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 #ifdef RTS_SUPPORTS_THREADS
 141   ASSERT(rts_n_free_capabilities > 0);
 142 #endif
 143 #if !defined(SMP)
 144   rts_n_free_capabilities = 0;
 145   *cap = &MainCapability;
 146   handleSignalsInThisThread();
 147 #else
 148   *cap = free_capabilities;
 149   free_capabilities = (*cap)->link;
 150   rts_n_free_capabilities--;
 151 #endif
 152     IF_DEBUG(scheduler,
 153              fprintf(stderr,"worker thread (%p): got capability\n",
 154                 osThreadId()));
 155 }
 156
 157 /*
 158  * Function:  releaseCapability(Capability*)
 159  *
 160  * Purpose:   Letting go of a capability. Causes a
 161  *            'returning worker' thread or a 'waiting worker'
 162  *            to wake up, in that order.
 163  *
 164  */
 165 void releaseCapability(Capability* cap
 166 #if !defined(SMP)
 167                        STG_UNUSED
 168 #endif
 169 )
 170 {       // Precondition: sched_mutex must be held
 171 #if defined(RTS_SUPPORTS_THREADS)
 172 #ifndef SMP
 173   ASSERT(rts_n_free_capabilities == 0);
 174 #endif
 175   /* Check to see whether a worker thread can be given
 176      the go-ahead to return the result of an external call..*/
 177   if (rts_n_waiting_workers > 0) {
 178     /* Decrement the counter here to avoid livelock where the
 179      * thread that is yielding its capability will repeatedly
 180      * signal returning_worker_cond.
 181      */
 182 #if defined(SMP)
 183         // SMP variant untested
 184     cap->link = returning_capabilities;
 185     returning_capabilities = cap;
 186 #else
 187 #endif
 188     rts_n_waiting_workers--;
 189     signalCondition(&returning_worker_cond);
 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   }
 201 #endif
 202     IF_DEBUG(scheduler,
 203              fprintf(stderr,"worker thread (%p): released capability\n",
 204                 osThreadId()));
 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            fprintf(stderr,"worker (%p): returning, waiting for lock.\n", osThreadId()));
 246   IF_DEBUG(scheduler,
 247            fprintf(stderr,"worker (%p): returning; workers waiting: %d\n",
 248                    osThreadId(), rts_n_waiting_workers));
 249   if ( noCapabilities() ) {
 250     rts_n_waiting_workers++;
 251     wakeBlockedWorkerThread();
 252     context_switch = 1; // make sure it's our turn soon
 253     waitCondition(&returning_worker_cond, pMutex);
 254 #if defined(SMP)
 255     *pCap = returning_capabilities;
 256     returning_capabilities = (*pCap)->link;
 257 #else
 258     *pCap = &MainCapability;
 259     ASSERT(rts_n_free_capabilities == 0);
 260     handleSignalsInThisThread();
 261 #endif
 262   } else {
 263     grabCapability(pCap);
 264   }
 265   return;
 266 }
 267
 268
 269 /* -----------------------------------------------------------------------------
 270    Yielding/waiting for capabilities
 271    -------------------------------------------------------------------------- */
 272
 273 /*
 274  * Function: yieldToReturningWorker(Mutex*,Capability*,Condition*)
 275  *
 276  * Purpose:  when, upon entry to the Scheduler, an OS worker thread
 277  *           spots that one or more threads are blocked waiting for
 278  *           permission to return back their result, it gives up
 279  *           its Capability.
 280  *           Immediately afterwards, it tries to reaquire the Capabilty
 281  *           using waitForWorkCapability.
 282  *
 283  *
 284  * Pre-condition:  pMutex is assumed held and the thread possesses
 285  *                 a Capability.
 286  * Post-condition: pMutex is held and the thread possesses
 287  *                 a Capability.
