ghc/rts/Capability.c

   1 /* ---------------------------------------------------------------------------
   2  *
   3  * (c) The GHC Team, 2002
   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).
  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 #include "PosixSource.h"
  19 #include "Rts.h"
  20 #include "RtsUtils.h"
  21 #include "OSThreads.h"
  22 #include "Capability.h"
  23 #include "Schedule.h"  /* to get at EMPTY_RUN_QUEUE() */
  24 #include "Signals.h" /* to get at handleSignalsInThisThread() */
  25
  26 #if !defined(SMP)
  27 Capability MainCapability;     /* for non-SMP, we have one global capability */
  28 #endif
  29
  30 nat rts_n_free_capabilities;
  31
  32 #if defined(RTS_SUPPORTS_THREADS)
  33 /* returning_worker_cond: when a worker thread returns from executing an
  34  * external call, it needs to wait for an RTS Capability before passing
  35  * on the result of the call to the Haskell thread that made it.
  36  *
  37  * returning_worker_cond is signalled in Capability.releaseCapability().
  38  *
  39  */
  40 Condition returning_worker_cond = INIT_COND_VAR;
  41
  42 /*
  43  * To avoid starvation of threads blocked on worker_thread_cond,
  44  * the task(s) that enter the Scheduler will check to see whether
  45  * there are one or more worker threads blocked waiting on
  46  * returning_worker_cond.
  47  */
  48 nat rts_n_waiting_workers = 0;
  49
  50 /* thread_ready_cond: when signalled, a thread has become runnable for a
  51  * task to execute.
  52  *
  53  * In the non-SMP case, it also implies that the thread that is woken up has
  54  * exclusive access to the RTS and all its data structures (that are not
  55  * locked by the Scheduler's mutex).
  56  *
  57  * thread_ready_cond is signalled whenever noCapabilities doesn't hold.
  58  *
  59  */
  60 Condition thread_ready_cond = INIT_COND_VAR;
  61
  62 /*
  63  * To be able to make an informed decision about whether or not
  64  * to create a new task when making an external call, keep track of
  65  * the number of tasks currently blocked waiting on thread_ready_cond.
  66  * (if > 0 => no need for a new task, just unblock an existing one).
  67  *
  68  * waitForWorkCapability() takes care of keeping it up-to-date;
  69  * Task.startTask() uses its current value.
  70  */
  71 nat rts_n_waiting_tasks = 0;
  72
  73 static Condition *passTarget = NULL;
  74 static rtsBool passingCapability = rtsFalse;
  75 #endif
  76
  77 /* -----------------------------------------------------------------------------
  78    Initialisation
  79    -------------------------------------------------------------------------- */
  80 static
  81 void
  82 initCapability( Capability *cap )
  83 {
  84     cap->f.stgGCEnter1     = (F_)__stg_gc_enter_1;
  85     cap->f.stgGCFun        = (F_)__stg_gc_fun;
  86 }
  87
  88 #if defined(SMP)
  89 static void initCapabilities_(nat n);
  90 #endif
  91
  92 /*
  93  * Function:  initCapabilities()
  94  *
  95  * Purpose:   set up the Capability handling. For the SMP build,
  96  *            we keep a table of them, the size of which is
  97  *            controlled by the user via the RTS flag RtsFlags.ParFlags.nNodes
  98  *
  99  * Pre-conditions: no locks assumed held.
 100  */
 101 void
 102 initCapabilities()
 103 {
 104 #if defined(RTS_SUPPORTS_THREADS)
 105   initCondition(&returning_worker_cond);
 106   initCondition(&thread_ready_cond);
 107 #endif
 108
 109 #if defined(SMP)
 110   initCapabilities_(RtsFlags.ParFlags.nNodes);
 111 #else
 112   initCapability(&MainCapability);
 113   rts_n_free_capabilities = 1;
 114 #endif
 115
 116   return;
 117 }
 118
 119 #if defined(SMP)
 120 /* Free capability list. */
 121 static Capability *free_capabilities; /* Available capabilities for running threads */
 122 static Capability *returning_capabilities;
 123         /* Capabilities being passed to returning worker threads */
 124 #endif
 125
 126 /* -----------------------------------------------------------------------------
 127    Acquiring capabilities
 128    -------------------------------------------------------------------------- */
 129
 130 /*
 131  * Function:  grabCapability(Capability**)
 132  *
 133  * Purpose:   the act of grabbing a capability is easy; just
 134  *            remove one from the free capabilities list (which
 135  *            may just have one entry). In threaded builds, worker
 136  *            threads are prevented from doing so willy-nilly
 137  *            via the condition variables thread_ready_cond and
 138  *            returning_worker_cond.
