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 #endif /* CONFIG_SMP */
  37
  38 /*
  39  * Update the current task's runtime statistics. Skip current tasks that
  40  * are not in our scheduling class.
  41  */
  42 static void update_curr_rt(struct rq *rq)
  43 {
  44         struct task_struct *curr = rq->curr;
  45         u64 delta_exec;
  46
  47         if (!task_has_rt_policy(curr))
  48                 return;
  49
  50         delta_exec = rq->clock - curr->se.exec_start;
  51         if (unlikely((s64)delta_exec < 0))
  52                 delta_exec = 0;
  53
  54         schedstat_set(curr->se.exec_max, max(curr->se.exec_max, delta_exec));
  55
  56         curr->se.sum_exec_runtime += delta_exec;
  57         curr->se.exec_start = rq->clock;
  58         cpuacct_charge(curr, delta_exec);
  59 }
  60
  61 static inline void inc_rt_tasks(struct task_struct *p, struct rq *rq)
  62 {
  63         WARN_ON(!rt_task(p));
  64         rq->rt.rt_nr_running++;
  65 #ifdef CONFIG_SMP
  66         if (p->prio < rq->rt.highest_prio)
  67                 rq->rt.highest_prio = p->prio;
  68         if (rq->rt.rt_nr_running > 1)
  69                 rt_set_overload(rq);
  70 #endif /* CONFIG_SMP */
  71 }
  72
  73 static inline void dec_rt_tasks(struct task_struct *p, struct rq *rq)
  74 {
  75         WARN_ON(!rt_task(p));
  76         WARN_ON(!rq->rt.rt_nr_running);
  77         rq->rt.rt_nr_running--;
  78 #ifdef CONFIG_SMP
  79         if (rq->rt.rt_nr_running) {
  80                 struct rt_prio_array *array;
  81
  82                 WARN_ON(p->prio < rq->rt.highest_prio);
  83                 if (p->prio == rq->rt.highest_prio) {
  84                         /* recalculate */
  85                         array = &rq->rt.active;
  86                         rq->rt.highest_prio =
  87                                 sched_find_first_bit(array->bitmap);
  88                 } /* otherwise leave rq->highest prio alone */
  89         } else
  90                 rq->rt.highest_prio = MAX_RT_PRIO;
  91         if (rq->rt.rt_nr_running < 2)
  92                 rt_clear_overload(rq);
  93 #endif /* CONFIG_SMP */
  94 }
  95
  96 static void enqueue_task_rt(struct rq *rq, struct task_struct *p, int wakeup)
  97 {
  98         struct rt_prio_array *array = &rq->rt.active;
  99
 100         list_add_tail(&p->run_list, array->queue + p->prio);
 101         __set_bit(p->prio, array->bitmap);
 102         inc_cpu_load(rq, p->se.load.weight);
 103
 104         inc_rt_tasks(p, rq);
 105 }
 106
 107 /*
 108  * Adding/removing a task to/from a priority array:
 109  */
 110 static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int sleep)
 111 {
 112         struct rt_prio_array *array = &rq->rt.active;
 113
 114         update_curr_rt(rq);
 115
 116         list_del(&p->run_list);
 117         if (list_empty(array->queue + p->prio))
 118                 __clear_bit(p->prio, array->bitmap);
 119         dec_cpu_load(rq, p->se.load.weight);
 120
 121         dec_rt_tasks(p, rq);
 122 }
 123
 124 /*
 125  * Put task to the end of the run list without the overhead of dequeue
 126  * followed by enqueue.
