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