kernel/sched_rt.c

   1 /*
   2  * Real-Time Scheduling Class (mapped to the SCHED_FIFO and SCHED_RR
   3  * policies)
   4  */
   5
   6 #ifdef CONFIG_SMP
   7 static cpumask_t rt_overload_mask;
   8 static atomic_t rto_count;
   9 static inline int rt_overloaded(void)
  10 {
  11         return atomic_read(&rto_count);
  12 }
  13 static inline cpumask_t *rt_overload(void)
  14 {
  15         return &rt_overload_mask;
  16 }
  17 static inline void rt_set_overload(struct rq *rq)
  18 {
  19         cpu_set(rq->cpu, rt_overload_mask);
  20         /*
  21          * Make sure the mask is visible before we set
  22          * the overload count. That is checked to determine
  23          * if we should look at the mask. It would be a shame
  24          * if we looked at the mask, but the mask was not
  25          * updated yet.
  26          */
  27         wmb();
  28         atomic_inc(&rto_count);
  29 }
  30 static inline void rt_clear_overload(struct rq *rq)
  31 {
  32         /* the order here really doesn't matter */
  33         atomic_dec(&rto_count);
  34         cpu_clear(rq->cpu, rt_overload_mask);
  35 }
  36
  37 static void update_rt_migration(struct rq *rq)
  38 {
  39         if (rq->rt.rt_nr_migratory && (rq->rt.rt_nr_running > 1))
  40                 rt_set_overload(rq);
  41         else
  42                 rt_clear_overload(rq);
  43 }
  44 #endif /* CONFIG_SMP */
  45
  46 /*
  47  * Update the current task's runtime statistics. Skip current tasks that
  48  * are not in our scheduling class.
  49  */
  50 static void update_curr_rt(struct rq *rq)
  51 {
  52         struct task_struct *curr = rq->curr;
  53         u64 delta_exec;
  54
  55         if (!task_has_rt_policy(curr))
  56                 return;
  57
  58         delta_exec = rq->clock - curr->se.exec_start;
  59         if (unlikely((s64)delta_exec < 0))
  60                 delta_exec = 0;
  61
  62         schedstat_set(curr->se.exec_max, max(curr->se.exec_max, delta_exec));
  63
  64         curr->se.sum_exec_runtime += delta_exec;
  65         curr->se.exec_start = rq->clock;
  66         cpuacct_charge(curr, delta_exec);
  67 }
  68
  69 static inline void inc_rt_tasks(struct task_struct *p, struct rq *rq)
  70 {
  71         WARN_ON(!rt_task(p));
  72         rq->rt.rt_nr_running++;
  73 #ifdef CONFIG_SMP
  74         if (p->prio < rq->rt.highest_prio)
  75                 rq->rt.highest_prio = p->prio;
  76         if (p->nr_cpus_allowed > 1)
  77                 rq->rt.rt_nr_migratory++;
  78
  79         update_rt_migration(rq);
  80 #endif /* CONFIG_SMP */
  81 }
  82
  83 static inline void dec_rt_tasks(struct task_struct *p, struct rq *rq)
  84 {
  85         WARN_ON(!rt_task(p));
  86         WARN_ON(!rq->rt.rt_nr_running);
  87         rq->rt.rt_nr_running--;
  88 #ifdef CONFIG_SMP
  89         if (rq->rt.rt_nr_running) {
  90                 struct rt_prio_array *array;
  91
  92                 WARN_ON(p->prio < rq->rt.highest_prio);
  93                 if (p->prio == rq->rt.highest_prio) {
  94                         /* recalculate */
  95                         array = &rq->rt.active;
  96                         rq->rt.highest_prio =
  97                                 sched_find_first_bit(array->bitmap);
  98                 } /* otherwise leave rq->highest prio alone */
  99         } else
 100                 rq->rt.highest_prio = MAX_RT_PRIO;
 101         if (p->nr_cpus_allowed > 1)
 102                 rq->rt.rt_nr_migratory--;
 103
 104         update_rt_migration(rq);
 105 #endif /* CONFIG_SMP */
 106 }
 107
 108 static void enqueue_task_rt(struct rq *rq, struct task_struct *p, int wakeup)
 109 {
 110         struct rt_prio_array *array = &rq->rt.active;
 111
 112         list_add_tail(&p->run_list, array->queue + p->prio);
 113         __set_bit(p->prio, array->bitmap);
 114         inc_cpu_load(rq, p->se.load.weight);
 115
 116         inc_rt_tasks(p, rq);
 117 }
 118
 119 /*
 120  * Adding/removing a task to/from a priority array:
 121  */
 122 static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int sleep)
 123 {
 124         struct rt_prio_array *array = &rq->rt.active;
 125
 126         update_curr_rt(rq);
 127
 128         list_del(&p->run_list);
 129         if (list_empty(array->queue + p->prio))
 130                 __clear_bit(p->prio, array->bitmap);
 131         dec_cpu_load(rq, p->se.load.weight);
 132
 133         dec_rt_tasks(p, rq);
 134 }
 135
 136 /*
 137  * Put task to the end of the run list without the overhead of dequeue
 138  * followed by enqueue.