 139  *
 140  */
 141 void grabCapability(Capability** cap)
 142 {
 143 #if !defined(SMP)
 144   ASSERT(rts_n_free_capabilities == 1);
 145   rts_n_free_capabilities = 0;
 146   *cap = &MainCapability;
 147   handleSignalsInThisThread();
 148 #else
 149   *cap = free_capabilities;
 150   free_capabilities = (*cap)->link;
 151   rts_n_free_capabilities--;
 152 #endif
 153   IF_DEBUG(scheduler, sched_belch("worker: got capability"));
 154 }
 155
 156 /*
 157  * Function:  releaseCapability(Capability*)
 158  *
 159  * Purpose:   Letting go of a capability. Causes a
 160  *            'returning worker' thread or a 'waiting worker'
 161  *            to wake up, in that order.
 162  *
 163  */
 164 void releaseCapability(Capability* cap
 165 #if !defined(SMP)
 166                        STG_UNUSED
 167 #endif
 168 )
 169 {       // Precondition: sched_mutex must be 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 #else
 186 #endif
 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 ( !EMPTY_RUN_QUEUE() )*/ {
 191 #if defined(SMP)
 192     cap->link = free_capabilities;
 193     free_capabilities = cap;
 194     rts_n_free_capabilities++;
 195 #else
 196     rts_n_free_capabilities = 1;
 197 #endif
 198     /* Signal that a capability is available */
 199     signalCondition(&thread_ready_cond);
 200     startSchedulerTaskIfNecessary();  // if there is more work to be done,
 201                                       // we'll need a new thread
 202     IF_DEBUG(scheduler, sched_belch("worker: released capability"));
 203   }
 204 #endif
 205   return;
 206 }
 207
 208 #if defined(RTS_SUPPORTS_THREADS)
 209 /*
 210  * When a native thread has completed the execution of an external
 211  * call, it needs to communicate the result back. This is done
 212  * as follows:
 213  *
 214  *  - in resumeThread(), the thread calls grabReturnCapability().
 215  *  - If no capabilities are readily available, grabReturnCapability()
 216  *    increments a counter rts_n_waiting_workers, and blocks
 217  *    waiting for the condition returning_worker_cond to become
 218  *    signalled.
 219  *  - upon entry to the Scheduler, a worker thread checks the
 220  *    value of rts_n_waiting_workers. If > 0, the worker thread
 221  *    will yield its capability to let a returning worker thread
 222  *    proceed with returning its result -- this is done via
 223  *    yieldToReturningWorker().
 224  *  - the worker thread that yielded its capability then tries
 225  *    to re-grab a capability and re-enter the Scheduler.
 226  */
 227
 228 /*
 229  * Function: grabReturnCapability(Capability**)
 230  *
 231  * Purpose:  when an OS thread returns from an external call,
 232  * it calls grabReturnCapability() (via Schedule.resumeThread())
 233  * to wait for permissions to enter the RTS & communicate the
 234  * result of the external call back to the Haskell thread that
 235  * made it.
 236  *
 237  * Pre-condition:  pMutex is held.
 238  * Post-condition: pMutex is still held and a capability has
 239  *                 been assigned to the worker thread.
 240  */
 241 void
 242 grabReturnCapability(Mutex* pMutex, Capability** pCap)
 243 {
 244   IF_DEBUG(scheduler,
 245            sched_belch("worker: returning; workers waiting: %d",
 246                        rts_n_waiting_workers));
 247   if ( noCapabilities() || passingCapability ) {
 248     rts_n_waiting_workers++;
 249     wakeBlockedWorkerThread();
 250     context_switch = 1; // make sure it's our turn soon
 251     waitCondition(&returning_worker_cond, pMutex);
 252 #if defined(SMP)
 253     *pCap = returning_capabilities;
 254     returning_capabilities = (*pCap)->link;
 255 #else
 256     *pCap = &MainCapability;
 257     ASSERT(rts_n_free_capabilities == 0);
 258     handleSignalsInThisThread();
 259 #endif
 260   } else {
 261     grabCapability(pCap);
 262   }
 263   return;
 264 }
 265
 266
 267 /* -----------------------------------------------------------------------------
 268    Yielding/waiting for capabilities
 269    -------------------------------------------------------------------------- */
 270
 271 /*
 272  * Function: yieldToReturningWorker(Mutex*,Capability*,Condition*)
 273  *
 274  * Purpose:  when, upon entry to the Scheduler, an OS worker thread
 275  *           spots that one or more threads are blocked waiting for
 276  *           permission to return back their result, it gives up
 277  *           its Capability.
 278  *           Immediately afterwards, it tries to reaquire the Capabilty
 279  *           using waitForWorkCapability.
 280  *
 281  *
 282  * Pre-condition:  pMutex is assumed held and the thread possesses
 283  *                 a Capability.
 284  * Post-condition: pMutex is held and the thread possesses
 285  *                 a Capability.
