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 "OSThreads.h"
  24 #include "Capability.h"
  25 #include "Schedule.h"
  26
  27 #if !defined(SMP)
  28 Capability MainCapability;     // for non-SMP, we have one global capability
  29 #endif
  30
  31 nat n_capabilities;
  32 Capability *capabilities = NULL;
  33
  34 // Holds the Capability which last became free.  This is used so that
  35 // an in-call has a chance of quickly finding a free Capability.
  36 // Maintaining a global free list of Capabilities would require global
  37 // locking, so we don't do that.
  38 Capability *last_free_capability;
  39
  40 #ifdef SMP
  41 #define UNUSED_IF_NOT_SMP
  42 #else
  43 #define UNUSED_IF_NOT_SMP STG_UNUSED
  44 #endif
  45
  46 #ifdef RTS_USER_SIGNALS
  47 #define UNUSED_IF_NOT_THREADS
  48 #else
  49 #define UNUSED_IF_NOT_THREADS STG_UNUSED
  50 #endif
  51
  52
  53 STATIC_INLINE rtsBool
  54 globalWorkToDo (void)
  55 {
  56     return blackholes_need_checking
  57         || interrupted
  58 #if defined(RTS_USER_SIGNALS)
  59         || signals_pending()
  60 #endif
  61         ;
  62 }
  63
  64 #if defined(THREADED_RTS)
  65 STATIC_INLINE rtsBool
  66 anyWorkForMe( Capability *cap, Task *task )
  67 {
  68     // If the run queue is not empty, then we only wake up the guy who
  69     // can run the thread at the head, even if there is some other
  70     // reason for this task to run (eg. interrupted=rtsTrue).
  71     if (!emptyRunQueue(cap)) {
  72         if (cap->run_queue_hd->bound == NULL) {
  73             return (task->tso == NULL);
  74         } else {
  75             return (cap->run_queue_hd->bound == task);
  76         }
  77     }
  78     return globalWorkToDo();
  79 }
  80 #endif
  81
  82 /* -----------------------------------------------------------------------------
  83  * Manage the returning_tasks lists.
  84  *
  85  * These functions require cap->lock
  86  * -------------------------------------------------------------------------- */
  87
  88 #if defined(THREADED_RTS)
  89 STATIC_INLINE void
  90 newReturningTask (Capability *cap, Task *task)
  91 {
  92     ASSERT_LOCK_HELD(&cap->lock);
  93     ASSERT(task->return_link == NULL);
  94     if (cap->returning_tasks_hd) {
  95         ASSERT(cap->returning_tasks_tl->return_link == NULL);
  96         cap->returning_tasks_tl->return_link = task;
  97     } else {
  98         cap->returning_tasks_hd = task;
  99     }
 100     cap->returning_tasks_tl = task;
 101 }
 102
 103 STATIC_INLINE Task *
 104 popReturningTask (Capability *cap)
 105 {
 106     ASSERT_LOCK_HELD(&cap->lock);
 107     Task *task;
 108     task = cap->returning_tasks_hd;
 109     ASSERT(task);
 110     cap->returning_tasks_hd = task->return_link;
 111     if (!cap->returning_tasks_hd) {
 112         cap->returning_tasks_tl = NULL;
 113     }
 114     task->return_link = NULL;
 115     return task;
 116 }
 117 #endif
 118
 119 /* ----------------------------------------------------------------------------
 120  * Initialisation
 121  *
 122  * The Capability is initially marked not free.
