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 #include "Signals.h" /* to get at handleSignalsInThisThread() */
  26
  27 #if !defined(SMP)
  28 Capability MainCapability;     /* for non-SMP, we have one global capability */
  29 #endif
  30
  31 #if defined(RTS_SUPPORTS_THREADS)
  32
  33 nat rts_n_free_capabilities;
  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 noCapabilities doesn't hold.
  60  *
  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 #ifdef SMP
  80 #define UNUSED_IF_NOT_SMP
  81 #else
  82 #define UNUSED_IF_NOT_SMP STG_UNUSED
  83 #endif
  84
  85 /* ----------------------------------------------------------------------------
  86    Initialisation
  87    ------------------------------------------------------------------------- */
  88
  89 static void
  90 initCapability( Capability *cap )
  91 {
  92     cap->f.stgGCEnter1     = (F_)__stg_gc_enter_1;
  93     cap->f.stgGCFun        = (F_)__stg_gc_fun;
  94 }
  95
  96 #if defined(SMP)
  97 static void initCapabilities_(nat n);
  98 #endif
  99
 100 /* ---------------------------------------------------------------------------
 101  * Function:  initCapabilities()
 102  *
 103  * Purpose:   set up the Capability handling. For the SMP build,
 104  *            we keep a table of them, the size of which is
 105  *            controlled by the user via the RTS flag RtsFlags.ParFlags.nNodes
 106  *
 107  * ------------------------------------------------------------------------- */
 108 void
 109 initCapabilities( void )
 110 {
 111 #if defined(SMP)
 112   initCapabilities_(RtsFlags.ParFlags.nNodes);
 113 #else
 114   initCapability(&MainCapability);
 115 #endif
 116
 117 #if defined(RTS_SUPPORTS_THREADS)
 118   initCondition(&returning_worker_cond);
 119   initCondition(&thread_ready_cond);
 120   rts_n_free_capabilities = 1;
 121 #endif
 122
 123   return;
 124 }
 125
 126 #if defined(SMP)
 127 /* Free capability list. */
 128 static Capability *free_capabilities; /* Available capabilities for running threads */
 129 static Capability *returning_capabilities;
 130         /* Capabilities being passed to returning worker threads */
 131 #endif
 132
 133 /* ----------------------------------------------------------------------------
 134    grabCapability( Capability** )
 135
 136    (only externally visible when !RTS_SUPPORTS_THREADS.  In the
 137    threaded RTS, clients must use waitFor*Capability()).
 138    ------------------------------------------------------------------------- */
 139
 140 void
 141 grabCapability( Capability** cap )
 142 {
 143 #if !defined(SMP)
 144 #if defined(RTS_SUPPORTS_THREADS)
 145   ASSERT(rts_n_free_capabilities == 1);
 146   rts_n_free_capabilities = 0;
 147 #endif
 148   *cap = &MainCapability;
 149   handleSignalsInThisThread();
 150 #else
 151   *cap = free_capabilities;
 152   free_capabilities = (*cap)->link;
 153   rts_n_free_capabilities--;
 154 #endif
 155   IF_DEBUG(scheduler, sched_belch("worker: got capability"));
 156 }
 157
 158 /* ----------------------------------------------------------------------------
 159  * Function:  releaseCapability(Capability*)
 160  *
 161  * Purpose:   Letting go of a capability. Causes a
 162  *            'returning worker' thread or a 'waiting worker'
 163  *            to wake up, in that order.
 164  * ------------------------------------------------------------------------- */
 165
 166 void
 167 releaseCapability( Capability* cap UNUSED_IF_NOT_SMP )
 168 {
 169     // Precondition: sched_mutex is held.
 170 #if defined(RTS_SUPPORTS_THREADS)
 171 #ifndef SMP
 172     ASSERT(rts_n_free_capabilities == 0);
 173 #endif
 174     // Check to see whether a worker thread can be given
 175     // the go-ahead to return the result of an external call..
 176     if (rts_n_waiting_workers > 0) {
 177         // Decrement the counter here to avoid livelock where the
 178         // thread that is yielding its capability will repeatedly
 179         // signal returning_worker_cond.
