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         if (rts_n_waiting_tasks > 0) {
 210             signalCondition(&thread_ready_cond);
 211         }
 212         startSchedulerTaskIfNecessary();
 213         IF_DEBUG(scheduler, sched_belch("worker: released capability"));
 214     }
 215 #endif
 216     return;
 217 }
 218
 219 #if defined(RTS_SUPPORTS_THREADS)
 220 /*
 221  * When a native thread has completed the execution of an external
 222  * call, it needs to communicate the result back. This is done
 223  * as follows:
 224  *
 225  *  - in resumeThread(), the thread calls waitForReturnCapability().
 226  *  - If no capabilities are readily available, waitForReturnCapability()
 227  *    increments a counter rts_n_waiting_workers, and blocks
 228  *    waiting for the condition returning_worker_cond to become
 229  *    signalled.
 230  *  - upon entry to the Scheduler, a worker thread checks the
 231  *    value of rts_n_waiting_workers. If > 0, the worker thread
 232  *    will yield its capability to let a returning worker thread
 233  *    proceed with returning its result -- this is done via
 234  *    yieldToReturningWorker().
 235  *  - the worker thread that yielded its capability then tries
 236  *    to re-grab a capability and re-enter the Scheduler.
 237  */
 238
 239 /* ----------------------------------------------------------------------------
 240  * waitForReturnCapability( Mutext *pMutex, Capability** )
 241  *
 242  * Purpose:  when an OS thread returns from an external call,
 243  * it calls grabReturnCapability() (via Schedule.resumeThread())
 244  * to wait for permissions to enter the RTS & communicate the
 245  * result of the external call back to the Haskell thread that
 246  * made it.
 247  *
 248  * ------------------------------------------------------------------------- */
 249
 250 void
 251 waitForReturnCapability( Mutex* pMutex, Capability** pCap )
 252 {
 253     // Pre-condition: pMutex is held.
 254
 255     IF_DEBUG(scheduler,
 256              sched_belch("worker: returning; workers waiting: %d",
 257                          rts_n_waiting_workers));
 258
 259     if ( noCapabilities() || passingCapability ) {
 260         rts_n_waiting_workers++;
 261         wakeBlockedWorkerThread();
 262         context_switch = 1;     // make sure it's our turn soon
 263         waitCondition(&returning_worker_cond, pMutex);
 264 #if defined(SMP)
 265         *pCap = returning_capabilities;
 266         returning_capabilities = (*pCap)->link;
 267 #else
 268         *pCap = &MainCapability;
 269         ASSERT(rts_n_free_capabilities == 0);
 270         handleSignalsInThisThread();
 271 #endif
 272     } else {
 273         grabCapability(pCap);
 274     }
 275
 276     // Post-condition: pMutex is held, pCap points to a capability
 277     // which is now held by the current thread.
 278     return;
 279 }
 280
 281
 282 /* ----------------------------------------------------------------------------
 283  * yieldCapability( Mutex* pMutex, Capability** pCap )
 284  * ------------------------------------------------------------------------- */
 285
 286 void
 287 yieldCapability( Capability** pCap )
 288 {
 289     // Pre-condition:  pMutex is assumed held, the current thread
 290     // holds the capability pointed to by pCap.
 291
 292     if ( rts_n_waiting_workers > 0 || passingCapability ) {
 293         IF_DEBUG(scheduler, sched_belch("worker: giving up capability"));
 294         releaseCapability(*pCap);
 295         *pCap = NULL;
 296     }
 297
 298     // Post-condition:  pMutex is assumed held, and either:
 299     //
 300     //  1. *pCap is NULL, in which case the current thread does not
 301     //     hold a capability now, or
 302     //  2. *pCap is not NULL, in which case the current thread still
 303     //     holds the capability.