 123  * ------------------------------------------------------------------------- */
 124
 125 static void
 126 initCapability( Capability *cap, nat i )
 127 {
 128         nat g;
 129
 130     cap->no = i;
 131     cap->in_haskell        = rtsFalse;
 132
 133     cap->run_queue_hd      = END_TSO_QUEUE;
 134     cap->run_queue_tl      = END_TSO_QUEUE;
 135
 136 #if defined(THREADED_RTS)
 137     initMutex(&cap->lock);
 138     cap->running_task      = NULL; // indicates cap is free
 139     cap->spare_workers     = NULL;
 140     cap->suspended_ccalling_tasks = NULL;
 141     cap->returning_tasks_hd = NULL;
 142     cap->returning_tasks_tl = NULL;
 143 #endif
 144
 145     cap->f.stgGCEnter1     = (F_)__stg_gc_enter_1;
 146     cap->f.stgGCFun        = (F_)__stg_gc_fun;
 147
 148     cap->mut_lists  = stgMallocBytes(sizeof(bdescr *) *
 149                                      RtsFlags.GcFlags.generations,
 150                                      "initCapability");
 151         for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
 152               cap->mut_lists[g] = NULL;
 153     }
 154 }
 155
 156 /* ---------------------------------------------------------------------------
 157  * Function:  initCapabilities()
 158  *
 159  * Purpose:   set up the Capability handling. For the SMP build,
 160  *            we keep a table of them, the size of which is
 161  *            controlled by the user via the RTS flag -N.
 162  *
 163  * ------------------------------------------------------------------------- */
 164 void
 165 initCapabilities( void )
 166 {
 167 #if defined(SMP)
 168     nat i,n;
 169
 170     n_capabilities = n = RtsFlags.ParFlags.nNodes;
 171     capabilities = stgMallocBytes(n * sizeof(Capability), "initCapabilities");
 172
 173     for (i = 0; i < n; i++) {
 174         initCapability(&capabilities[i], i);
 175     }
 176
 177     IF_DEBUG(scheduler, sched_belch("allocated %d capabilities", n));
 178 #else
 179     n_capabilities = 1;
 180     capabilities = &MainCapability;
 181     initCapability(&MainCapability, 0);
 182 #endif
 183
 184     // There are no free capabilities to begin with.  We will start
 185     // a worker Task to each Capability, which will quickly put the
 186     // Capability on the free list when it finds nothing to do.
 187     last_free_capability = &capabilities[0];
 188 }
 189
 190 /* ----------------------------------------------------------------------------
 191  * Give a Capability to a Task.  The task must currently be sleeping
 192  * on its condition variable.
 193  *
 194  * Requires cap->lock (modifies cap->running_task).
 195  *
 196  * When migrating a Task, the migrater must take task->lock before
 197  * modifying task->cap, to synchronise with the waking up Task.
 198  * Additionally, the migrater should own the Capability (when
 199  * migrating the run queue), or cap->lock (when migrating
 200  * returning_workers).
 201  *
 202  * ------------------------------------------------------------------------- */
 203
 204 #if defined(THREADED_RTS)
 205 STATIC_INLINE void
 206 giveCapabilityToTask (Capability *cap, Task *task)
 207 {
 208     ASSERT_LOCK_HELD(&cap->lock);
 209     ASSERT(task->cap == cap);
 210     // We are not modifying task->cap, so we do not need to take task->lock.
 211     IF_DEBUG(scheduler,
 212              sched_belch("passing capability %d to %s %p",
 213                          cap->no, task->tso ? "bound task" : "worker",
 214                          (void *)task->id));
 215     ACQUIRE_LOCK(&task->lock);
 216     task->wakeup = rtsTrue;
 217     // the wakeup flag is needed because signalCondition() doesn't
 218     // flag the condition if the thread is already runniing, but we want
 219     // it to be sticky.
 220     signalCondition(&task->cond);
 221     RELEASE_LOCK(&task->lock);
 222 }
 223 #endif
 224
 225 /* ----------------------------------------------------------------------------
 226  * Function:  releaseCapability(Capability*)
 227  *
 228  * Purpose:   Letting go of a capability. Causes a
 229  *            'returning worker' thread or a 'waiting worker'
 230  *            to wake up, in that order.
 231  * ------------------------------------------------------------------------- */
 232
 233 #if defined(THREADED_RTS)
 234 void
 235 releaseCapability_ (Capability* cap)
 236 {
 237     Task *task;
 238
 239     task = cap->running_task;
 240
 241     ASSERT_CAPABILITY_INVARIANTS(cap,task);
 242
 243     cap->running_task = NULL;
 244
 245     // Check to see whether a worker thread can be given
 246     // the go-ahead to return the result of an external call..
