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 #endif
  73
  74 /* -----------------------------------------------------------------------------
  75    Initialisation
  76    -------------------------------------------------------------------------- */
  77 static
  78 void
  79 initCapability( Capability *cap )
  80 {
  81     cap->f.stgGCEnter1     = (F_)__stg_gc_enter_1;
  82     cap->f.stgGCFun        = (F_)__stg_gc_fun;
  83 }
  84
  85 #if defined(SMP)
  86 static void initCapabilities_(nat n);
  87 #endif
  88
  89 /*
  90  * Function:  initCapabilities()
  91  *
  92  * Purpose:   set up the Capability handling. For the SMP build,
  93  *            we keep a table of them, the size of which is
  94  *            controlled by the user via the RTS flag RtsFlags.ParFlags.nNodes
  95  *
  96  * Pre-conditions: no locks assumed held.
  97  */
  98 void
  99 initCapabilities()
 100 {
 101 #if defined(RTS_SUPPORTS_THREADS)
 102   initCondition(&returning_worker_cond);
 103   initCondition(&thread_ready_cond);
 104 #endif
 105
 106 #if defined(SMP)
 107   initCapabilities_(RtsFlags.ParFlags.nNodes);
 108 #else
 109   initCapability(&MainCapability);
 110   rts_n_free_capabilities = 1;
 111 #endif
 112
 113   return;
 114 }
 115
 116 #if defined(SMP)
 117 /* Free capability list. */
 118 static Capability *free_capabilities; /* Available capabilities for running threads */
 119 static Capability *returning_capabilities;
 120         /* Capabilities being passed to returning worker threads */
 121 #endif
 122
 123 /* -----------------------------------------------------------------------------
 124    Acquiring capabilities
 125    -------------------------------------------------------------------------- */
 126
 127 /*
 128  * Function:  grabCapability(Capability**)
 129  *
 130  * Purpose:   the act of grabbing a capability is easy; just
 131  *            remove one from the free capabilities list (which
 132  *            may just have one entry). In threaded builds, worker
 133  *            threads are prevented from doing so willy-nilly
 134  *            via the condition variables thread_ready_cond and
 135  *            returning_worker_cond.
 136  *
 137  */
 138 void grabCapability(Capability** cap)
 139 {
 140 #if !defined(SMP)
 141   ASSERT(rts_n_free_capabilities == 1);
 142   rts_n_free_capabilities = 0;
 143   *cap = &MainCapability;
 144   handleSignalsInThisThread();
 145 #else
 146   *cap = free_capabilities;
 147   free_capabilities = (*cap)->link;
 148   rts_n_free_capabilities--;
 149 #endif
 150 #ifdef RTS_SUPPORTS_THREADS
 151     IF_DEBUG(scheduler, sched_belch("worker: got capability"));
 152 #endif
 153 }
 154
 155 /*
 156  * Function:  releaseCapability(Capability*)
 157  *
 158  * Purpose:   Letting go of a capability. Causes a
 159  *            'returning worker' thread or a 'waiting worker'
 160  *            to wake up, in that order.
 161  *
 162  */
 163 void releaseCapability(Capability* cap
 164 #if !defined(SMP)
 165                        STG_UNUSED
 166 #endif
 167 )
 168 {       // Precondition: sched_mutex must be held
 169 #if defined(RTS_SUPPORTS_THREADS)
 170 #ifndef SMP
 171   ASSERT(rts_n_free_capabilities == 0);
 172 #endif
 173   /* Check to see whether a worker thread can be given
 174      the go-ahead to return the result of an external call..*/
 175   if (rts_n_waiting_workers > 0) {
 176     /* Decrement the counter here to avoid livelock where the
 177      * thread that is yielding its capability will repeatedly
 178      * signal returning_worker_cond.
 179      */
 180 #if defined(SMP)
 181         // SMP variant untested
 182     cap->link = returning_capabilities;
 183     returning_capabilities = cap;
 184 #else
 185 #endif
 186     rts_n_waiting_workers--;
 187     signalCondition(&returning_worker_cond);
 188     IF_DEBUG(scheduler, sched_belch("worker: released capability to returning worker"));
 189   } else /*if ( !EMPTY_RUN_QUEUE() )*/ {
 190 #if defined(SMP)
 191     cap->link = free_capabilities;
 192     free_capabilities = cap;
 193     rts_n_free_capabilities++;
 194 #else
 195     rts_n_free_capabilities = 1;
 196 #endif
 197     /* Signal that a capability is available */
 198     signalCondition(&thread_ready_cond);
 199     startSchedulerTaskIfNecessary();  // if there is more work to be done,
 200                                       // we'll need a new thread
 201   }
 202 #endif
 203 #ifdef RTS_SUPPORTS_THREADS
 204     IF_DEBUG(scheduler, sched_belch("worker: released capability"));
 205 #endif
 206   return;
 207 }
 208
 209 #if defined(RTS_SUPPORTS_THREADS)
 210 /*
 211  * When a native thread has completed the execution of an external
 212  * call, it needs to communicate the result back. This is done
 213  * as follows:
 214  *
 215  *  - in resumeThread(), the thread calls grabReturnCapability().