 139  */
 140 static void requeue_task_rt(struct rq *rq, struct task_struct *p)
 141 {
 142         struct rt_prio_array *array = &rq->rt.active;
 143
 144         list_move_tail(&p->run_list, array->queue + p->prio);
 145 }
 146
 147 static void
 148 yield_task_rt(struct rq *rq)
 149 {
 150         requeue_task_rt(rq, rq->curr);
 151 }
 152
 153 #ifdef CONFIG_SMP
 154 static int select_task_rq_rt(struct task_struct *p, int sync)
 155 {
 156         return task_cpu(p);
 157 }
 158 #endif /* CONFIG_SMP */
 159
 160 /*
 161  * Preempt the current task with a newly woken task if needed:
 162  */
 163 static void check_preempt_curr_rt(struct rq *rq, struct task_struct *p)
 164 {
 165         if (p->prio < rq->curr->prio)
 166                 resched_task(rq->curr);
 167 }
 168
 169 static struct task_struct *pick_next_task_rt(struct rq *rq)
 170 {
 171         struct rt_prio_array *array = &rq->rt.active;
 172         struct task_struct *next;
 173         struct list_head *queue;
 174         int idx;
 175
 176         idx = sched_find_first_bit(array->bitmap);
 177         if (idx >= MAX_RT_PRIO)
 178                 return NULL;
 179
 180         queue = array->queue + idx;
 181         next = list_entry(queue->next, struct task_struct, run_list);
 182
 183         next->se.exec_start = rq->clock;
 184
 185         return next;
 186 }
 187
 188 static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
 189 {
 190         update_curr_rt(rq);
 191         p->se.exec_start = 0;
 192 }
 193
 194 #ifdef CONFIG_SMP
 195 /* Only try algorithms three times */
 196 #define RT_MAX_TRIES 3
 197
 198 static int double_lock_balance(struct rq *this_rq, struct rq *busiest);
 199 static void deactivate_task(struct rq *rq, struct task_struct *p, int sleep);
 200
 201 static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
 202 {
 203         if (!task_running(rq, p) &&
 204             (cpu < 0 || cpu_isset(cpu, p->cpus_allowed)) &&
 205             (p->nr_cpus_allowed > 1))
 206                 return 1;
 207         return 0;
 208 }
 209
 210 /* Return the second highest RT task, NULL otherwise */
 211 static struct task_struct *pick_next_highest_task_rt(struct rq *rq,
 212                                                      int cpu)
 213 {
 214         struct rt_prio_array *array = &rq->rt.active;
 215         struct task_struct *next;
 216         struct list_head *queue;
 217         int idx;
 218
 219         assert_spin_locked(&rq->lock);
 220
 221         if (likely(rq->rt.rt_nr_running < 2))
 222                 return NULL;
 223
 224         idx = sched_find_first_bit(array->bitmap);
 225         if (unlikely(idx >= MAX_RT_PRIO)) {
 226                 WARN_ON(1); /* rt_nr_running is bad */
 227                 return NULL;
 228         }
 229
 230         queue = array->queue + idx;
 231         BUG_ON(list_empty(queue));
 232
 233         next = list_entry(queue->next, struct task_struct, run_list);
 234         if (unlikely(pick_rt_task(rq, next, cpu)))
 235                 goto out;
 236
 237         if (queue->next->next != queue) {
 238                 /* same prio task */
 239                 next = list_entry(queue->next->next, struct task_struct, run_list);
 240                 if (pick_rt_task(rq, next, cpu))
 241                         goto out;
 242         }
 243
 244  retry:
 245         /* slower, but more flexible */
 246         idx = find_next_bit(array->bitmap, MAX_RT_PRIO, idx+1);
 247         if (unlikely(idx >= MAX_RT_PRIO))