 247     if (cap->returning_tasks_hd != NULL) {
 248         giveCapabilityToTask(cap,cap->returning_tasks_hd);
 249         // The Task pops itself from the queue (see waitForReturnCapability())
 250         return;
 251     }
 252
 253     // If the next thread on the run queue is a bound thread,
 254     // give this Capability to the appropriate Task.
 255     if (!emptyRunQueue(cap) && cap->run_queue_hd->bound) {
 256         // Make sure we're not about to try to wake ourselves up
 257         ASSERT(task != cap->run_queue_hd->bound);
 258         task = cap->run_queue_hd->bound;
 259         giveCapabilityToTask(cap,task);
 260         return;
 261     }
 262
 263     // If we have an unbound thread on the run queue, or if there's
 264     // anything else to do, give the Capability to a worker thread.
 265     if (!emptyRunQueue(cap) || globalWorkToDo()) {
 266         if (cap->spare_workers) {
 267             giveCapabilityToTask(cap,cap->spare_workers);
 268             // The worker Task pops itself from the queue;
 269             return;
 270         }
 271
 272         // Create a worker thread if we don't have one.  If the system
 273         // is interrupted, we only create a worker task if there
 274         // are threads that need to be completed.  If the system is
 275         // shutting down, we never create a new worker.
 276         if (!shutting_down_scheduler) {
 277             IF_DEBUG(scheduler,
 278                      sched_belch("starting new worker on capability %d", cap->no));
 279             startWorkerTask(cap, workerStart);
 280             return;
 281         }
 282     }
 283
 284     last_free_capability = cap;
 285     IF_DEBUG(scheduler, sched_belch("freeing capability %d", cap->no));
 286 }
 287
 288 void
 289 releaseCapability (Capability* cap UNUSED_IF_NOT_THREADS)
 290 {
 291     ACQUIRE_LOCK(&cap->lock);
 292     releaseCapability_(cap);
 293     RELEASE_LOCK(&cap->lock);
 294 }
 295
 296 static void
 297 releaseCapabilityAndQueueWorker (Capability* cap UNUSED_IF_NOT_THREADS)
 298 {
 299     Task *task;
 300
 301     ACQUIRE_LOCK(&cap->lock);
 302
 303     task = cap->running_task;
 304
 305     // If the current task is a worker, save it on the spare_workers
 306     // list of this Capability.  A worker can mark itself as stopped,
 307     // in which case it is not replaced on the spare_worker queue.
 308     // This happens when the system is shutting down (see
 309     // Schedule.c:workerStart()).
 310     // Also, be careful to check that this task hasn't just exited
 311     // Haskell to do a foreign call (task->suspended_tso).
 312     if (!isBoundTask(task) && !task->stopped && !task->suspended_tso) {
 313         task->next = cap->spare_workers;
 314         cap->spare_workers = task;
 315     }
 316     // Bound tasks just float around attached to their TSOs.
 317
 318     releaseCapability_(cap);
 319
 320     RELEASE_LOCK(&cap->lock);
 321 }
 322 #endif
 323
 324 /* ----------------------------------------------------------------------------
 325  * waitForReturnCapability( Task *task )
 326  *
 327  * Purpose:  when an OS thread returns from an external call,
 328  * it calls waitForReturnCapability() (via Schedule.resumeThread())
 329  * to wait for permission to enter the RTS & communicate the
 330  * result of the external call back to the Haskell thread that
 331  * made it.
 332  *
 333  * ------------------------------------------------------------------------- */
 334 void
 335 waitForReturnCapability (Capability **pCap,
 336                          Task *task UNUSED_IF_NOT_THREADS)
 337 {
 338 #if !defined(THREADED_RTS)
 339
 340     MainCapability.running_task = task;
 341     task->cap = &MainCapability;
 342     *pCap = &MainCapability;
 343
 344 #else
 345     Capability *cap = *pCap;
 346
 347     if (cap == NULL) {
 348         // Try last_free_capability first
 349         cap = last_free_capability;
 350         if (!cap->running_task) {
 351             nat i;
 352             // otherwise, search for a free capability
 353             for (i = 0; i < n_capabilities; i++) {
 354                 cap = &capabilities[i];
 355                 if (!cap->running_task) {
 356                     break;
 357                 }
 358             }
 359             // Can't find a free one, use last_free_capability.
