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         rts_n_free_capabilities++;
 216         IF_DEBUG(scheduler, sched_belch("worker: released capability, passing it"));
 217
 218     } else {
 219         rts_n_free_capabilities++;
 220         // Signal that a capability is available
 221         if (rts_n_waiting_tasks > 0 && ANY_WORK_TO_DO()) {
 222             signalCondition(&thread_ready_cond);
 223         }
 224         startSchedulerTaskIfNecessary();
 225         IF_DEBUG(scheduler, sched_belch("worker: released capability"));
 226     }
 227 #endif
 228     return;
 229 }
 230
 231 #if defined(RTS_SUPPORTS_THREADS)
 232 /*
 233  * When a native thread has completed the execution of an external
 234  * call, it needs to communicate the result back. This is done
 235  * as follows:
 236  *
 237  *  - in resumeThread(), the thread calls waitForReturnCapability().
 238  *  - If no capabilities are readily available, waitForReturnCapability()
 239  *    increments a counter rts_n_waiting_workers, and blocks
 240  *    waiting for the condition returning_worker_cond to become
 241  *    signalled.
 242  *  - upon entry to the Scheduler, a worker thread checks the
 243  *    value of rts_n_waiting_workers. If > 0, the worker thread
 244  *    will yield its capability to let a returning worker thread
 245  *    proceed with returning its result -- this is done via
 246  *    yieldToReturningWorker().
 247  *  - the worker thread that yielded its capability then tries
 248  *    to re-grab a capability and re-enter the Scheduler.
 249  */
 250
 251 /* ----------------------------------------------------------------------------
 252  * waitForReturnCapability( Mutext *pMutex, Capability** )
 253  *
 254  * Purpose:  when an OS thread returns from an external call,
 255  * it calls grabReturnCapability() (via Schedule.resumeThread())
 256  * to wait for permissions to enter the RTS & communicate the
 257  * result of the external call back to the Haskell thread that
 258  * made it.
 259  *
 260  * ------------------------------------------------------------------------- */
 261
 262 void
 263 waitForReturnCapability( Mutex* pMutex, Capability** pCap )
 264 {
 265     // Pre-condition: pMutex is held.
 266
 267     IF_DEBUG(scheduler,
 268              sched_belch("worker: returning; workers waiting: %d",
 269                          rts_n_waiting_workers));
 270
 271     if ( noCapabilities() || passingCapability ) {
 272         rts_n_waiting_workers++;
 273         context_switch = 1;     // make sure it's our turn soon
 274         waitCondition(&returning_worker_cond, pMutex);
 275 #if defined(SMP)
 276         *pCap = free_capabilities;
 277         free_capabilities = (*pCap)->link;
 278         ASSERT(pCap != NULL);
 279 #else
 280         *pCap = &MainCapability;
 281         ASSERT(rts_n_free_capabilities == 0);
 282 #endif
 283     } else {
 284         grabCapability(pCap);
 285     }
 286
 287     // Post-condition: pMutex is held, pCap points to a capability
 288     // which is now held by the current thread.
 289     return;
 290 }
 291
 292
 293 /* ----------------------------------------------------------------------------
 294  * yieldCapability( Mutex* pMutex, Capability** pCap )
 295  * ------------------------------------------------------------------------- */
 296
 297 void
 298 yieldCapability( Capability** pCap )
 299 {
 300     // Pre-condition:  pMutex is assumed held, the current thread
 301     // holds the capability pointed to by pCap.
 302
 303     if ( rts_n_waiting_workers > 0 || passingCapability || !ANY_WORK_TO_DO()) {
 304         IF_DEBUG(scheduler,
 305                  if (rts_n_waiting_workers > 0) {
 306                      sched_belch("worker: giving up capability (returning wkr)");
 307                  } else if (passingCapability) {
 308                      sched_belch("worker: giving up capability (passing capability)");
 309                  } else {
 310                      sched_belch("worker: giving up capability (no threads to run)");
 311                  }
 312             );
 313         releaseCapability(*pCap);
 314         *pCap = NULL;
 315     }
 316
 317     // Post-condition:  either:
 318     //
 319     //  1. *pCap is NULL, in which case the current thread does not
 320     //     hold a capability now, or
 321     //  2. *pCap is not NULL, in which case the current thread still
 322     //     holds the capability.