 304     //
 305     return;
 306 }
 307
 308
 309 /* ----------------------------------------------------------------------------
 310  * waitForCapability( Mutex*, Capability**, Condition* )
 311  *
 312  * Purpose:  wait for a Capability to become available. In
 313  *           the process of doing so, updates the number
 314  *           of tasks currently blocked waiting for a capability/more
 315  *           work. That counter is used when deciding whether or
 316  *           not to create a new worker thread when an external
 317  *           call is made.
 318  *           If pThreadCond is not NULL, a capability can be specifically
 319  *           passed to this thread using passCapability.
 320  * ------------------------------------------------------------------------- */
 321
 322 void
 323 waitForCapability( Mutex* pMutex, Capability** pCap, Condition* pThreadCond )
 324 {
 325     // Pre-condition: pMutex is held.
 326
 327     while ( noCapabilities() ||
 328             (passingCapability && passTarget != pThreadCond)) {
 329         IF_DEBUG(scheduler,
 330                  sched_belch("worker: wait for capability (cond: %p)",
 331                              pThreadCond));
 332
 333         if (pThreadCond != NULL) {
 334             waitCondition(pThreadCond, pMutex);
 335             IF_DEBUG(scheduler, sched_belch("worker: get passed capability"));
 336         } else {
 337             rts_n_waiting_tasks++;
 338             waitCondition(&thread_ready_cond, pMutex);
 339             rts_n_waiting_tasks--;
 340             IF_DEBUG(scheduler, sched_belch("worker: get normal capability"));
 341         }
 342     }
 343     passingCapability = rtsFalse;
 344     grabCapability(pCap);
 345
 346     // Post-condition: pMutex is held and *pCap is held by the current thread
 347     return;
 348 }
 349
 350 /* ----------------------------------------------------------------------------
 351    passCapability, passCapabilityToWorker
 352    ------------------------------------------------------------------------- */
 353
 354 void
 355 passCapability( Condition *pTargetThreadCond )
 356 {
 357     // Pre-condition: pMutex is held and cap is held by the current thread
 358
 359     passTarget = pTargetThreadCond;
 360     passingCapability = rtsTrue;
 361     IF_DEBUG(scheduler, sched_belch("worker: passCapability"));
 362
 363     // Post-condition: pMutex is held; cap is still held, but will be
 364     //                 passed to the target thread when next released.
 365 }
 366
 367 void
 368 passCapabilityToWorker( void )
 369 {
 370     // Pre-condition: pMutex is held and cap is held by the current thread
 371
 372     passTarget = NULL;
 373     passingCapability = rtsTrue;
 374     IF_DEBUG(scheduler, sched_belch("worker: passCapabilityToWorker"));
 375
 376     // Post-condition: pMutex is held; cap is still held, but will be
 377     //                 passed to a worker thread when next released.
 378 }
 379
 380 #endif /* RTS_SUPPORTS_THREADS */
 381
 382 /* ------------------------------------------------------------------------- */
 383
 384 #if defined(SMP)
 385 /*
 386  * Function: initCapabilities_(nat)
 387  *
 388  * Purpose:  upon startup, allocate and fill in table
 389  *           holding 'n' Capabilities. Only for SMP, since
 390  *           it is the only build that supports multiple
 391  *           capabilities within the RTS.
 392  */
 393 static void
 394 initCapabilities_(nat n)
 395 {
 396   nat i;
 397   Capability *cap, *prev;
 398   cap  = NULL;
 399   prev = NULL;
 400   for (i = 0; i < n; i++) {
 401     cap = stgMallocBytes(sizeof(Capability), "initCapabilities");
 402     initCapability(cap);
 403     cap->link = prev;
 404     prev = cap;
 405   }
 406   free_capabilities = cap;
 407   rts_n_free_capabilities = n;
 408   returning_capabilities = NULL;
 409   IF_DEBUG(scheduler,
 410            sched_belch("allocated %d capabilities", n_free_capabilities));
 411 }
 412 #endif /* SMP */
 413