 360             cap = last_free_capability;
 361         }
 362
 363         // record the Capability as the one this Task is now assocated with.
 364         task->cap = cap;
 365
 366     } else {
 367         ASSERT(task->cap == cap);
 368     }
 369
 370     ACQUIRE_LOCK(&cap->lock);
 371
 372     IF_DEBUG(scheduler,
 373              sched_belch("returning; I want capability %d", cap->no));
 374
 375     if (!cap->running_task) {
 376         // It's free; just grab it
 377         cap->running_task = task;
 378         RELEASE_LOCK(&cap->lock);
 379     } else {
 380         newReturningTask(cap,task);
 381         RELEASE_LOCK(&cap->lock);
 382
 383         for (;;) {
 384             ACQUIRE_LOCK(&task->lock);
 385             // task->lock held, cap->lock not held
 386             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 387             cap = task->cap;
 388             task->wakeup = rtsFalse;
 389             RELEASE_LOCK(&task->lock);
 390
 391             // now check whether we should wake up...
 392             ACQUIRE_LOCK(&cap->lock);
 393             if (cap->running_task == NULL) {
 394                 if (cap->returning_tasks_hd != task) {
 395                     giveCapabilityToTask(cap,cap->returning_tasks_hd);
 396                     RELEASE_LOCK(&cap->lock);
 397                     continue;
 398                 }
 399                 cap->running_task = task;
 400                 popReturningTask(cap);
 401                 RELEASE_LOCK(&cap->lock);
 402                 break;
 403             }
 404             RELEASE_LOCK(&cap->lock);
 405         }
 406
 407     }
 408
 409     ASSERT_CAPABILITY_INVARIANTS(cap,task);
 410
 411     IF_DEBUG(scheduler,
 412              sched_belch("returning; got capability %d", cap->no));
 413
 414     *pCap = cap;
 415 #endif
 416 }
 417
 418 #if defined(THREADED_RTS)
 419 /* ----------------------------------------------------------------------------
 420  * yieldCapability
 421  * ------------------------------------------------------------------------- */
 422
 423 void
 424 yieldCapability (Capability** pCap, Task *task)
 425 {
 426     Capability *cap = *pCap;
 427
 428     // The fast path; no locking
 429     if ( cap->returning_tasks_hd == NULL && anyWorkForMe(cap,task) )
 430         return;
 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         releaseCapabilityAndQueueWorker(cap);
 438
 439         for (;;) {
 440             ACQUIRE_LOCK(&task->lock);
 441             // task->lock held, cap->lock not held
 442             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 443             cap = task->cap;
 444             task->wakeup = rtsFalse;
 445             RELEASE_LOCK(&task->lock);
 446
 447             IF_DEBUG(scheduler, sched_belch("woken up on capability %d", cap->no));
 448             ACQUIRE_LOCK(&cap->lock);
 449             if (cap->running_task != NULL) {
 450                 RELEASE_LOCK(&cap->lock);
 451                 continue;
 452             }
 453
 454             if (task->tso == NULL) {
 455                 ASSERT(cap->spare_workers != NULL);
 456                 // if we're not at the front of the queue, release it
 457                 // again.  This is unlikely to happen.
 458                 if (cap->spare_workers != task) {
 459                     giveCapabilityToTask(cap,cap->spare_workers);
 460                     RELEASE_LOCK(&cap->lock);
 461                     continue;
 462                 }
 463                 cap->spare_workers = task->next;
 464                 task->next = NULL;
 465             }
 466             cap->running_task = task;
 467             RELEASE_LOCK(&cap->lock);
 468             break;
 469         }
 470
 471         IF_DEBUG(scheduler, sched_belch("got capability %d", cap->no));
 472         ASSERT(cap->running_task == task);
 473     }
 474
 475     *pCap = cap;
 476
 477     ASSERT_CAPABILITY_INVARIANTS(cap,task);
 478
 479     return;
 480 }
 481
 482 /* ----------------------------------------------------------------------------
 483  * prodCapabilities
 484  *
 485  * Used to indicate that the interrupted flag is now set, or some
 486  * other global condition that might require waking up a Task on each
 487  * Capability.