 127  */
 128 static void requeue_task_rt(struct rq *rq, struct task_struct *p)
 129 {
 130         struct rt_prio_array *array = &rq->rt.active;
 131
 132         list_move_tail(&p->run_list, array->queue + p->prio);
 133 }
 134
 135 static void
 136 yield_task_rt(struct rq *rq)
 137 {
 138         requeue_task_rt(rq, rq->curr);
 139 }
 140
 141 /*
 142  * Preempt the current task with a newly woken task if needed:
 143  */
 144 static void check_preempt_curr_rt(struct rq *rq, struct task_struct *p)
 145 {
 146         if (p->prio < rq->curr->prio)
 147                 resched_task(rq->curr);
 148 }
 149
 150 static struct task_struct *pick_next_task_rt(struct rq *rq)
 151 {
 152         struct rt_prio_array *array = &rq->rt.active;
 153         struct task_struct *next;
 154         struct list_head *queue;
 155         int idx;
 156
 157         idx = sched_find_first_bit(array->bitmap);
 158         if (idx >= MAX_RT_PRIO)
 159                 return NULL;
 160
 161         queue = array->queue + idx;
 162         next = list_entry(queue->next, struct task_struct, run_list);
 163
 164         next->se.exec_start = rq->clock;
 165
 166         return next;
 167 }
 168
 169 static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
 170 {
 171         update_curr_rt(rq);
 172         p->se.exec_start = 0;
 173 }
 174
 175 #ifdef CONFIG_SMP
 176 /* Only try algorithms three times */
 177 #define RT_MAX_TRIES 3
 178
 179 static int double_lock_balance(struct rq *this_rq, struct rq *busiest);
 180 static void deactivate_task(struct rq *rq, struct task_struct *p, int sleep);
 181
 182 /* Return the second highest RT task, NULL otherwise */
 183 static struct task_struct *pick_next_highest_task_rt(struct rq *rq)
 184 {
 185         struct rt_prio_array *array = &rq->rt.active;
 186         struct task_struct *next;
 187         struct list_head *queue;
 188         int idx;
 189
 190         assert_spin_locked(&rq->lock);
 191
 192         if (likely(rq->rt.rt_nr_running < 2))
 193                 return NULL;
 194
 195         idx = sched_find_first_bit(array->bitmap);
 196         if (unlikely(idx >= MAX_RT_PRIO)) {
 197                 WARN_ON(1); /* rt_nr_running is bad */
 198                 return NULL;
 199         }
 200
 201         queue = array->queue + idx;
 202         next = list_entry(queue->next, struct task_struct, run_list);
 203         if (unlikely(next != rq->curr))
 204                 return next;
 205
 206         if (queue->next->next != queue) {
 207                 /* same prio task */
 208                 next = list_entry(queue->next->next, struct task_struct, run_list);
 209                 return next;
 210         }
 211
 212         /* slower, but more flexible */
 213         idx = find_next_bit(array->bitmap, MAX_RT_PRIO, idx+1);
 214         if (unlikely(idx >= MAX_RT_PRIO)) {
 215                 WARN_ON(1); /* rt_nr_running was 2 and above! */
 216                 return NULL;
 217         }
 218
 219         queue = array->queue + idx;
 220         next = list_entry(queue->next, struct task_struct, run_list);
 221
 222         return next;
 223 }
 224
 225 static DEFINE_PER_CPU(cpumask_t, local_cpu_mask);
 226
 227 /* Will lock the rq it finds */
 228 static struct rq *find_lock_lowest_rq(struct task_struct *task,
 229                                       struct rq *this_rq)
 230 {
 231         struct rq *lowest_rq = NULL;
 232         int cpu;
 233         int tries;
 234         cpumask_t *cpu_mask = &__get_cpu_var(local_cpu_mask);
 235
 236         cpus_and(*cpu_mask, cpu_online_map, task->cpus_allowed);
 237
 238         for (tries = 0; tries < RT_MAX_TRIES; tries++) {
 239                 /*
 240                  * Scan each rq for the lowest prio.
 241                  */
 242                 for_each_cpu_mask(cpu, *cpu_mask) {
 243                         struct rq *rq = &per_cpu(runqueues, cpu);
 244
 245                         if (cpu == this_rq->cpu)
 246                                 continue;
 247
 248                         /* We look for lowest RT prio or non-rt CPU */
 249                         if (rq->rt.highest_prio >= MAX_RT_PRIO) {
 250                                 lowest_rq = rq;
 251                                 break;
 252                         }
 253
 254                         /* no locking for now */
 255                         if (rq->rt.highest_prio > task->prio &&
 256                             (!lowest_rq || rq->rt.highest_prio > lowest_rq->rt.highest_prio)) {
 257                                 lowest_rq = rq;
 258                         }
 259                 }
 260
 261                 if (!lowest_rq)
 262                         break;
 263
 264                 /* if the prio of this runqueue changed, try again */
 265                 if (double_lock_balance(this_rq, lowest_rq)) {
 266                         /*
 267                          * We had to unlock the run queue. In
 268                          * the mean time, task could have
 269                          * migrated already or had its affinity changed.
 270                          * Also make sure that it wasn't scheduled on its rq.
 271                          */
 272                         if (unlikely(task_rq(task) != this_rq ||
 273                                      !cpu_isset(lowest_rq->cpu, task->cpus_allowed) ||
 274                                      task_running(this_rq, task) ||
 275                                      !task->se.on_rq)) {
 276                                 spin_unlock(&lowest_rq->lock);
 277                                 lowest_rq = NULL;
 278                                 break;
 279                         }
 280                 }
 281
 282                 /* If this rq is still suitable use it. */
 283                 if (lowest_rq->rt.highest_prio > task->prio)
 284                         break;
 285
 286                 /* try again */
 287                 spin_unlock(&lowest_rq->lock);
 288                 lowest_rq = NULL;
 289         }
 290
 291         return lowest_rq;
 292 }
 293
 294 /*
 295  * If the current CPU has more than one RT task, see if the non
 296  * running task can migrate over to a CPU that is running a task
 297  * of lesser priority.
