ghc/rts/Capability.c

   1 /* ---------------------------------------------------------------------------
   2  * (c) The GHC Team, 2003
   3  *
   4  * Capabilities
   5  *
   6  * A Capability represent the token required to execute STG code,
   7  * and all the state an OS thread/task needs to run Haskell code:
   8  * its STG registers, a pointer to its TSO, a nursery etc. During
   9  * STG execution, a pointer to the capabilitity is kept in a
  10  * register (BaseReg).
  11  *
  12  * Only in an SMP build will there be multiple capabilities, for
  13  * the threaded RTS and other non-threaded builds, there is only
  14  * one global capability, namely MainCapability.
  15  *
  16  * --------------------------------------------------------------------------*/
  17
  18 #include "PosixSource.h"
  19 #include "Rts.h"
  20 #include "RtsUtils.h"
  21 #include "RtsFlags.h"
  22 #include "OSThreads.h"
  23 #include "Capability.h"
  24 #include "Schedule.h"  /* to get at EMPTY_RUN_QUEUE() */
  25
  26 #if !defined(SMP)
  27 Capability MainCapability;     /* for non-SMP, we have one global capability */
  28 #endif
  29
  30 Capability *capabilities = NULL;
  31 nat rts_n_free_capabilities;
  32
  33 #if defined(RTS_SUPPORTS_THREADS)
  34
  35 /* returning_worker_cond: when a worker thread returns from executing an
  36  * external call, it needs to wait for an RTS Capability before passing
  37  * on the result of the call to the Haskell thread that made it.
  38  *
  39  * returning_worker_cond is signalled in Capability.releaseCapability().
  40  *
  41  */
  42 Condition returning_worker_cond = INIT_COND_VAR;
  43
  44 /*
  45  * To avoid starvation of threads blocked on worker_thread_cond,
  46  * the task(s) that enter the Scheduler will check to see whether
  47  * there are one or more worker threads blocked waiting on
  48  * returning_worker_cond.
  49  */
  50 nat rts_n_waiting_workers = 0;
  51
  52 /* thread_ready_cond: when signalled, a thread has become runnable for a
  53  * task to execute.
  54  *
  55  * In the non-SMP case, it also implies that the thread that is woken up has
  56  * exclusive access to the RTS and all its data structures (that are not
  57  * locked by the Scheduler's mutex).
  58  *
  59  * thread_ready_cond is signalled whenever
  60  *      !noCapabilities && !EMPTY_RUN_QUEUE().
  61  */
  62 Condition thread_ready_cond = INIT_COND_VAR;
  63
  64 /*
  65  * To be able to make an informed decision about whether or not
  66  * to create a new task when making an external call, keep track of
  67  * the number of tasks currently blocked waiting on thread_ready_cond.
  68  * (if > 0 => no need for a new task, just unblock an existing one).
  69  *
  70  * waitForWorkCapability() takes care of keeping it up-to-date;
  71  * Task.startTask() uses its current value.
  72  */
  73 nat rts_n_waiting_tasks = 0;
  74
  75 static Condition *passTarget = NULL;
  76 static rtsBool passingCapability = rtsFalse;
  77 #endif
  78
  79 #if defined(SMP)
  80 /*
  81  * Free capability list.