 216  *  - If no capabilities are readily available, grabReturnCapability()
 217  *    increments a counter rts_n_waiting_workers, and blocks
 218  *    waiting for the condition returning_worker_cond to become
 219  *    signalled.
 220  *  - upon entry to the Scheduler, a worker thread checks the
 221  *    value of rts_n_waiting_workers. If > 0, the worker thread
 222  *    will yield its capability to let a returning worker thread
 223  *    proceed with returning its result -- this is done via
 224  *    yieldToReturningWorker().
 225  *  - the worker thread that yielded its capability then tries
 226  *    to re-grab a capability and re-enter the Scheduler.
 227  */
 228
 229 /*
 230  * Function: grabReturnCapability(Capability**)
 231  *
 232  * Purpose:  when an OS thread returns from an external call,
 233  * it calls grabReturnCapability() (via Schedule.resumeThread())
 234  * to wait for permissions to enter the RTS & communicate the
 235  * result of the external call back to the Haskell thread that
 236  * made it.
 237  *
 238  * Pre-condition:  pMutex is held.
 239  * Post-condition: pMutex is still held and a capability has
 240  *                 been assigned to the worker thread.
 241  */
 242 void
 243 grabReturnCapability(Mutex* pMutex, Capability** pCap)
 244 {
 245   IF_DEBUG(scheduler,
 246            sched_belch("worker: returning; workers waiting: %d",
 247                        rts_n_waiting_workers));
 248   if ( noCapabilities() ) {
 249     rts_n_waiting_workers++;
 250     wakeBlockedWorkerThread();
 251     context_switch = 1; // make sure it's our turn soon
 252     waitCondition(&returning_worker_cond, pMutex);
 253 #if defined(SMP)
 254     *pCap = returning_capabilities;
 255     returning_capabilities = (*pCap)->link;
 256 #else
 257     *pCap = &MainCapability;
 258     ASSERT(rts_n_free_capabilities == 0);
 259     handleSignalsInThisThread();
 260 #endif
 261   } else {
 262     grabCapability(pCap);
 263   }
 264   return;
 265 }
 266
 267
 268 /* -----------------------------------------------------------------------------
 269    Yielding/waiting for capabilities
 270    -------------------------------------------------------------------------- */
 271
 272 /*
 273  * Function: yieldToReturningWorker(Mutex*,Capability*,Condition*)
 274  *
 275  * Purpose:  when, upon entry to the Scheduler, an OS worker thread
 276  *           spots that one or more threads are blocked waiting for
 277  *           permission to return back their result, it gives up
 278  *           its Capability.
 279  *           Immediately afterwards, it tries to reaquire the Capabilty
 280  *           using waitForWorkCapability.
 281  *
 282  *
 283  * Pre-condition:  pMutex is assumed held and the thread possesses
 284  *                 a Capability.
 285  * Post-condition: pMutex is held and the thread possesses
 286  *                 a Capability.
 287  */
 288 void
 289 yieldToReturningWorker(Mutex* pMutex, Capability** pCap, Condition* pThreadCond)
 290 {
 291   if ( rts_n_waiting_workers > 0 ) {
 292     IF_DEBUG(scheduler, sched_belch("worker: giving up capability"));
 293     releaseCapability(*pCap);
 294         /* And wait for work */
 295     waitForWorkCapability(pMutex, pCap, pThreadCond);
 296     IF_DEBUG(scheduler,
 297              sched_belch("worker: got back capability (after yieldToReturningWorker)"));
 298   }
 299   return;
 300 }
 301
 302
 303 /*
 304  * Function: waitForWorkCapability(Mutex*, Capability**, Condition*)
 305  *
 306  * Purpose:  wait for a Capability to become available. In
 307  *           the process of doing so, updates the number
 308  *           of tasks currently blocked waiting for a capability/more
 309  *           work. That counter is used when deciding whether or
 310  *           not to create a new worker thread when an external
 311  *           call is made.