 298  */
 299 static int push_rt_task(struct rq *this_rq)
 300 {
 301         struct task_struct *next_task;
 302         struct rq *lowest_rq;
 303         int ret = 0;
 304         int paranoid = RT_MAX_TRIES;
 305
 306         assert_spin_locked(&this_rq->lock);
 307
 308         next_task = pick_next_highest_task_rt(this_rq);
 309         if (!next_task)
 310                 return 0;
 311
 312  retry:
 313         if (unlikely(next_task == this_rq->curr))
 314                 return 0;
 315
 316         /*
 317          * It's possible that the next_task slipped in of
 318          * higher priority than current. If that's the case
 319          * just reschedule current.
 320          */
 321         if (unlikely(next_task->prio < this_rq->curr->prio)) {
 322                 resched_task(this_rq->curr);
 323                 return 0;
 324         }
 325
 326         /* We might release this_rq lock */
 327         get_task_struct(next_task);
 328
 329         /* find_lock_lowest_rq locks the rq if found */
 330         lowest_rq = find_lock_lowest_rq(next_task, this_rq);
 331         if (!lowest_rq) {
 332                 struct task_struct *task;
 333                 /*
 334                  * find lock_lowest_rq releases this_rq->lock
 335                  * so it is possible that next_task has changed.
 336                  * If it has, then try again.
 337                  */
 338                 task = pick_next_highest_task_rt(this_rq);
 339                 if (unlikely(task != next_task) && task && paranoid--) {
 340                         put_task_struct(next_task);
 341                         next_task = task;
 342                         goto retry;
 343                 }
 344                 goto out;
 345         }
 346
 347         assert_spin_locked(&lowest_rq->lock);
 348
 349         deactivate_task(this_rq, next_task, 0);
 350         set_task_cpu(next_task, lowest_rq->cpu);
 351         activate_task(lowest_rq, next_task, 0);
 352
 353         resched_task(lowest_rq->curr);
 354
 355         spin_unlock(&lowest_rq->lock);
 356
 357         ret = 1;
 358 out:
 359         put_task_struct(next_task);
 360
 361         return ret;
 362 }
 363
 364 /*
 365  * TODO: Currently we just use the second highest prio task on
 366  *       the queue, and stop when it can't migrate (or there's
 367  *       no more RT tasks).  There may be a case where a lower
 368  *       priority RT task has a different affinity than the
 369  *       higher RT task. In this case the lower RT task could
 370  *       possibly be able to migrate where as the higher priority
 371  *       RT task could not.  We currently ignore this issue.
 372  *       Enhancements are welcome!
 373  */
 374 static void push_rt_tasks(struct rq *rq)
 375 {
 376         /* push_rt_task will return true if it moved an RT */
 377         while (push_rt_task(rq))
 378                 ;
 379 }
 380
 381 static void schedule_tail_balance_rt(struct rq *rq)
 382 {
 383         /*
 384          * If we have more than one rt_task queued, then
 385          * see if we can push the other rt_tasks off to other CPUS.
 386          * Note we may release the rq lock, and since
 387          * the lock was owned by prev, we need to release it
 388          * first via finish_lock_switch and then reaquire it here.