  82  */
  83 Capability *free_capabilities;
  84 #endif
  85
  86 #ifdef SMP
  87 #define UNUSED_IF_NOT_SMP
  88 #else
  89 #define UNUSED_IF_NOT_SMP STG_UNUSED
  90 #endif
  91
  92 #if defined(RTS_USER_SIGNALS)
  93 #define ANY_WORK_TO_DO() (!EMPTY_RUN_QUEUE() || interrupted || blackholes_need_checking || signals_pending())
  94 #else
  95 #define ANY_WORK_TO_DO() (!EMPTY_RUN_QUEUE() || interrupted || blackholes_need_checking)
  96 #endif
  97
  98 /* ----------------------------------------------------------------------------
  99    Initialisation
 100    ------------------------------------------------------------------------- */
 101
 102 static void
 103 initCapability( Capability *cap )
 104 {
 105     cap->f.stgGCEnter1     = (F_)__stg_gc_enter_1;
 106     cap->f.stgGCFun        = (F_)__stg_gc_fun;
 107 }
 108
 109 /* ---------------------------------------------------------------------------
 110  * Function:  initCapabilities()
 111  *
 112  * Purpose:   set up the Capability handling. For the SMP build,
 113  *            we keep a table of them, the size of which is
 114  *            controlled by the user via the RTS flag RtsFlags.ParFlags.nNodes
 115  *
 116  * ------------------------------------------------------------------------- */
 117 void
 118 initCapabilities( void )
 119 {
 120 #if defined(SMP)
 121     nat i,n;
 122
 123     n = RtsFlags.ParFlags.nNodes;
 124     capabilities = stgMallocBytes(n * sizeof(Capability), "initCapabilities");
 125
 126     for (i = 0; i < n; i++) {
 127         initCapability(&capabilities[i]);
 128         capabilities[i].link = &capabilities[i+1];
 129     }
 130     capabilities[n-1].link = NULL;
 131
 132     free_capabilities = &capabilities[0];
 133     rts_n_free_capabilities = n;
 134
 135     IF_DEBUG(scheduler, sched_belch("allocated %d capabilities", n));
 136 #else
 137     capabilities = &MainCapability;
 138     initCapability(&MainCapability);
 139     rts_n_free_capabilities = 1;
 140 #endif
 141
 142 #if defined(RTS_SUPPORTS_THREADS)
 143     initCondition(&returning_worker_cond);
 144     initCondition(&thread_ready_cond);
 145 #endif
 146 }
 147
 148 /* ----------------------------------------------------------------------------
 149    grabCapability( Capability** )
 150
 151    (only externally visible when !RTS_SUPPORTS_THREADS.  In the
 152    threaded RTS, clients must use waitFor*Capability()).
 153    ------------------------------------------------------------------------- */
 154
 155 #if defined(RTS_SUPPORTS_THREADS)
 156 static
 157 #endif
 158 void
 159 grabCapability( Capability** cap )
 160 {
 161 #if defined(SMP)
 162   ASSERT(rts_n_free_capabilities > 0);
 163   *cap = free_capabilities;
 164   free_capabilities = (*cap)->link;
 165   rts_n_free_capabilities--;
 166 #else
 167 # if defined(RTS_SUPPORTS_THREADS)
 168   ASSERT(rts_n_free_capabilities == 1);
 169   rts_n_free_capabilities = 0;
 170 # endif
 171   *cap = &MainCapability;
 172 #endif
 173 #if defined(RTS_SUPPORTS_THREADS)
 174   IF_DEBUG(scheduler, sched_belch("worker: got capability"));
 175 #endif
 176 }
 177
 178 /* ----------------------------------------------------------------------------
 179  * Function:  releaseCapability(Capability*)
 180  *
 181  * Purpose:   Letting go of a capability. Causes a
 182  *            'returning worker' thread or a 'waiting worker'
 183  *            to wake up, in that order.
 184  * ------------------------------------------------------------------------- */
 185
 186 void
 187 releaseCapability( Capability* cap UNUSED_IF_NOT_SMP )
 188 {
 189     // Precondition: sched_mutex is held.
 190 #if defined(RTS_SUPPORTS_THREADS)
 191 #if !defined(SMP)
 192     ASSERT(rts_n_free_capabilities == 0);
 193 #endif
 194 #if defined(SMP)
 195     cap->link = free_capabilities;
 196     free_capabilities = cap;
 197 #endif
 198     // Check to see whether a worker thread can be given
 199     // the go-ahead to return the result of an external call..
 200     if (rts_n_waiting_workers > 0) {
 201         // Decrement the counter here to avoid livelock where the
 202         // thread that is yielding its capability will repeatedly
 203         // signal returning_worker_cond.
