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     cap->mut_lists  = stgMallocBytes(sizeof(bdescr *) *
 147                                      RtsFlags.GcFlags.generations,
 148                                      "initCapability");
 149 }
 150
 151 /* ---------------------------------------------------------------------------
 152  * Function:  initCapabilities()
 153  *
 154  * Purpose:   set up the Capability handling. For the SMP build,
 155  *            we keep a table of them, the size of which is
 156  *            controlled by the user via the RTS flag -N.
 157  *
 158  * ------------------------------------------------------------------------- */
 159 void
 160 initCapabilities( void )
 161 {
 162 #if defined(SMP)
 163     nat i,n;
 164
 165     n_capabilities = n = RtsFlags.ParFlags.nNodes;
 166     capabilities = stgMallocBytes(n * sizeof(Capability), "initCapabilities");
 167
 168     for (i = 0; i < n; i++) {
 169         initCapability(&capabilities[i], i);
 170     }
 171
 172     IF_DEBUG(scheduler, sched_belch("allocated %d capabilities", n));
 173 #else
 174     n_capabilities = 1;
 175     capabilities = &MainCapability;
 176     initCapability(&MainCapability, 0);
 177 #endif
 178
 179     // There are no free capabilities to begin with.  We will start
 180     // a worker Task to each Capability, which will quickly put the
 181     // Capability on the free list when it finds nothing to do.
 182     last_free_capability = &capabilities[0];
 183 }
 184
 185 /* ----------------------------------------------------------------------------
 186  * Give a Capability to a Task.  The task must currently be sleeping
 187  * on its condition variable.
 188  *
 189  * Requires cap->lock (modifies cap->running_task).
 190  *
 191  * When migrating a Task, the migrater must take task->lock before
 192  * modifying task->cap, to synchronise with the waking up Task.
 193  * Additionally, the migrater should own the Capability (when
 194  * migrating the run queue), or cap->lock (when migrating
 195  * returning_workers).
 196  *
 197  * ------------------------------------------------------------------------- */
 198
 199 #if defined(THREADED_RTS)
 200 STATIC_INLINE void
 201 giveCapabilityToTask (Capability *cap, Task *task)
 202 {
 203     ASSERT_LOCK_HELD(&cap->lock);
 204     ASSERT(task->cap == cap);
 205     // We are not modifying task->cap, so we do not need to take task->lock.
 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_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) || 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 UNUSED_IF_NOT_THREADS)
 285 {
 286     ACQUIRE_LOCK(&cap->lock);
 287     releaseCapability_(cap);
 288     RELEASE_LOCK(&cap->lock);
 289 }
 290
 291 static void
 292 releaseCapabilityAndQueueWorker (Capability* cap UNUSED_IF_NOT_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,
 331                          Task *task UNUSED_IF_NOT_THREADS)
 332 {
 333 #if !defined(THREADED_RTS)
 334
 335     MainCapability.running_task = task;
 336     task->cap = &MainCapability;
 337     *pCap = &MainCapability;
 338
 339 #else
 340     Capability *cap = *pCap;
 341
 342     if (cap == NULL) {
 343         // Try last_free_capability first
 344         cap = last_free_capability;
 345         if (!cap->running_task) {
 346             nat i;
 347             // otherwise, search for a free capability
 348             for (i = 0; i < n_capabilities; i++) {
 349                 cap = &capabilities[i];
 350                 if (!cap->running_task) {
 351                     break;
 352                 }
 353             }
 354             // Can't find a free one, use last_free_capability.
 355             cap = last_free_capability;
 356         }
 357
 358         // record the Capability as the one this Task is now assocated with.