 248                 return NULL;
 249
 250         queue = array->queue + idx;
 251         BUG_ON(list_empty(queue));
 252
 253         list_for_each_entry(next, queue, run_list) {
 254                 if (pick_rt_task(rq, next, cpu))
 255                         goto out;
 256         }
 257
 258         goto retry;
 259
 260  out:
 261         return next;
 262 }
 263
 264 static DEFINE_PER_CPU(cpumask_t, local_cpu_mask);
 265
 266 /* Will lock the rq it finds */
 267 static struct rq *find_lock_lowest_rq(struct task_struct *task,
 268                                       struct rq *this_rq)
 269 {
 270         struct rq *lowest_rq = NULL;
 271         int cpu;
 272         int tries;
 273         cpumask_t *cpu_mask = &__get_cpu_var(local_cpu_mask);
 274
 275         cpus_and(*cpu_mask, cpu_online_map, task->cpus_allowed);
 276
 277         for (tries = 0; tries < RT_MAX_TRIES; tries++) {
 278                 /*
 279                  * Scan each rq for the lowest prio.
 280                  */
 281                 for_each_cpu_mask(cpu, *cpu_mask) {
 282                         struct rq *rq = &per_cpu(runqueues, cpu);
 283
 284                         if (cpu == this_rq->cpu)
 285                                 continue;
 286
 287                         /* We look for lowest RT prio or non-rt CPU */
 288                         if (rq->rt.highest_prio >= MAX_RT_PRIO) {
 289                                 lowest_rq = rq;
 290                                 break;
 291                         }
 292
 293                         /* no locking for now */
 294                         if (rq->rt.highest_prio > task->prio &&
 295                             (!lowest_rq || rq->rt.highest_prio > lowest_rq->rt.highest_prio)) {
 296                                 lowest_rq = rq;
 297                         }
 298                 }
 299
 300                 if (!lowest_rq)
 301                         break;
 302
 303                 /* if the prio of this runqueue changed, try again */
 304                 if (double_lock_balance(this_rq, lowest_rq)) {
 305                         /*
 306                          * We had to unlock the run queue. In
 307                          * the mean time, task could have
 308                          * migrated already or had its affinity changed.
 309                          * Also make sure that it wasn't scheduled on its rq.
 310                          */
 311                         if (unlikely(task_rq(task) != this_rq ||
 312                                      !cpu_isset(lowest_rq->cpu, task->cpus_allowed) ||
 313                                      task_running(this_rq, task) ||
 314                                      !task->se.on_rq)) {
 315                                 spin_unlock(&lowest_rq->lock);
 316                                 lowest_rq = NULL;
 317                                 break;
 318                         }
 319                 }
 320
 321                 /* If this rq is still suitable use it. */
 322                 if (lowest_rq->rt.highest_prio > task->prio)
 323                         break;
 324
 325                 /* try again */
 326                 spin_unlock(&lowest_rq->lock);
 327                 lowest_rq = NULL;
 328         }
 329
 330         return lowest_rq;
 331 }
 332
 333 /*
 334  * If the current CPU has more than one RT task, see if the non
 335  * running task can migrate over to a CPU that is running a task
 336  * of lesser priority.
