kernel/time/tick-sched.c

   1 /*
   2  *  linux/kernel/time/tick-sched.c
   3  *
   4  *  Copyright(C) 2005-2006, Thomas Gleixner <tglx@linutronix.de>
   5  *  Copyright(C) 2005-2007, Red Hat, Inc., Ingo Molnar
   6  *  Copyright(C) 2006-2007  Timesys Corp., Thomas Gleixner
   7  *
   8  *  No idle tick implementation for low and high resolution timers
   9  *
  10  *  Started by: Thomas Gleixner and Ingo Molnar
  11  *
  12  *  Distribute under GPLv2.
  13  */
  14 #include <linux/cpu.h>
  15 #include <linux/err.h>
  16 #include <linux/hrtimer.h>
  17 #include <linux/interrupt.h>
  18 #include <linux/kernel_stat.h>
  19 #include <linux/percpu.h>
  20 #include <linux/profile.h>
  21 #include <linux/sched.h>
  22 #include <linux/tick.h>
  23
  24 #include <asm/irq_regs.h>
  25
  26 #include "tick-internal.h"
  27
  28 /*
  29  * Per cpu nohz control structure
  30  */
  31 static DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched);
  32
  33 /*
  34  * The time, when the last jiffy update happened. Protected by xtime_lock.
  35  */
  36 static ktime_t last_jiffies_update;
  37
  38 struct tick_sched *tick_get_tick_sched(int cpu)
  39 {
  40         return &per_cpu(tick_cpu_sched, cpu);
  41 }
  42
  43 /*
  44  * Must be called with interrupts disabled !
  45  */
  46 static void tick_do_update_jiffies64(ktime_t now)
  47 {
  48         unsigned long ticks = 0;
  49         ktime_t delta;
  50
  51         /* Reevalute with xtime_lock held */
  52         write_seqlock(&xtime_lock);
  53
  54         delta = ktime_sub(now, last_jiffies_update);
  55         if (delta.tv64 >= tick_period.tv64) {
  56
  57                 delta = ktime_sub(delta, tick_period);
  58                 last_jiffies_update = ktime_add(last_jiffies_update,
  59                                                 tick_period);
  60
  61                 /* Slow path for long timeouts */
  62                 if (unlikely(delta.tv64 >= tick_period.tv64)) {
  63                         s64 incr = ktime_to_ns(tick_period);
  64
  65                         ticks = ktime_divns(delta, incr);
  66
  67                         last_jiffies_update = ktime_add_ns(last_jiffies_update,
  68                                                            incr * ticks);
  69                 }
  70                 do_timer(++ticks);
  71         }
  72         write_sequnlock(&xtime_lock);
  73 }
  74
  75 /*
  76  * Initialize and return retrieve the jiffies update.
  77  */
  78 static ktime_t tick_init_jiffy_update(void)
  79 {
  80         ktime_t period;
  81
  82         write_seqlock(&xtime_lock);
  83         /* Did we start the jiffies update yet ? */
  84         if (last_jiffies_update.tv64 == 0)
  85                 last_jiffies_update = tick_next_period;
  86         period = last_jiffies_update;
  87         write_sequnlock(&xtime_lock);
  88         return period;
  89 }
  90
  91 /*
  92  * NOHZ - aka dynamic tick functionality
  93  */
  94 #ifdef CONFIG_NO_HZ
  95 /*
  96  * NO HZ enabled ?
  97  */
  98 static int tick_nohz_enabled __read_mostly  = 1;
  99
 100 /*
 101  * Enable / Disable tickless mode
 102  */
 103 static int __init setup_tick_nohz(char *str)
 104 {
 105         if (!strcmp(str, "off"))
 106                 tick_nohz_enabled = 0;
 107         else if (!strcmp(str, "on"))
 108                 tick_nohz_enabled = 1;
 109         else
 110                 return 0;
 111         return 1;
 112 }
 113
 114 __setup("nohz=", setup_tick_nohz);
 115
 116 /**
 117  * tick_nohz_update_jiffies - update jiffies when idle was interrupted
 118  *
 119  * Called from interrupt entry when the CPU was idle
 120  *
 121  * In case the sched_tick was stopped on this CPU, we have to check if jiffies
 122  * must be updated. Otherwise an interrupt handler could use a stale jiffy
 123  * value. We do this unconditionally on any cpu, as we don't know whether the
 124  * cpu, which has the update task assigned is in a long sleep.