 359         task->cap = cap;
 360
 361     } else {
 362         ASSERT(task->cap == cap);
 363     }
 364
 365     ACQUIRE_LOCK(&cap->lock);
 366
 367     IF_DEBUG(scheduler,
 368              sched_belch("returning; I want capability %d", cap->no));
 369
 370     if (!cap->running_task) {
 371         // It's free; just grab it
 372         cap->running_task = task;
 373         RELEASE_LOCK(&cap->lock);
 374     } else {
 375         newReturningTask(cap,task);
 376         RELEASE_LOCK(&cap->lock);
 377
 378         for (;;) {
 379             ACQUIRE_LOCK(&task->lock);
 380             // task->lock held, cap->lock not held
 381             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 382             cap = task->cap;
 383             task->wakeup = rtsFalse;
 384             RELEASE_LOCK(&task->lock);
 385
 386             // now check whether we should wake up...
 387             ACQUIRE_LOCK(&cap->lock);
 388             if (cap->running_task == NULL) {
 389                 if (cap->returning_tasks_hd != task) {
 390                     giveCapabilityToTask(cap,cap->returning_tasks_hd);
 391                     RELEASE_LOCK(&cap->lock);
 392                     continue;
 393                 }
 394                 cap->running_task = task;
 395                 popReturningTask(cap);
 396                 RELEASE_LOCK(&cap->lock);
 397                 break;
 398             }
 399             RELEASE_LOCK(&cap->lock);
 400         }
 401
 402     }
 403
 404     ASSERT_CAPABILITY_INVARIANTS(cap,task);
 405
 406     IF_DEBUG(scheduler,
 407              sched_belch("returning; got capability %d", cap->no));
 408
 409     *pCap = cap;
 410 #endif
 411 }
 412
 413 #if defined(THREADED_RTS)
 414 /* ----------------------------------------------------------------------------
 415  * yieldCapability
 416  * ------------------------------------------------------------------------- */
 417
 418 void
 419 yieldCapability (Capability** pCap, Task *task)
 420 {
 421     Capability *cap = *pCap;
 422
 423     // The fast path; no locking
 424     if ( cap->returning_tasks_hd == NULL && anyWorkForMe(cap,task) )
 425         return;
 426
 427     while ( cap->returning_tasks_hd != NULL || !anyWorkForMe(cap,task) ) {
 428         IF_DEBUG(scheduler, sched_belch("giving up capability %d", cap->no));
 429
 430         // We must now release the capability and wait to be woken up
 431         // again.
 432         releaseCapabilityAndQueueWorker(cap);
 433
 434         for (;;) {
 435             ACQUIRE_LOCK(&task->lock);
 436             // task->lock held, cap->lock not held
 437             if (!task->wakeup) waitCondition(&task->cond, &task->lock);
 438             cap = task->cap;
 439             task->wakeup = rtsFalse;
 440             RELEASE_LOCK(&task->lock);
 441
 442             IF_DEBUG(scheduler, sched_belch("woken up on capability %d", cap->no));
 443             ACQUIRE_LOCK(&cap->lock);
 444             if (cap->running_task != NULL) {
 445                 RELEASE_LOCK(&cap->lock);
 446                 continue;
 447             }
 448
 449             if (task->tso == NULL) {
 450                 ASSERT(cap->spare_workers != NULL);
 451                 // if we're not at the front of the queue, release it
 452                 // again.  This is unlikely to happen.
 453                 if (cap->spare_workers != task) {
 454                     giveCapabilityToTask(cap,cap->spare_workers);
 455                     RELEASE_LOCK(&cap->lock);
 456                     continue;
 457                 }
 458                 cap->spare_workers = task->next;
 459                 task->next = NULL;
 460             }
 461             cap->running_task = task;
 462             RELEASE_LOCK(&cap->lock);
 463             break;
 464         }
 465
 466         IF_DEBUG(scheduler, sched_belch("got capability %d", cap->no));
 467         ASSERT(cap->running_task == task);
 468     }
 469
 470     *pCap = cap;
 471
 472     ASSERT_CAPABILITY_INVARIANTS(cap,task);
 473
 474     return;
 475 }
 476
 477 /* ----------------------------------------------------------------------------
 478  * prodCapabilities
 479  *
 480  * Used to indicate that the interrupted flag is now set, or some
 481  * other global condition that might require waking up a Task on each
 482  * Capability.