 180
 181 #if defined(SMP)
 182         // SMP variant untested
 183         cap->link = returning_capabilities;
 184         returning_capabilities = cap;
 185 #endif
 186
 187         rts_n_waiting_workers--;
 188         signalCondition(&returning_worker_cond);
 189         IF_DEBUG(scheduler, sched_belch("worker: released capability to returning worker"));
 190     } else if (passingCapability) {
 191         if (passTarget == NULL) {
 192             signalCondition(&thread_ready_cond);
 193             startSchedulerTaskIfNecessary();
 194         } else {
 195             signalCondition(passTarget);
 196         }
 197         rts_n_free_capabilities = 1;
 198         IF_DEBUG(scheduler, sched_belch("worker: released capability, passing it"));
 199
 200     } else {
 201 #if defined(SMP)
 202         cap->link = free_capabilities;
 203         free_capabilities = cap;
 204         rts_n_free_capabilities++;
 205 #else
 206         rts_n_free_capabilities = 1;
 207 #endif
 208         // Signal that a capability is available
 209         signalCondition(&thread_ready_cond);
 210         startSchedulerTaskIfNecessary();
 211         IF_DEBUG(scheduler, sched_belch("worker: released capability"));
 212     }
 213 #endif
 214     return;
 215 }
 216
 217 #if defined(RTS_SUPPORTS_THREADS)
 218 /*
 219  * When a native thread has completed the execution of an external
 220  * call, it needs to communicate the result back. This is done
 221  * as follows:
 222  *
 223  *  - in resumeThread(), the thread calls waitForReturnCapability().
 224  *  - If no capabilities are readily available, waitForReturnCapability()
 225  *    increments a counter rts_n_waiting_workers, and blocks
 226  *    waiting for the condition returning_worker_cond to become
 227  *    signalled.
 228  *  - upon entry to the Scheduler, a worker thread checks the
 229  *    value of rts_n_waiting_workers. If > 0, the worker thread
 230  *    will yield its capability to let a returning worker thread
 231  *    proceed with returning its result -- this is done via
 232  *    yieldToReturningWorker().
 233  *  - the worker thread that yielded its capability then tries
 234  *    to re-grab a capability and re-enter the Scheduler.
 235  */
 236
 237 /* ----------------------------------------------------------------------------
 238  * waitForReturnCapability( Mutext *pMutex, Capability** )
 239  *
 240  * Purpose:  when an OS thread returns from an external call,
 241  * it calls grabReturnCapability() (via Schedule.resumeThread())
 242  * to wait for permissions to enter the RTS & communicate the
 243  * result of the external call back to the Haskell thread that
 244  * made it.
 245  *
 246  * ------------------------------------------------------------------------- */
 247
 248 void
 249 waitForReturnCapability( Mutex* pMutex, Capability** pCap )
 250 {
 251     // Pre-condition: pMutex is held.
 252
 253     IF_DEBUG(scheduler,
 254              sched_belch("worker: returning; workers waiting: %d",
 255                          rts_n_waiting_workers));
 256
 257     if ( noCapabilities() || passingCapability ) {
 258         rts_n_waiting_workers++;
 259         wakeBlockedWorkerThread();
 260         context_switch = 1;     // make sure it's our turn soon
 261         waitCondition(&returning_worker_cond, pMutex);
 262 #if defined(SMP)
 263         *pCap = returning_capabilities;
 264         returning_capabilities = (*pCap)->link;
 265 #else
 266         *pCap = &MainCapability;
 267         ASSERT(rts_n_free_capabilities == 0);
 268         handleSignalsInThisThread();
 269 #endif
 270     } else {
 271         grabCapability(pCap);
 272     }
 273
 274     // Post-condition: pMutex is held, pCap points to a capability
 275     // which is now held by the current thread.
 276     return;
 277 }
 278
 279
 280 /* ----------------------------------------------------------------------------
 281  * yieldCapability( Mutex* pMutex, Capability** pCap )
 282  * ------------------------------------------------------------------------- */
 283
 284 void
 285 yieldCapability( Capability** pCap )
 286 {
 287     // Pre-condition:  pMutex is assumed held, the current thread
 288     // holds the capability pointed to by pCap.