 125  */
 126 void tick_nohz_update_jiffies(void)
 127 {
 128         int cpu = smp_processor_id();
 129         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 130         unsigned long flags;
 131         ktime_t now;
 132
 133         if (!ts->tick_stopped)
 134                 return;
 135
 136         touch_softlockup_watchdog();
 137
 138         cpu_clear(cpu, nohz_cpu_mask);
 139         now = ktime_get();
 140         ts->idle_waketime = now;
 141
 142         local_irq_save(flags);
 143         tick_do_update_jiffies64(now);
 144         local_irq_restore(flags);
 145 }
 146
 147 void tick_nohz_stop_idle(int cpu)
 148 {
 149         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 150
 151         if (ts->idle_active) {
 152                 ktime_t now, delta;
 153                 now = ktime_get();
 154                 delta = ktime_sub(now, ts->idle_entrytime);
 155                 ts->idle_lastupdate = now;
 156                 ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
 157                 ts->idle_active = 0;
 158         }
 159 }
 160
 161 static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
 162 {
 163         ktime_t now, delta;
 164
 165         now = ktime_get();
 166         if (ts->idle_active) {
 167                 delta = ktime_sub(now, ts->idle_entrytime);
 168                 ts->idle_lastupdate = now;
 169                 ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
 170         }
 171         ts->idle_entrytime = now;
 172         ts->idle_active = 1;
 173         return now;
 174 }
 175
 176 u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
 177 {
 178         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 179
 180         *last_update_time = ktime_to_us(ts->idle_lastupdate);
 181         return ktime_to_us(ts->idle_sleeptime);
 182 }
 183
 184 /**
 185  * tick_nohz_stop_sched_tick - stop the idle tick from the idle task
 186  *
 187  * When the next event is more than a tick into the future, stop the idle tick
 188  * Called either from the idle loop or from irq_exit() when an idle period was
 189  * just interrupted by an interrupt which did not cause a reschedule.
 190  */
 191 void tick_nohz_stop_sched_tick(void)
 192 {
 193         unsigned long seq, last_jiffies, next_jiffies, delta_jiffies, flags;
 194         unsigned long rt_jiffies;
 195         struct tick_sched *ts;
 196         ktime_t last_update, expires, now;
 197         struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
 198         int cpu;
 199
 200         local_irq_save(flags);
 201
 202         cpu = smp_processor_id();
 203         ts = &per_cpu(tick_cpu_sched, cpu);
 204         now = tick_nohz_start_idle(ts);
 205
 206         /*
 207          * If this cpu is offline and it is the one which updates
 208          * jiffies, then give up the assignment and let it be taken by
 209          * the cpu which runs the tick timer next. If we don't drop
 210          * this here the jiffies might be stale and do_timer() never
 211          * invoked.
 212          */
 213         if (unlikely(!cpu_online(cpu))) {
 214                 if (cpu == tick_do_timer_cpu)
 215                         tick_do_timer_cpu = -1;
 216         }
 217
 218         if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
 219                 goto end;
 220
 221         if (need_resched())
 222                 goto end;
 223
 224         if (unlikely(local_softirq_pending())) {
 225                 static int ratelimit;
 226
 227                 if (ratelimit < 10) {
 228                         printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n",
 229                                local_softirq_pending());
 230                         ratelimit++;
 231                 }
 232         }
 233
 234         ts->idle_calls++;
 235         /* Read jiffies and the time when jiffies were updated last */
 236         do {
 237                 seq = read_seqbegin(&xtime_lock);
 238                 last_update = last_jiffies_update;
 239                 last_jiffies = jiffies;
 240         } while (read_seqretry(&xtime_lock, seq));
 241
 242         /* Get the next timer wheel timer */
 243         next_jiffies = get_next_timer_interrupt(last_jiffies);
 244         delta_jiffies = next_jiffies - last_jiffies;
 245
 246         rt_jiffies = rt_needs_cpu(cpu);
 247         if (rt_jiffies && rt_jiffies < delta_jiffies)
 248                 delta_jiffies = rt_jiffies;
 249
 250         if (rcu_needs_cpu(cpu))
 251                 delta_jiffies = 1;
 252         /*
 253          * Do not stop the tick, if we are only one off
 254          * or if the cpu is required for rcu
 255          */
 256         if (!ts->tick_stopped && delta_jiffies == 1)
 257                 goto out;
 258
 259         /* Schedule the tick, if we are at least one jiffie off */
 260         if ((long)delta_jiffies >= 1) {
 261
 262                 if (delta_jiffies > 1)
 263                         cpu_set(cpu, nohz_cpu_mask);
 264                 /*
 265                  * nohz_stop_sched_tick can be called several times before
 266                  * the nohz_restart_sched_tick is called. This happens when
 267                  * interrupts arrive which do not cause a reschedule. In the
 268                  * first call we save the current tick time, so we can restart
 269                  * the scheduler tick in nohz_restart_sched_tick.
