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