 389          */
 390         if (unlikely(rq->rt.rt_nr_running > 1)) {
 391                 spin_lock_irq(&rq->lock);
 392                 push_rt_tasks(rq);
 393                 spin_unlock_irq(&rq->lock);
 394         }
 395 }
 396
 397 /*
 398  * Load-balancing iterator. Note: while the runqueue stays locked
 399  * during the whole iteration, the current task might be
 400  * dequeued so the iterator has to be dequeue-safe. Here we
 401  * achieve that by always pre-iterating before returning
 402  * the current task:
 403  */
 404 static struct task_struct *load_balance_start_rt(void *arg)
 405 {
 406         struct rq *rq = arg;
 407         struct rt_prio_array *array = &rq->rt.active;
 408         struct list_head *head, *curr;
 409         struct task_struct *p;
 410         int idx;
 411
 412         idx = sched_find_first_bit(array->bitmap);
 413         if (idx >= MAX_RT_PRIO)
 414                 return NULL;
 415
 416         head = array->queue + idx;
 417         curr = head->prev;
 418
 419         p = list_entry(curr, struct task_struct, run_list);
 420
 421         curr = curr->prev;
 422
 423         rq->rt.rt_load_balance_idx = idx;
 424         rq->rt.rt_load_balance_head = head;
 425         rq->rt.rt_load_balance_curr = curr;
 426
 427         return p;
 428 }
 429
 430 static struct task_struct *load_balance_next_rt(void *arg)
 431 {
 432         struct rq *rq = arg;
 433         struct rt_prio_array *array = &rq->rt.active;
 434         struct list_head *head, *curr;
 435         struct task_struct *p;
 436         int idx;
 437
 438         idx = rq->rt.rt_load_balance_idx;
 439         head = rq->rt.rt_load_balance_head;
 440         curr = rq->rt.rt_load_balance_curr;
 441
 442         /*
 443          * If we arrived back to the head again then
 444          * iterate to the next queue (if any):
 445          */
 446         if (unlikely(head == curr)) {
 447                 int next_idx = find_next_bit(array->bitmap, MAX_RT_PRIO, idx+1);
 448
 449                 if (next_idx >= MAX_RT_PRIO)
 450                         return NULL;
 451
 452                 idx = next_idx;
 453                 head = array->queue + idx;
 454                 curr = head->prev;
 455
 456                 rq->rt.rt_load_balance_idx = idx;
 457                 rq->rt.rt_load_balance_head = head;
 458         }
 459
 460         p = list_entry(curr, struct task_struct, run_list);
 461
 462         curr = curr->prev;
 463
 464         rq->rt.rt_load_balance_curr = curr;
 465
 466         return p;
 467 }
 468
 469 static unsigned long
 470 load_balance_rt(struct rq *this_rq, int this_cpu, struct rq *busiest,
 471                 unsigned long max_load_move,
 472                 struct sched_domain *sd, enum cpu_idle_type idle,
 473                 int *all_pinned, int *this_best_prio)
 474 {
 475         struct rq_iterator rt_rq_iterator;
 476
 477         rt_rq_iterator.start = load_balance_start_rt;
 478         rt_rq_iterator.next = load_balance_next_rt;
 479         /* pass 'busiest' rq argument into
 480          * load_balance_[start|next]_rt iterators
 481          */
 482         rt_rq_iterator.arg = busiest;
 483
 484         return balance_tasks(this_rq, this_cpu, busiest, max_load_move, sd,
 485                              idle, all_pinned, this_best_prio, &rt_rq_iterator);
 486 }
 487
 488 static int
 489 move_one_task_rt(struct rq *this_rq, int this_cpu, struct rq *busiest,
 490                  struct sched_domain *sd, enum cpu_idle_type idle)
 491 {
 492         struct rq_iterator rt_rq_iterator;
 493
 494         rt_rq_iterator.start = load_balance_start_rt;
 495         rt_rq_iterator.next = load_balance_next_rt;
 496         rt_rq_iterator.arg = busiest;
 497
 498         return iter_move_one_task(this_rq, this_cpu, busiest, sd, idle,
 499                                   &rt_rq_iterator);
 500 }
 501 #else /* CONFIG_SMP */
 502 # define schedule_tail_balance_rt(rq)   do { } while (0)
 503 #endif /* CONFIG_SMP */
 504
 505 static void task_tick_rt(struct rq *rq, struct task_struct *p)
 506 {
 507         update_curr_rt(rq);
 508
 509         /*
 510          * RR tasks need a special form of timeslice management.
 511          * FIFO tasks have no timeslices.
 512          */
 513         if (p->policy != SCHED_RR)
 514                 return;
 515
 516         if (--p->time_slice)
 517                 return;
 518
 519         p->time_slice = DEF_TIMESLICE;
 520
 521         /*
 522          * Requeue to the end of queue if we are not the only element
 523          * on the queue:
 524          */
 525         if (p->run_list.prev != p->run_list.next) {
 526                 requeue_task_rt(rq, p);
 527                 set_tsk_need_resched(p);
 528         }
 529 }
 530
 531 static void set_curr_task_rt(struct rq *rq)
 532 {
 533         struct task_struct *p = rq->curr;
 534
 535         p->se.exec_start = rq->clock;
 536 }
 537
 538 const struct sched_class rt_sched_class = {
 539         .next                   = &fair_sched_class,
 540         .enqueue_task           = enqueue_task_rt,
 541         .dequeue_task           = dequeue_task_rt,
 542         .yield_task             = yield_task_rt,
 543
 544         .check_preempt_curr     = check_preempt_curr_rt,
 545
 546         .pick_next_task         = pick_next_task_rt,
 547         .put_prev_task          = put_prev_task_rt,
 548
 549 #ifdef CONFIG_SMP
 550         .load_balance           = load_balance_rt,
 551         .move_one_task          = move_one_task_rt,
 552 #endif
 553
 554         .set_curr_task          = set_curr_task_rt,
 555         .task_tick              = task_tick_rt,
 556 };