 289
 290     if ( rts_n_waiting_workers > 0 || passingCapability ) {
 291         IF_DEBUG(scheduler, sched_belch("worker: giving up capability"));
 292         releaseCapability(*pCap);
 293         *pCap = NULL;
 294     }
 295
 296     // Post-condition:  pMutex is assumed held, and either:
 297     //
 298     //  1. *pCap is NULL, in which case the current thread does not
 299     //     hold a capability now, or
 300     //  2. *pCap is not NULL, in which case the current thread still
 301     //     holds the capability.
 302     //
 303     return;
 304 }
 305
 306
 307 /* ----------------------------------------------------------------------------
 308  * waitForCapability( Mutex*, Capability**, Condition* )
 309  *
 310  * Purpose:  wait for a Capability to become available. In
 311  *           the process of doing so, updates the number
 312  *           of tasks currently blocked waiting for a capability/more
 313  *           work. That counter is used when deciding whether or
 314  *           not to create a new worker thread when an external
 315  *           call is made.
 316  *           If pThreadCond is not NULL, a capability can be specifically
 317  *           passed to this thread using passCapability.
 318  * ------------------------------------------------------------------------- */
 319
 320 void
 321 waitForCapability( Mutex* pMutex, Capability** pCap, Condition* pThreadCond )
 322 {
 323     // Pre-condition: pMutex is held.
 324
 325     while ( noCapabilities() ||
 326             (passingCapability && passTarget != pThreadCond)) {
 327         IF_DEBUG(scheduler,
 328                  sched_belch("worker: wait for capability (cond: %p)",
 329                              pThreadCond));
 330
 331         if (pThreadCond != NULL) {
 332             waitCondition(pThreadCond, pMutex);
 333             IF_DEBUG(scheduler, sched_belch("worker: get passed capability"));
 334         } else {
 335             rts_n_waiting_tasks++;
 336             waitCondition(&thread_ready_cond, pMutex);
 337             rts_n_waiting_tasks--;
 338             IF_DEBUG(scheduler, sched_belch("worker: get normal capability"));
 339         }
 340     }
 341     passingCapability = rtsFalse;
 342     grabCapability(pCap);
 343
 344     // Post-condition: pMutex is held and *pCap is held by the current thread
 345     return;
 346 }
 347
 348 /* ----------------------------------------------------------------------------
 349    passCapability, passCapabilityToWorker
 350    ------------------------------------------------------------------------- */
 351
 352 void
 353 passCapability( Condition *pTargetThreadCond )
 354 {
 355     // Pre-condition: pMutex is held and cap is held by the current thread
 356
 357     passTarget = pTargetThreadCond;
 358     passingCapability = rtsTrue;
 359     IF_DEBUG(scheduler, sched_belch("worker: passCapability"));
 360
 361     // Post-condition: pMutex is held; cap is still held, but will be
 362     //                 passed to the target thread when next released.
 363 }
 364
 365 void
 366 passCapabilityToWorker( void )
 367 {
 368     // Pre-condition: pMutex is held and cap is held by the current thread
 369
 370     passTarget = NULL;
 371     passingCapability = rtsTrue;
 372     IF_DEBUG(scheduler, sched_belch("worker: passCapabilityToWorker"));
 373
 374     // Post-condition: pMutex is held; cap is still held, but will be
 375     //                 passed to a worker thread when next released.
 376 }
 377
 378 #endif /* RTS_SUPPORTS_THREADS */
 379
 380 /* ------------------------------------------------------------------------- */
 381
 382 #if defined(SMP)
 383 /*
 384  * Function: initCapabilities_(nat)
 385  *
 386  * Purpose:  upon startup, allocate and fill in table
 387  *           holding 'n' Capabilities. Only for SMP, since
 388  *           it is the only build that supports multiple
 389  *           capabilities within the RTS.
 390  */
 391 static void
 392 initCapabilities_(nat n)
 393 {
 394   nat i;
 395   Capability *cap, *prev;
 396   cap  = NULL;
 397   prev = NULL;
 398   for (i = 0; i < n; i++) {
 399     cap = stgMallocBytes(sizeof(Capability), "initCapabilities");
 400     initCapability(cap);
 401     cap->link = prev;
 402     prev = cap;
 403   }
 404   free_capabilities = cap;
 405   rts_n_free_capabilities = n;
 406   returning_capabilities = NULL;
 407   IF_DEBUG(scheduler,
 408            sched_belch("allocated %d capabilities", n_free_capabilities));
 409 }
 410 #endif /* SMP */
 411