 337  */
 338 static int push_rt_task(struct rq *rq)
 339 {
 340         struct task_struct *next_task;
 341         struct rq *lowest_rq;
 342         int ret = 0;
 343         int paranoid = RT_MAX_TRIES;
 344
 345         assert_spin_locked(&rq->lock);
 346
 347         next_task = pick_next_highest_task_rt(rq, -1);
 348         if (!next_task)
 349                 return 0;
 350
 351  retry:
 352         if (unlikely(next_task == rq->curr)) {
 353                 WARN_ON(1);
 354                 return 0;
 355         }
 356
 357         /*
 358          * It's possible that the next_task slipped in of
 359          * higher priority than current. If that's the case
 360          * just reschedule current.
 361          */
 362         if (unlikely(next_task->prio < rq->curr->prio)) {
 363                 resched_task(rq->curr);
 364                 return 0;
 365         }
 366
 367         /* We might release rq lock */
 368         get_task_struct(next_task);
 369
 370         /* find_lock_lowest_rq locks the rq if found */
 371         lowest_rq = find_lock_lowest_rq(next_task, rq);
 372         if (!lowest_rq) {
 373                 struct task_struct *task;
 374                 /*
 375                  * find lock_lowest_rq releases rq->lock
 376                  * so it is possible that next_task has changed.
 377                  * If it has, then try again.
 378                  */
 379                 task = pick_next_highest_task_rt(rq, -1);
 380                 if (unlikely(task != next_task) && task && paranoid--) {
 381                         put_task_struct(next_task);
 382                         next_task = task;
 383                         goto retry;
 384                 }
 385                 goto out;
 386         }
 387
 388         assert_spin_locked(&lowest_rq->lock);
 389
 390         deactivate_task(rq, next_task, 0);
 391         set_task_cpu(next_task, lowest_rq->cpu);
 392         activate_task(lowest_rq, next_task, 0);
 393
 394         resched_task(lowest_rq->curr);
 395
 396         spin_unlock(&lowest_rq->lock);
 397
 398         ret = 1;
 399 out:
 400         put_task_struct(next_task);
 401
 402         return ret;
 403 }
 404
 405 /*
 406  * TODO: Currently we just use the second highest prio task on
 407  *       the queue, and stop when it can't migrate (or there's
 408  *       no more RT tasks).  There may be a case where a lower
 409  *       priority RT task has a different affinity than the
 410  *       higher RT task. In this case the lower RT task could
 411  *       possibly be able to migrate where as the higher priority
 412  *       RT task could not.  We currently ignore this issue.
 413  *       Enhancements are welcome!
 414  */
 415 static void push_rt_tasks(struct rq *rq)
 416 {
 417         /* push_rt_task will return true if it moved an RT */
 418         while (push_rt_task(rq))
 419                 ;
 420 }
 421
 422 static int pull_rt_task(struct rq *this_rq)
 423 {
 424         struct task_struct *next;
 425         struct task_struct *p;
 426         struct rq *src_rq;
 427         cpumask_t *rto_cpumask;
 428         int this_cpu = this_rq->cpu;
 429         int cpu;
 430         int ret = 0;
 431
 432         assert_spin_locked(&this_rq->lock);
 433
 434         /*
 435          * If cpusets are used, and we have overlapping
 436          * run queue cpusets, then this algorithm may not catch all.
 437          * This is just the price you pay on trying to keep
 438          * dirtying caches down on large SMP machines.
 439          */
 440         if (likely(!rt_overloaded()))
 441                 return 0;
 442
 443         next = pick_next_task_rt(this_rq);
 444
 445         rto_cpumask = rt_overload();
 446
 447         for_each_cpu_mask(cpu, *rto_cpumask) {
 448                 if (this_cpu == cpu)
 449                         continue;
 450
 451                 src_rq = cpu_rq(cpu);
 452                 if (unlikely(src_rq->rt.rt_nr_running <= 1)) {
 453                         /*
 454                          * It is possible that overlapping cpusets
 455                          * will miss clearing a non overloaded runqueue.
 456                          * Clear it now.
 457                          */
 458                         if (double_lock_balance(this_rq, src_rq)) {
 459                                 /* unlocked our runqueue lock */
 460                                 struct task_struct *old_next = next;
 461                                 next = pick_next_task_rt(this_rq);
 462                                 if (next != old_next)
 463                                         ret = 1;
 464                         }
 465                         if (likely(src_rq->rt.rt_nr_running <= 1))
 466                                 /*
 467                                  * Small chance that this_rq->curr changed
 468                                  * but it's really harmless here.