 288  */
 289 void
 290 yieldToReturningWorker(Mutex* pMutex, Capability** pCap, Condition* pThreadCond)
 291 {
 292   if ( rts_n_waiting_workers > 0 ) {
 293     IF_DEBUG(scheduler,
 294              fprintf(stderr,"worker thread (%p): giving up RTS token\n", osThreadId()));
 295     releaseCapability(*pCap);
 296         /* And wait for work */
 297     waitForWorkCapability(pMutex, pCap, pThreadCond);
 298     IF_DEBUG(scheduler,
 299              fprintf(stderr,"worker thread (%p): got back RTS token (after yieldToReturningWorker)\n",
 300                 osThreadId()));
 301   }
 302   return;
 303 }
 304
 305
 306 /*
 307  * Function: waitForWorkCapability(Mutex*, Capability**, Condition*)
 308  *
 309  * Purpose:  wait for a Capability to become available. In
 310  *           the process of doing so, updates the number
 311  *           of tasks currently blocked waiting for a capability/more
 312  *           work. That counter is used when deciding whether or
 313  *           not to create a new worker thread when an external
 314  *           call is made.
 315  *           If pThreadCond is not NULL, a capability can be specifically
 316  *           passed to this thread using passCapability.
 317  *
 318  * Pre-condition: pMutex is held.
 319  * Post-condition: pMutex is held and *pCap is held by the current thread
 320  */
 321
 322 static Condition *passTarget = NULL;
 323
 324 void
 325 waitForWorkCapability(Mutex* pMutex, Capability** pCap, Condition* pThreadCond)
 326 {
 327 #ifdef SMP
 328   #error SMP version not implemented
 329 #endif
 330   IF_DEBUG(scheduler,
 331            fprintf(stderr,"worker thread (%p): wait for cap (cond: %p)\n",
 332               osThreadId(),pThreadCond));
 333   while ( noCapabilities() || (pThreadCond && passTarget != pThreadCond)
 334       || (!pThreadCond && passTarget)) {
 335     if(pThreadCond)
 336     {
 337       waitCondition(pThreadCond, pMutex);
 338       IF_DEBUG(scheduler,
 339                fprintf(stderr,"worker thread (%p): get passed capability\n",
 340                   osThreadId()));
 341     }
 342     else
 343     {
 344       rts_n_waiting_tasks++;
 345       waitCondition(&thread_ready_cond, pMutex);
 346       rts_n_waiting_tasks--;
 347       IF_DEBUG(scheduler,
 348                fprintf(stderr,"worker thread (%p): get normal capability\n",
 349                   osThreadId()));
 350     }
 351   }
 352   passTarget = NULL;
 353   grabCapability(pCap);
 354   return;
 355 }
 356
 357 /*
 358  * Function: passCapability(Mutex*, Capability*, Condition*)
 359  *
 360  * Purpose:  Let go of the capability and make sure the thread associated
 361  *           with the Condition pTargetThreadCond gets it next.
 362  *
 363  * Pre-condition: pMutex is held and cap is held by the current thread
 364  * Post-condition: pMutex is held; cap will be grabbed by the "target"
 365  *                 thread when pMutex is released.
 366  */
 367
 368 void
 369 passCapability(Mutex* pMutex, Capability* cap, Condition *pTargetThreadCond)
 370 {
 371 #ifdef SMP
 372   #error SMP version not implemented
 373 #endif
 374     rts_n_free_capabilities = 1;
 375     signalCondition(pTargetThreadCond);
 376     passTarget = pTargetThreadCond;
 377     IF_DEBUG(scheduler,
 378              fprintf(stderr,"worker thread (%p): passCapability\n",
 379                 osThreadId()));
 380 }
 381
 382
 383 #endif /* RTS_SUPPORTS_THREADS */
 384
 385 #if defined(SMP)
 386 /*
 387  * Function: initCapabilities_(nat)
 388  *
 389  * Purpose:  upon startup, allocate and fill in table
 390  *           holding 'n' Capabilities. Only for SMP, since
 391  *           it is the only build that supports multiple
 392  *           capabilities within the RTS.
 393  */
 394 static void
 395 initCapabilities_(nat n)
 396 {
 397   nat i;
 398   Capability *cap, *prev;
 399   cap  = NULL;
 400   prev = NULL;
 401   for (i = 0; i < n; i++) {
 402     cap = stgMallocBytes(sizeof(Capability), "initCapabilities");
 403     initCapability(cap);
 404     cap->link = prev;
 405     prev = cap;
 406   }
 407   free_capabilities = cap;
 408   rts_n_free_capabilities = n;
 409   returning_capabilities = NULL;
 410   IF_DEBUG(scheduler,fprintf(stderr,"scheduler: Allocated %d capabilities\n", n_free_capabilities););
 411 }
 412 #endif /* SMP */
 413