 488  * ------------------------------------------------------------------------- */
 489
 490 static void
 491 prodCapabilities(rtsBool all)
 492 {
 493     nat i;
 494     Capability *cap;
 495     Task *task;
 496
 497     for (i=0; i < n_capabilities; i++) {
 498         cap = &capabilities[i];
 499         ACQUIRE_LOCK(&cap->lock);
 500         if (!cap->running_task) {
 501             if (cap->spare_workers) {
 502                 task = cap->spare_workers;
 503                 ASSERT(!task->stopped);
 504                 giveCapabilityToTask(cap,task);
 505                 if (!all) {
 506                     RELEASE_LOCK(&cap->lock);
 507                     return;
 508                 }
 509             }
 510         }
 511         RELEASE_LOCK(&cap->lock);
 512     }
 513 }
 514
 515 void
 516 prodAllCapabilities (void)
 517 {
 518     prodCapabilities(rtsTrue);
 519 }
 520
 521 /* ----------------------------------------------------------------------------
 522  * prodOneCapability
 523  *
 524  * Like prodAllCapabilities, but we only require a single Task to wake
 525  * up in order to service some global event, such as checking for
 526  * deadlock after some idle time has passed.
 527  * ------------------------------------------------------------------------- */
 528
 529 void
 530 prodOneCapability (void)
 531 {
 532     prodCapabilities(rtsFalse);
 533 }
 534
 535 /* ----------------------------------------------------------------------------
 536  * shutdownCapability
 537  *
 538  * At shutdown time, we want to let everything exit as cleanly as
 539  * possible.  For each capability, we let its run queue drain, and
 540  * allow the workers to stop.
 541  *
 542  * This function should be called when interrupted and
 543  * shutting_down_scheduler = rtsTrue, thus any worker that wakes up
 544  * will exit the scheduler and call taskStop(), and any bound thread
 545  * that wakes up will return to its caller.  Runnable threads are
 546  * killed.
 547  *
 548  * ------------------------------------------------------------------------- */
 549
 550 void
 551 shutdownCapability (Capability *cap, Task *task)
 552 {
 553     nat i;
 554
 555     ASSERT(interrupted && shutting_down_scheduler);
 556
 557     task->cap = cap;
 558
 559     for (i = 0; i < 50; i++) {
 560         IF_DEBUG(scheduler, sched_belch("shutting down capability %d, attempt %d", cap->no, i));
 561         ACQUIRE_LOCK(&cap->lock);
 562         if (cap->running_task) {
 563             RELEASE_LOCK(&cap->lock);
 564             IF_DEBUG(scheduler, sched_belch("not owner, yielding"));
 565             yieldThread();
 566             continue;
 567         }
 568         cap->running_task = task;
 569         if (!emptyRunQueue(cap) || cap->spare_workers) {
 570             IF_DEBUG(scheduler, sched_belch("runnable threads or workers still alive, yielding"));
 571             releaseCapability_(cap); // this will wake up a worker
 572             RELEASE_LOCK(&cap->lock);
 573             yieldThread();
 574             continue;
 575         }
 576         IF_DEBUG(scheduler, sched_belch("capability %d is stopped.", cap->no));
 577         RELEASE_LOCK(&cap->lock);
 578         break;
 579     }
 580     // we now have the Capability, its run queue and spare workers
 581     // list are both empty.
 582 }
 583
 584 /* ----------------------------------------------------------------------------
 585  * tryGrabCapability
 586  *
 587  * Attempt to gain control of a Capability if it is free.
 588  *
 589  * ------------------------------------------------------------------------- */
 590
 591 rtsBool
 592 tryGrabCapability (Capability *cap, Task *task)
 593 {
 594     if (cap->running_task != NULL) return rtsFalse;
 595     ACQUIRE_LOCK(&cap->lock);
 596     if (cap->running_task != NULL) {
 597         RELEASE_LOCK(&cap->lock);
 598         return rtsFalse;
 599     }
 600     task->cap = cap;
 601     cap->running_task = task;
 602     RELEASE_LOCK(&cap->lock);
 603     return rtsTrue;
 604 }
 605
 606
 607 #endif /* THREADED_RTS */
 608
 609