 469                                  */
 470                                 rt_clear_overload(this_rq);
 471                         else
 472                                 /*
 473                                  * Heh, the src_rq is now overloaded, since
 474                                  * we already have the src_rq lock, go straight
 475                                  * to pulling tasks from it.
 476                                  */
 477                                 goto try_pulling;
 478                         spin_unlock(&src_rq->lock);
 479                         continue;
 480                 }
 481
 482                 /*
 483                  * We can potentially drop this_rq's lock in
 484                  * double_lock_balance, and another CPU could
 485                  * steal our next task - hence we must cause
 486                  * the caller to recalculate the next task
 487                  * in that case:
 488                  */
 489                 if (double_lock_balance(this_rq, src_rq)) {
 490                         struct task_struct *old_next = next;
 491                         next = pick_next_task_rt(this_rq);
 492                         if (next != old_next)
 493                                 ret = 1;
 494                 }
 495
 496                 /*
 497                  * Are there still pullable RT tasks?
 498                  */
 499                 if (src_rq->rt.rt_nr_running <= 1) {
 500                         spin_unlock(&src_rq->lock);
 501                         continue;
 502                 }
 503
 504  try_pulling:
 505                 p = pick_next_highest_task_rt(src_rq, this_cpu);
 506
 507                 /*
 508                  * Do we have an RT task that preempts
 509                  * the to-be-scheduled task?
 510                  */
 511                 if (p && (!next || (p->prio < next->prio))) {
 512                         WARN_ON(p == src_rq->curr);
 513                         WARN_ON(!p->se.on_rq);
 514
 515                         /*
 516                          * There's a chance that p is higher in priority
 517                          * than what's currently running on its cpu.
 518                          * This is just that p is wakeing up and hasn't
 519                          * had a chance to schedule. We only pull
 520                          * p if it is lower in priority than the
 521                          * current task on the run queue or
 522                          * this_rq next task is lower in prio than
 523                          * the current task on that rq.
 524                          */
 525                         if (p->prio < src_rq->curr->prio ||
 526                             (next && next->prio < src_rq->curr->prio))
 527                                 goto bail;
 528
 529                         ret = 1;
 530
 531                         deactivate_task(src_rq, p, 0);
 532                         set_task_cpu(p, this_cpu);
 533                         activate_task(this_rq, p, 0);
 534                         /*
 535                          * We continue with the search, just in
 536                          * case there's an even higher prio task
 537                          * in another runqueue. (low likelyhood
 538                          * but possible)
 539                          */
 540
 541                         /*
 542                          * Update next so that we won't pick a task
 543                          * on another cpu with a priority lower (or equal)
 544                          * than the one we just picked.
 545                          */
 546                         next = p;
 547
 548                 }
 549  bail:
 550                 spin_unlock(&src_rq->lock);
 551         }
 552
 553         return ret;
 554 }
 555
 556 static void schedule_balance_rt(struct rq *rq,
 557                                 struct task_struct *prev)
 558 {
 559         /* Try to pull RT tasks here if we lower this rq's prio */
 560         if (unlikely(rt_task(prev)) &&
 561             rq->rt.highest_prio > prev->prio)
 562                 pull_rt_task(rq);
 563 }
 564
 565 static void schedule_tail_balance_rt(struct rq *rq)
 566 {
 567         /*
 568          * If we have more than one rt_task queued, then
 569          * see if we can push the other rt_tasks off to other CPUS.
 570          * Note we may release the rq lock, and since
 571          * the lock was owned by prev, we need to release it
 572          * first via finish_lock_switch and then reaquire it here.
