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