 483  * ------------------------------------------------------------------------- */
 484
 485 static void
 486 prodCapabilities(rtsBool all)
 487 {
 488     nat i;
 489     Capability *cap;
 490     Task *task;
 491
 492     for (i=0; i < n_capabilities; i++) {
 493         cap = &capabilities[i];
 494         ACQUIRE_LOCK(&cap->lock);
 495         if (!cap->running_task) {
 496             if (cap->spare_workers) {
 497                 task = cap->spare_workers;
 498                 ASSERT(!task->stopped);
 499                 giveCapabilityToTask(cap,task);
 500                 if (!all) {
 501                     RELEASE_LOCK(&cap->lock);
 502                     return;
 503                 }
 504             }
 505         }
 506         RELEASE_LOCK(&cap->lock);
 507     }
 508 }
 509
 510 void
 511 prodAllCapabilities (void)
 512 {
 513     prodCapabilities(rtsTrue);
 514 }
 515
 516 /* ----------------------------------------------------------------------------
 517  * prodOneCapability
 518  *
 519  * Like prodAllCapabilities, but we only require a single Task to wake
 520  * up in order to service some global event, such as checking for
 521  * deadlock after some idle time has passed.
 522  * ------------------------------------------------------------------------- */
 523
 524 void
 525 prodOneCapability (void)
 526 {
 527     prodCapabilities(rtsFalse);
 528 }
 529
 530 /* ----------------------------------------------------------------------------
 531  * shutdownCapability
 532  *
 533  * At shutdown time, we want to let everything exit as cleanly as
 534  * possible.  For each capability, we let its run queue drain, and
 535  * allow the workers to stop.
 536  *
 537  * This function should be called when interrupted and
 538  * shutting_down_scheduler = rtsTrue, thus any worker that wakes up
 539  * will exit the scheduler and call taskStop(), and any bound thread
 540  * that wakes up will return to its caller.  Runnable threads are
 541  * killed.
 542  *
 543  * ------------------------------------------------------------------------- */
 544
 545 void
 546 shutdownCapability (Capability *cap, Task *task)
 547 {
 548     nat i;
 549
 550     ASSERT(interrupted && shutting_down_scheduler);
 551
 552     task->cap = cap;
 553
 554     for (i = 0; i < 50; i++) {
 555         IF_DEBUG(scheduler, sched_belch("shutting down capability %d, attempt %d", cap->no, i));
 556         ACQUIRE_LOCK(&cap->lock);
 557         if (cap->running_task) {
 558             RELEASE_LOCK(&cap->lock);
 559             IF_DEBUG(scheduler, sched_belch("not owner, yielding"));
 560             yieldThread();
 561             continue;
 562         }
 563         cap->running_task = task;
 564         if (!emptyRunQueue(cap) || cap->spare_workers) {
 565             IF_DEBUG(scheduler, sched_belch("runnable threads or workers still alive, yielding"));
 566             releaseCapability_(cap); // this will wake up a worker
 567             RELEASE_LOCK(&cap->lock);
 568             yieldThread();
 569             continue;
 570         }
 571         IF_DEBUG(scheduler, sched_belch("capability %d is stopped.", cap->no));
 572         RELEASE_LOCK(&cap->lock);
 573         break;
 574     }
 575     // we now have the Capability, its run queue and spare workers
 576     // list are both empty.
 577 }
 578
 579 /* ----------------------------------------------------------------------------
 580  * tryGrabCapability
 581  *
 582  * Attempt to gain control of a Capability if it is free.
 583  *
 584  * ------------------------------------------------------------------------- */
 585
 586 rtsBool
 587 tryGrabCapability (Capability *cap, Task *task)
 588 {
 589     if (cap->running_task != NULL) return rtsFalse;
 590     ACQUIRE_LOCK(&cap->lock);
 591     if (cap->running_task != NULL) {
 592         RELEASE_LOCK(&cap->lock);
 593         return rtsFalse;
 594     }
 595     task->cap = cap;
 596     cap->running_task = task;
 597     RELEASE_LOCK(&cap->lock);
 598     return rtsTrue;
 599 }
 600
 601
 602 #endif /* THREADED_RTS */
 603
 604