 286  */
 287 void
 288 yieldToReturningWorker(Mutex* pMutex, Capability** pCap, Condition* pThreadCond)
 289 {
 290   if ( rts_n_waiting_workers > 0 ) {
 291     IF_DEBUG(scheduler, sched_belch("worker: giving up capability"));
 292     releaseCapability(*pCap);
 293         /* And wait for work */
 294     waitForWorkCapability(pMutex, pCap, pThreadCond);
 295     IF_DEBUG(scheduler,
 296              sched_belch("worker: got back capability (after yieldToReturningWorker)"));
 297   }
 298   return;
 299 }
 300
 301
 302 /*
 303  * Function: waitForWorkCapability(Mutex*, Capability**, Condition*)
 304  *
 305  * Purpose:  wait for a Capability to become available. In
 306  *           the process of doing so, updates the number
 307  *           of tasks currently blocked waiting for a capability/more
 308  *           work. That counter is used when deciding whether or
 309  *           not to create a new worker thread when an external
 310  *           call is made.
 311  *           If pThreadCond is not NULL, a capability can be specifically
 312  *           passed to this thread using passCapability.
 313  *
 314  * Pre-condition: pMutex is held.
 315  * Post-condition: pMutex is held and *pCap is held by the current thread
 316  */
 317
 318 void
 319 waitForWorkCapability(Mutex* pMutex, Capability** pCap, Condition* pThreadCond)
 320 {
 321 #ifdef SMP
 322   #error SMP version not implemented
 323 #endif
 324   while ( noCapabilities() || (passingCapability && passTarget != pThreadCond)) {
 325     IF_DEBUG(scheduler,
 326              sched_belch("worker: wait for capability (cond: %p)",
 327                          pThreadCond));
 328     if(pThreadCond)
 329     {
 330       waitCondition(pThreadCond, pMutex);
 331       IF_DEBUG(scheduler, sched_belch("worker: get passed capability"));
 332     }
 333     else
 334     {
 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   return;
 344 }
 345
 346 /*
 347  * Function: passCapability(Mutex*, Capability*, Condition*)
 348  *
 349  * Purpose:  Let go of the capability and make sure the thread associated
 350  *           with the Condition pTargetThreadCond gets it next.
 351  *
 352  * Pre-condition: pMutex is held and cap is held by the current thread
 353  * Post-condition: pMutex is held; cap will be grabbed by the "target"
 354  *                 thread when pMutex is released.
 355  */
 356
 357 void
 358 passCapability(Mutex* pMutex, Capability* cap, Condition *pTargetThreadCond)
 359 {
 360 #ifdef SMP
 361   #error SMP version not implemented
 362 #endif
 363     ASSERT(rts_n_free_capabilities == 0);
 364     rts_n_free_capabilities = 1;
 365     signalCondition(pTargetThreadCond);
 366     passTarget = pTargetThreadCond;
 367     passingCapability = rtsTrue;
 368     IF_DEBUG(scheduler, sched_belch("worker: passCapability"));
 369 }
 370
 371 /*
 372  * Function: passCapabilityToWorker(Mutex*, Capability*)
 373  *
 374  * Purpose:  Let go of the capability and make sure that a
 375  *           "plain" worker thread (not a bound thread) gets it next.
 376  *
 377  * Pre-condition: pMutex is held and cap is held by the current thread
 378  * Post-condition: pMutex is held; cap will be grabbed by the "target"
 379  *                 thread when pMutex is released.
 380  */
 381
 382 void
 383 passCapabilityToWorker(Mutex* pMutex, Capability* cap)
 384 {
 385 #ifdef SMP
 386   #error SMP version not implemented
 387 #endif
 388     rts_n_free_capabilities = 1;
 389     signalCondition(&thread_ready_cond);
 390     startSchedulerTaskIfNecessary();
 391     passTarget = NULL;
 392     passingCapability = rtsTrue;
 393     IF_DEBUG(scheduler, sched_belch("worker: passCapabilityToWorker"));
 394 }
 395
 396
 397
 398 #endif /* RTS_SUPPORTS_THREADS */
 399
 400 #if defined(SMP)
 401 /*
 402  * Function: initCapabilities_(nat)
 403  *
 404  * Purpose:  upon startup, allocate and fill in table
 405  *           holding 'n' Capabilities. Only for SMP, since
 406  *           it is the only build that supports multiple
 407  *           capabilities within the RTS.
 408  */
 409 static void
 410 initCapabilities_(nat n)
 411 {
 412   nat i;
 413   Capability *cap, *prev;
 414   cap  = NULL;
 415   prev = NULL;
 416   for (i = 0; i < n; i++) {
 417     cap = stgMallocBytes(sizeof(Capability), "initCapabilities");
 418     initCapability(cap);
 419     cap->link = prev;
 420     prev = cap;
 421   }
 422   free_capabilities = cap;
 423   rts_n_free_capabilities = n;
 424   returning_capabilities = NULL;
 425   IF_DEBUG(scheduler,
 426            sched_belch("allocated %d capabilities", n_free_capabilities));
 427 }
 428 #endif /* SMP */
 429