread())
 339  * to wait for permission to enter the RTS & communicate the
 340  * result of the external call back to the Haskell thread that
 341  * made it.
 342  *
 343  * ------------------------------------------------------------------------- */
 344 void
 345 waitForReturnCapability (Capability **pCap,
 346                          Task *task UNUSED_IF_NOT_THREADS)
 347 {
 348 #if !defined(THREADED_RTS)
 349
 350     MainCapability.running_task = task;
 351     task->cap = &MainCapability;
 352     *pCap = &MainCapability;
 353
 354 #else
 355     Capability *cap = *pCap;
 356
 357     if (cap == NULL) {
 358         // Try last_free_capability first
 359         cap = last_free_capability;
 360         if (!cap->running_task) {
 361             nat i;
 362             // otherwise, search for a free capability
 363             for (i = 0; i < n_capabilities; i++) {
 364                 cap = &capabilities[i];
 365                 if (!cap->running_task) {
 366                     break;
 367                 }
 368             }
 369             // Can't find a free one, use last_free_capability.
 370             cap = last_free_capability;
 371         }
 372
 373         // record the Capability as the one this Task is now assocated with.
 374         task->cap = cap;
 375
 376     } else {
 377         ASSERT(task->cap == cap);
 378     }
 379
 380     ACQUIRE_LOCK(&cap->lock);
 381
 382     IF_DEBUG(scheduler,
 383              sched_belch("returning; I want capability %d", cap->no));
 384
 385     if (!cap->running_task) {
 386         // It's free; just grab it
 387         cap->running_task = task;
 388         RELEASE_LOCK(&cap->lock);
 389     } else {
 390         newReturningTask(cap,task);
 391         RELEASE_LOCK(&cap->lock);
 392
 393         for (;;) {
 394             ACQUIRE_LOCK(&task->lock);
 395             // task->lock held, cap->lock not held
 396             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 397             cap = task->cap;
 398             task->wakeup = rtsFalse;
 399             RELEASE_LOCK(&task->lock);
 400
 401             // now check whether we should wake up...
 402             ACQUIRE_LOCK(&cap->lock);
 403             if (cap->running_task == NULL) {
 404                 if (cap->returning_tasks_hd != task) {
 405                     giveCapabilityToTask(cap,cap->returning_tasks_hd);
 406                     RELEASE_LOCK(&cap->lock);
 407                     continue;
 408                 }
 409                 cap->running_task = task;
 410                 popReturningTask(cap);
 411                 RELEASE_LOCK(&cap->lock);
 412                 break;
 413             }
 414             RELEASE_LOCK(&cap->lock);
 415         }
 416
 417     }
 418
 419     ASSERT_CAPABILITY_INVARIANTS(cap,task);
 420
 421     IF_DEBUG(scheduler,
 422              sched_belch("returning; got capability %d", cap->no));
 423
 424     *pCap = cap;
 425 #endif
 426 }
 427
 428 #if defined(THREADED_RTS)
 429 /* ----------------------------------------------------------------------------
 430  * yieldCapability
 431  * ------------------------------------------------------------------------- */
 432
 433 void
 434 yieldCapability (Capability** pCap, Task *task)
 435 {
 436     Capability *cap = *pCap;
 437
 438     // The fast path; no locking
 439     if ( cap->returning_tasks_hd == NULL && anyWorkForMe(cap,task) )
 440         return;
 441
 442     while ( cap->returning_tasks_hd != NULL || !anyWorkForMe(cap,task) ) {
 443         IF_DEBUG(scheduler, sched_belch("giving up capability %d", cap->no));
 444
 445         // We must now release the capability and wait to be woken up
 446         // again.
 447         releaseCapabilityAndQueueWorker(cap);
 448
 449         for (;;) {
 450             ACQUIRE_LOCK(&task->lock);
 451             // task->lock held, cap->lock not held
 452             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 453             cap = task->cap;
 454             task->wakeup = rtsFalse;
 455             RELEASE_LOCK(&task->lock);
 456
 457             IF_DEBUG(scheduler, sched_belch("woken up on capability %d", cap->no));
 458             ACQUIRE_LOCK(&cap->lock);
 459             if (cap->running_task != NULL) {
 460                 RELEASE_LOCK(&cap->lock);
 461                 continue;
 462             }
 463
 464             if (task->tso == NULL) {
 465                 ASSERT(cap->spare_workers != NULL);
 466                 // if we're not at the front of the queue, release it
 467                 // again.  This is unlikely to happen.