 323     //
 324     return;
 325 }
 326
 327
 328 /* ----------------------------------------------------------------------------
 329  * waitForCapability( Mutex*, Capability**, Condition* )
 330  *
 331  * Purpose:  wait for a Capability to become available. In
 332  *           the process of doing so, updates the number
 333  *           of tasks currently blocked waiting for a capability/more
 334  *           work. That counter is used when deciding whether or
 335  *           not to create a new worker thread when an external
 336  *           call is made.
 337  *           If pThreadCond is not NULL, a capability can be specifically
 338  *           passed to this thread using passCapability.
 339  * ------------------------------------------------------------------------- */
 340
 341 void
 342 waitForCapability( Mutex* pMutex, Capability** pCap, Condition* pThreadCond )
 343 {
 344     // Pre-condition: pMutex is held.
 345
 346     while ( noCapabilities() ||
 347             (passingCapability && passTarget != pThreadCond) ||
 348             !ANY_WORK_TO_DO()) {
 349         IF_DEBUG(scheduler,
 350                  sched_belch("worker: wait for capability (cond: %p)",
 351                              pThreadCond));
 352
 353         if (pThreadCond != NULL) {
 354             waitCondition(pThreadCond, pMutex);
 355             IF_DEBUG(scheduler, sched_belch("worker: get passed capability"));
 356         } else {
 357             rts_n_waiting_tasks++;
 358             waitCondition(&thread_ready_cond, pMutex);
 359             rts_n_waiting_tasks--;
 360             IF_DEBUG(scheduler, sched_belch("worker: get normal capability"));
 361         }
 362     }
 363     passingCapability = rtsFalse;
 364     grabCapability(pCap);
 365
 366     // Post-condition: pMutex is held and *pCap is held by the current thread
 367     return;
 368 }
 369
 370 /* ----------------------------------------------------------------------------
 371    passCapability, passCapabilityToWorker
 372    ------------------------------------------------------------------------- */
 373
 374 void
 375 passCapability( Condition *pTargetThreadCond )
 376 {
 377     // Pre-condition: pMutex is held and cap is held by the current thread
 378
 379     passTarget = pTargetThreadCond;
 380     passingCapability = rtsTrue;
 381     IF_DEBUG(scheduler, sched_belch("worker: passCapability"));
 382
 383     // Post-condition: pMutex is held; cap is still held, but will be
 384     //                 passed to the target thread when next released.
 385 }
 386
 387 void
 388 passCapabilityToWorker( void )
 389 {
 390     // Pre-condition: pMutex is held and cap is held by the current thread
 391
 392     passTarget = NULL;
 393     passingCapability = rtsTrue;
 394     IF_DEBUG(scheduler, sched_belch("worker: passCapabilityToWorker"));
 395
 396     // Post-condition: pMutex is held; cap is still held, but will be
 397     //                 passed to a worker thread when next released.
 398 }
 399
 400 #endif /* RTS_SUPPORTS_THREADS */
 401
 402 /* ----------------------------------------------------------------------------
 403    threadRunnable()
 404
 405    Signals that a thread has been placed on the run queue, so a worker
 406    might need to be woken up to run it.
 407
 408    ToDo: should check whether the thread at the front of the queue is
 409    bound, and if so wake up the appropriate worker.
 410    -------------------------------------------------------------------------- */
 411 void
 412 threadRunnable ( void )
 413 {
 414 #if defined(RTS_SUPPORTS_THREADS)
 415     if ( !noCapabilities() && ANY_WORK_TO_DO() && rts_n_waiting_tasks > 0 ) {
 416         signalCondition(&thread_ready_cond);
 417     }
 418     startSchedulerTaskIfNecessary();
 419 #endif
 420 }
 421
 422
 423 /* ----------------------------------------------------------------------------
 424    prodWorker()
 425
 426    Wake up... time to die.
 427    -------------------------------------------------------------------------- */
 428 void
 429 prodWorker ( void )
 430 {
 431 #if defined(RTS_SUPPORTS_THREADS)
 432     signalCondition(&thread_ready_cond);
 433 #endif
 434 }