 573          */
 574         if (unlikely(rq->rt.rt_nr_running > 1)) {
 575                 spin_lock_irq(&rq->lock);
 576                 push_rt_tasks(rq);
 577                 spin_unlock_irq(&rq->lock);
 578         }
 579 }
 580
 581
 582 static void wakeup_balance_rt(struct rq *rq, struct task_struct *p)
 583 {
 584         if (unlikely(rt_task(p)) &&
 585             !task_running(rq, p) &&
 586             (p->prio >= rq->curr->prio))
 587                 push_rt_tasks(rq);
 588 }
 589
 590 static unsigned long
 591 load_balance_rt(struct rq *this_rq, int this_cpu, struct rq *busiest,
 592                 unsigned long max_load_move,
 593                 struct sched_domain *sd, enum cpu_idle_type idle,
 594                 int *all_pinned, int *this_best_prio)
 595 {
 596         /* don't touch RT tasks */
 597         return 0;
 598 }
 599
 600 static int
 601 move_one_task_rt(struct rq *this_rq, int this_cpu, struct rq *busiest,
 602                  struct sched_domain *sd, enum cpu_idle_type idle)
 603 {
 604         /* don't touch RT tasks */
 605         return 0;
 606 }
 607 static void set_cpus_allowed_rt(struct task_struct *p, cpumask_t *new_mask)
 608 {
 609         int weight = cpus_weight(*new_mask);
 610
 611         BUG_ON(!rt_task(p));
 612
 613         /*
 614          * Update the migration status of the RQ if we have an RT task
 615          * which is running AND changing its weight value.
 616          */
 617         if (p->se.on_rq && (weight != p->nr_cpus_allowed)) {
 618                 struct rq *rq = task_rq(p);
 619
 620                 if ((p->nr_cpus_allowed <= 1) && (weight > 1))
 621                         rq->rt.rt_nr_migratory++;
 622                 else if((p->nr_cpus_allowed > 1) && (weight <= 1)) {
 623                         BUG_ON(!rq->rt.rt_nr_migratory);
 624                         rq->rt.rt_nr_migratory--;
 625                 }
 626
 627                 update_rt_migration(rq);
 628         }
 629
 630         p->cpus_allowed    = *new_mask;
 631         p->nr_cpus_allowed = weight;
 632 }
 633 #else /* CONFIG_SMP */
 634 # define schedule_tail_balance_rt(rq)   do { } while (0)
 635 # define schedule_balance_rt(rq, prev)  do { } while (0)
 636 # define wakeup_balance_rt(rq, p)       do { } while (0)
 637 #endif /* CONFIG_SMP */
 638
 639 static void task_tick_rt(struct rq *rq, struct task_struct *p)
 640 {
 641         update_curr_rt(rq);
 642
 643         /*
 644          * RR tasks need a special form of timeslice management.
 645          * FIFO tasks have no timeslices.
 646          */
 647         if (p->policy != SCHED_RR)
 648                 return;
 649
 650         if (--p->time_slice)
 651                 return;
 652
 653         p->time_slice = DEF_TIMESLICE;
 654
 655         /*
 656          * Requeue to the end of queue if we are not the only element
 657          * on the queue:
 658          */
 659         if (p->run_list.prev != p->run_list.next) {
 660                 requeue_task_rt(rq, p);
 661                 set_tsk_need_resched(p);
 662         }
 663 }
 664
 665 static void set_curr_task_rt(struct rq *rq)
 666 {
 667         struct task_struct *p = rq->curr;
 668
 669         p->se.exec_start = rq->clock;
 670 }
 671
 672 const struct sched_class rt_sched_class = {
 673         .next                   = &fair_sched_class,
 674         .enqueue_task           = enqueue_task_rt,
 675         .dequeue_task           = dequeue_task_rt,
 676         .yield_task             = yield_task_rt,
 677 #ifdef CONFIG_SMP
 678         .select_task_rq         = select_task_rq_rt,
 679 #endif /* CONFIG_SMP */
 680
 681         .check_preempt_curr     = check_preempt_curr_rt,
 682
 683         .pick_next_task         = pick_next_task_rt,
 684         .put_prev_task          = put_prev_task_rt,
 685
 686 #ifdef CONFIG_SMP
 687         .load_balance           = load_balance_rt,
 688         .move_one_task          = move_one_task_rt,
 689         .set_cpus_allowed       = set_cpus_allowed_rt,
 690 #endif
 691
 692         .set_curr_task          = set_curr_task_rt,
 693         .task_tick              = task_tick_rt,
 694 };