 204
 205         rts_n_waiting_workers--;
 206         signalCondition(&returning_worker_cond);
 207         IF_DEBUG(scheduler, sched_belch("worker: released capability to returning worker"));
 208     } else if (passingCapability) {
 209         if (passTarget == NULL) {
 210             signalCondition(&thread_ready_cond);
 211             startSchedulerTaskIfNecessary();
 212         } else {
 213             signalCondition(passTarget);
 214         }
 215 #if defined(SMP)
 216         rts_n_free_capabilities++;
 217 #else
 218         rts_n_free_capabilities = 1;
 219 #endif
 220         IF_DEBUG(scheduler, sched_belch("worker: released capability, passing it"));
 221
 222     } else {
 223 #if defined(SMP)
 224         rts_n_free_capabilities++;
 225 #else
 226         rts_n_free_capabilities = 1;
 227 #endif
 228         // Signal that a capability is available
 229         if (rts_n_waiting_tasks > 0 && ANY_WORK_TO_DO()) {
 230             signalCondition(&thread_ready_cond);
 231         }
 232         startSchedulerTaskIfNecessary();
 233         IF_DEBUG(scheduler, sched_belch("worker: released capability"));
 234     }
 235 #endif
 236     return;
 237 }
 238
 239 #if defined(RTS_SUPPORTS_THREADS)
 240 /*
 241  * When a native thread has completed the execution of an external
 242  * call, it needs to communicate the result back. This is done
 243  * as follows:
 244  *
 245  *  - in resumeThread(), the thread calls waitForReturnCapability().
 246  *  - If no capabilities are readily available, waitForReturnCapability()
 247  *    increments a counter rts_n_waiting_workers, and blocks
 248  *    waiting for the condition returning_worker_cond to become
 249  *    signalled.
 250  *  - upon entry to the Scheduler, a worker thread checks the
 251  *    value of rts_n_waiting_workers. If > 0, the worker thread
 252  *    will yield its capability to let a returning worker thread
 253  *    proceed with returning its result -- this is done via
 254  *    yieldToReturningWorker().
 255  *  - the worker thread that yielded its capability then tries
 256  *    to re-grab a capability and re-enter the Scheduler.
 257  */
 258
 259 /* ----------------------------------------------------------------------------
 260  * waitForReturnCapability( Mutext *pMutex, Capability** )
 261  *
 262  * Purpose:  when an OS thread returns from an external call,
 263  * it calls grabReturnCapability() (via Schedule.resumeThread())
 264  * to wait for permissions to enter the RTS & communicate the
 265  * result of the external call back to the Haskell thread that
 266  * made it.
 267  *
 268  * ------------------------------------------------------------------------- */
 269
 270 void
 271 waitForReturnCapability( Mutex* pMutex, Capability** pCap )
 272 {
 273     // Pre-condition: pMutex is held.
 274
 275     IF_DEBUG(scheduler,
 276              sched_belch("worker: returning; workers waiting: %d",
 277                          rts_n_waiting_workers));
 278
 279     if ( noCapabilities() || passingCapability ) {
 280         rts_n_waiting_workers++;
 281         context_switch = 1;     // make sure it's our turn soon
 282         waitCondition(&returning_worker_cond, pMutex);
 283 #if defined(SMP)
 284         *pCap = free_capabilities;
 285         free_capabilities = (*pCap)->link;
 286         ASSERT(pCap != NULL);
 287 #else
 288         *pCap = &MainCapability;
 289         ASSERT(rts_n_free_capabilities == 0);
 290 #endif
 291     } else {
 292         grabCapability(pCap);
 293     }
 294
 295     // Post-condition: pMutex is held, pCap points to a capability
 296     // which is now held by the current thread.
 297     return;
 298 }
 299
 300
 301 /* ----------------------------------------------------------------------------
 302  * yieldCapability( Mutex* pMutex, Capability** pCap )
 303  * ------------------------------------------------------------------------- */
 304
 305 void
 306 yieldCapability( Capability** pCap )
 307 {
 308     // Pre-condition:  pMutex is assumed held, the current thread
 309     // holds the capability pointed to by pCap.
 310
 311     if ( rts_n_waiting_workers > 0 || passingCapability || !ANY_WORK_TO_DO()) {
 312         IF_DEBUG(scheduler,
 313                  if (rts_n_waiting_workers > 0) {
 314                      sched_belch("worker: giving up capability (returning wkr)");
 315                  } else if (passingCapability) {
 316                      sched_belch("worker: giving up capability (passing capability)");
 317                  } else {
 318                      sched_belch("worker: giving up capability (no threads to run)");
 319                  }
 320             );
 321         releaseCapability(*pCap);
 322         *pCap = NULL;
 323     }
 324
 325     // Post-condition:  either:
 326     //
 327     //  1. *pCap is NULL, in which case the current thread does not
 328     //     hold a capability now, or
 329     //  2. *pCap is not NULL, in which case the current thread still
 330     //     holds the capability.