 312  *           If pThreadCond is not NULL, a capability can be specifically
 313  *           passed to this thread using passCapability.
 314  *
 315  * Pre-condition: pMutex is held.
 316  * Post-condition: pMutex is held and *pCap is held by the current thread
 317  */
 318
 319 static Condition *passTarget = NULL;
 320 static rtsBool passingCapability = rtsFalse;
 321
 322 void
 323 waitForWorkCapability(Mutex* pMutex, Capability** pCap, Condition* pThreadCond)
 324 {
 325 #ifdef SMP
 326   #error SMP version not implemented
 327 #endif
 328   while ( noCapabilities() || (passingCapability && passTarget != pThreadCond)) {
 329     IF_DEBUG(scheduler,
 330              sched_belch("worker: wait for capability (cond: %p)",
 331                          pThreadCond));
 332     if(pThreadCond)
 333     {
 334       waitCondition(pThreadCond, pMutex);
 335       IF_DEBUG(scheduler, sched_belch("worker: get passed capability"));
 336     }
 337     else
 338     {
 339       rts_n_waiting_tasks++;
 340       waitCondition(&thread_ready_cond, pMutex);
 341       rts_n_waiting_tasks--;
 342       IF_DEBUG(scheduler, sched_belch("worker: get normal capability"));
 343     }
 344   }
 345   passingCapability = rtsFalse;
 346   grabCapability(pCap);
 347   return;
 348 }
 349
 350 /*
 351  * Function: passCapability(Mutex*, Capability*, Condition*)
 352  *
 353  * Purpose:  Let go of the capability and make sure the thread associated
 354  *           with the Condition pTargetThreadCond gets it next.
 355  *
 356  * Pre-condition: pMutex is held and cap is held by the current thread
 357  * Post-condition: pMutex is held; cap will be grabbed by the "target"
 358  *                 thread when pMutex is released.
 359  */
 360
 361 void
 362 passCapability(Mutex* pMutex, Capability* cap, Condition *pTargetThreadCond)
 363 {
 364 #ifdef SMP
 365   #error SMP version not implemented
 366 #endif
 367     rts_n_free_capabilities = 1;
 368     signalCondition(pTargetThreadCond);
 369     passTarget = pTargetThreadCond;
 370     passingCapability = rtsTrue;
 371     IF_DEBUG(scheduler, sched_belch("worker: passCapability"));
 372 }
 373
 374 /*
 375  * Function: passCapabilityToWorker(Mutex*, Capability*)
 376  *
 377  * Purpose:  Let go of the capability and make sure that a
 378  *           "plain" worker thread (not a bound thread) gets it next.
 379  *
 380  * Pre-condition: pMutex is held and cap is held by the current thread
 381  * Post-condition: pMutex is held; cap will be grabbed by the "target"
 382  *                 thread when pMutex is released.
 383  */
 384
 385 void
 386 passCapabilityToWorker(Mutex* pMutex, Capability* cap)
 387 {
 388 #ifdef SMP
 389   #error SMP version not implemented
 390 #endif
 391     rts_n_free_capabilities = 1;
 392     signalCondition(&thread_ready_cond);
 393     startSchedulerTaskIfNecessary();
 394     passTarget = NULL;
 395     passingCapability = rtsTrue;
 396     IF_DEBUG(scheduler, sched_belch("worker: passCapabilityToWorker"));
 397 }
 398
 399
 400
 401 #endif /* RTS_SUPPORTS_THREADS */
 402
 403 #if defined(SMP)
 404 /*
 405  * Function: initCapabilities_(nat)
 406  *
 407  * Purpose:  upon startup, allocate and fill in table
 408  *           holding 'n' Capabilities. Only for SMP, since
 409  *           it is the only build that supports multiple
 410  *           capabilities within the RTS.
 411  */
 412 static void
 413 initCapabilities_(nat n)
 414 {
 415   nat i;
 416   Capability *cap, *prev;
 417   cap  = NULL;
 418   prev = NULL;
 419   for (i = 0; i < n; i++) {
 420     cap = stgMallocBytes(sizeof(Capability), "initCapabilities");
 421     initCapability(cap);
 422     cap->link = prev;
 423     prev = cap;
 424   }
 425   free_capabilities = cap;
 426   rts_n_free_capabilities = n;
 427   returning_capabilities = NULL;
 428   IF_DEBUG(scheduler,
 429            sched_belch("allocated %d capabilities", n_free_capabilities));
 430 }
 431 #endif /* SMP */
 432