 468                 if (cap->spare_workers != task) {
 469                     giveCapabilityToTask(cap,cap->spare_workers);
 470                     RELEASE_LOCK(&cap->lock);
 471                     continue;
 472                 }
 473                 cap->spare_workers = task->next;
 474                 task->next = NULL;
 475             }
 476             cap->running_task = task;
 477             RELEASE_LOCK(&cap->lock);
 478             break;
 479         }
 480
 481         IF_DEBUG(scheduler, sched_belch("got capability %d", cap->no));
 482         ASSERT(cap->running_task == task);
 483     }
 484
 485     *pCap = cap;
 486
 487     ASSERT_CAPABILITY_INVARIANTS(cap,task);
 488
 489     return;
 490 }
 491
 492 /* ----------------------------------------------------------------------------
 493  * prodCapabilities
 494  *
 495  * Used to indicate that the interrupted flag is now set, or some
 496  * other global condition that might require waking up a Task on each
 497  * Capability.
 498  * ------------------------------------------------------------------------- */
 499
 500 static void
 501 prodCapabilities(rtsBool all)
 502 {
 503     nat i;
 504     Capability *cap;
 505     Task *task;
 506
 507     for (i=0; i < n_capabilities; i++) {
 508         cap = &capabilities[i];
 509         ACQUIRE_LOCK(&cap->lock);
 510         if (!cap->running_task) {
 511             if (cap->spare_workers) {
 512                 task = cap->spare_workers;
 513                 ASSERT(!task->stopped);
 514                 giveCapabilityToTask(cap,task);
 515                 if (!all) {
 516                     RELEASE_LOCK(&cap->lock);
 517                     return;
 518                 }
 519             }
 520         }
 521         RELEASE_LOCK(&cap->lock);
 522     }
 523 }
 524
 525 void
 526 prodAllCapabilities (void)
 527 {
 528     prodCapabilities(rtsTrue);
 529 }
 530
 531 /* ----------------------------------------------------------------------------
 532  * prodOneCapability
 533  *
 534  * Like prodAllCapabilities, but we only require a single Task to wake
 535  * up in order to service some global event, such as checking for
 536  * deadlock after some idle time has passed.
 537  * ------------------------------------------------------------------------- */
 538
 539 void
 540 prodOneCapability (void)
 541 {
 542     prodCapabilities(rtsFalse);
 543 }
 544
 545 /* ----------------------------------------------------------------------------
 546  * shutdownCapability
 547  *
 548  * At shutdown time, we want to let everything exit as cleanly as
 549  * possible.  For each capability, we let its run queue drain, and
 550  * allow the workers to stop.
 551  *
 552  * This function should be called when interrupted and
 553  * shutting_down_scheduler = rtsTrue, thus any worker that wakes up
 554  * will exit the scheduler and call taskStop(), and any bound thread
 555  * that wakes up will return to its caller.  Runnable threads are
 556  * killed.
 557  *
 558  * ------------------------------------------------------------------------- */
 559
 560 void
 561 shutdownCapability (Capability *cap, Task *task)
 562 {
 563     nat i;
 564
 565     ASSERT(interrupted && shutting_down_scheduler);
 566
 567     task->cap = cap;
 568
 569     for (i = 0; i < 50; i++) {
 570         IF_DEBUG(scheduler, sched_belch("shutting down capability %d, attempt %d", cap->no, i));
 571         ACQUIRE_LOCK(&cap->lock);
 572         if (cap->running_task) {
 573             RELEASE_LOCK(&cap->lock);
 574             IF_DEBUG(scheduler, sched_belch("not owner, yielding"));
 575             yieldThread();
 576             continue;
 577         }
 578         cap->running_task = task;
 579         if (!emptyRunQueue(cap) || cap->spare_workers) {
 580             IF_DEBUG(scheduler, sched_belch("runnable threads or workers still alive, yielding"));
 581             releaseCapability_(cap); // this will wake up a worker
 582             RELEASE_LOCK(&cap->lock);
 583             yieldThread();
 584             continue;
 585         }
 586         IF_DEBUG(scheduler, sched_belch("capability %d is stopped.", cap->no));
 587         RELEASE_LOCK(&cap->lock);
 588         break;
 589     }
 590     // we now have the Capability, its run queue and spare workers
 591     // list are both empty.
 592 }
 593
 594 /* ----------------------------------------------------------------------------
 595  * tryGrabCapability
 596  *
 597  * Attempt to gain control of a Capability if it is free.
 598  *
 599  * ------------------------------------------------------------------------- */
 600
 601 rtsBool
 602 tryGrabCapability (Capability *cap, Task *task)
 603 {
 604     if (cap->running_task != NULL) return rtsFalse;
 605     ACQUIRE_LOCK(&cap->lock);
 606     if (cap->running_task != NULL) {
 607         RELEASE_LOCK(&cap->lock);
 608         return rtsFalse;
 609     }
 610     task->cap = cap;
 611     cap->running_task = task;
 612     RELEASE_LOCK(&cap->lock);
 613     return rtsTrue;
 614 }
 615
 616
 617 #endif /* THREADED_RTS */
 618
 619