 331     //
 332     return;
 333 }
 334
 335
 336 /* ----------------------------------------------------------------------------
 337  * waitForCapability( Mutex*, Capability**, Condition* )
 338  *
 339  * Purpose:  wait for a Capability to become available. In
 340  *           the process of doing so, updates the number
 341  *           of tasks currently blocked waiting for a capability/more
 342  *           work. That counter is used when deciding whether or
 343  *           not to create a new worker thread when an external
 344  *           call is made.
 345  *           If pThreadCond is not NULL, a capability can be specifically
 346  *           passed to this thread using passCapability.
 347  * ------------------------------------------------------------------------- */
 348
 349 void
 350 waitForCapability( Mutex* pMutex, Capability** pCap, Condition* pThreadCond )
 351 {
 352     // Pre-condition: pMutex is held.
 353
 354     while ( noCapabilities() ||
 355             (passingCapability && passTarget != pThreadCond) ||
 356             !ANY_WORK_TO_DO()) {
 357         IF_DEBUG(scheduler,
 358                  sched_belch("worker: wait for capability (cond: %p)",
 359                              pThreadCond));
 360
 361         if (pThreadCond != NULL) {
 362             waitCondition(pThreadCond, pMutex);
 363             IF_DEBUG(scheduler, sched_belch("worker: get passed capability"));
 364         } else {
 365             rts_n_waiting_tasks++;
 366             waitCondition(&thread_ready_cond, pMutex);
 367             rts_n_waiting_tasks--;
 368             IF_DEBUG(scheduler, sched_belch("worker: get normal capability"));
 369         }
 370     }
 371     passingCapability = rtsFalse;
 372     grabCapability(pCap);
 373
 374     // Post-condition: pMutex is held and *pCap is held by the current thread
 375     return;
 376 }
 377
 378 /* ----------------------------------------------------------------------------
 379    passCapability, passCapabilityToWorker
 380    ------------------------------------------------------------------------- */
 381
 382 void
 383 passCapability( Condition *pTargetThreadCond )
 384 {
 385     // Pre-condition: pMutex is held and cap is held by the current thread
 386
 387     passTarget = pTargetThreadCond;
 388     passingCapability = rtsTrue;
 389     IF_DEBUG(scheduler, sched_belch("worker: passCapability"));
 390
 391     // Post-condition: pMutex is held; cap is still held, but will be
 392     //                 passed to the target thread when next released.
 393 }
 394
 395 void
 396 passCapabilityToWorker( void )
 397 {
 398     // Pre-condition: pMutex is held and cap is held by the current thread
 399
 400     passTarget = NULL;
 401     passingCapability = rtsTrue;
 402     IF_DEBUG(scheduler, sched_belch("worker: passCapabilityToWorker"));
 403
 404     // Post-condition: pMutex is held; cap is still held, but will be
 405     //                 passed to a worker thread when next released.
 406 }
 407
 408 #endif /* RTS_SUPPORTS_THREADS */
 409
 410 /* ----------------------------------------------------------------------------
 411    threadRunnable()
 412
 413    Signals that a thread has been placed on the run queue, so a worker
 414    might need to be woken up to run it.
 415
 416    ToDo: should check whether the thread at the front of the queue is
 417    bound, and if so wake up the appropriate worker.
 418    -------------------------------------------------------------------------- */
 419 void
 420 threadRunnable ( void )
 421 {
 422 #if defined(RTS_SUPPORTS_THREADS)
 423     if ( !noCapabilities() && ANY_WORK_TO_DO() && rts_n_waiting_tasks > 0 ) {
 424         signalCondition(&thread_ready_cond);
 425     }
 426     startSchedulerTaskIfNecessary();
 427 #endif
 428 }
 429
 430
 431 /* ----------------------------------------------------------------------------
 432    prodWorker()
 433
 434    Wake up... time to die.
 435    -------------------------------------------------------------------------- */
 436 void
 437 prodWorker ( void )
 438 {
 439 #if defined(RTS_SUPPORTS_THREADS)
 440     signalCondition(&thread_ready_cond);
 441 #endif
 442 }