 270                  */
 271                 if (!ts->tick_stopped) {
 272                         if (select_nohz_load_balancer(1)) {
 273                                 /*
 274                                  * sched tick not stopped!
 275                                  */
 276                                 cpu_clear(cpu, nohz_cpu_mask);
 277                                 goto out;
 278                         }
 279
 280                         ts->idle_tick = ts->sched_timer.expires;
 281                         ts->tick_stopped = 1;
 282                         ts->idle_jiffies = last_jiffies;
 283                         rcu_enter_nohz();
 284                 }
 285
 286                 /*
 287                  * If this cpu is the one which updates jiffies, then
 288                  * give up the assignment and let it be taken by the
 289                  * cpu which runs the tick timer next, which might be
 290                  * this cpu as well. If we don't drop this here the
 291                  * jiffies might be stale and do_timer() never
 292                  * invoked.
 293                  */
 294                 if (cpu == tick_do_timer_cpu)
 295                         tick_do_timer_cpu = -1;
 296
 297                 ts->idle_sleeps++;
 298
 299                 /*
 300                  * delta_jiffies >= NEXT_TIMER_MAX_DELTA signals that
 301                  * there is no timer pending or at least extremly far
 302                  * into the future (12 days for HZ=1000). In this case
 303                  * we simply stop the tick timer:
 304                  */
 305                 if (unlikely(delta_jiffies >= NEXT_TIMER_MAX_DELTA)) {
 306                         ts->idle_expires.tv64 = KTIME_MAX;
 307                         if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
 308                                 hrtimer_cancel(&ts->sched_timer);
 309                         goto out;
 310                 }
 311
 312                 /*
 313                  * calculate the expiry time for the next timer wheel
 314                  * timer
 315                  */
 316                 expires = ktime_add_ns(last_update, tick_period.tv64 *
 317                                        delta_jiffies);
 318                 ts->idle_expires = expires;
 319
 320                 if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
 321                         hrtimer_start(&ts->sched_timer, expires,
 322                                       HRTIMER_MODE_ABS);
 323                         /* Check, if the timer was already in the past */
 324                         if (hrtimer_active(&ts->sched_timer))
 325                                 goto out;
 326                 } else if (!tick_program_event(expires, 0))
 327                                 goto out;
 328                 /*
 329                  * We are past the event already. So we crossed a
 330                  * jiffie boundary. Update jiffies and raise the
 331                  * softirq.
 332                  */
 333                 tick_do_update_jiffies64(ktime_get());
 334                 cpu_clear(cpu, nohz_cpu_mask);
 335         }
 336         raise_softirq_irqoff(TIMER_SOFTIRQ);
 337 out:
 338         ts->next_jiffies = next_jiffies;
 339         ts->last_jiffies = last_jiffies;
 340         ts->sleep_length = ktime_sub(dev->next_event, now);
 341 end:
 342         local_irq_restore(flags);
 343 }
 344
 345 /**
 346  * tick_nohz_get_sleep_length - return the length of the current sleep
 347  *
 348  * Called from power state control code with interrupts disabled
 349  */
 350 ktime_t tick_nohz_get_sleep_length(void)
 351 {
 352         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 353
 354         return ts->sleep_length;
 355 }
 356
 357 /**
 358  * tick_nohz_restart_sched_tick - restart the idle tick from the idle task
 359  *
 360  * Restart the idle tick when the CPU is woken up from idle
 361  */
 362 void tick_nohz_restart_sched_tick(void)
 363 {
 364         int cpu = smp_processor_id();
 365         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 366         unsigned long ticks;
 367         ktime_t now;
 368
 369         local_irq_disable();
 370         tick_nohz_stop_idle(cpu);
 371
 372         if (!ts->tick_stopped) {
 373                 local_irq_enable();
 374                 return;
 375         }
 376
 377         rcu_exit_nohz();
 378
 379         /* Update jiffies first */
 380         select_nohz_load_balancer(0);
 381         now = ktime_get();
 382         tick_do_update_jiffies64(now);
 383         cpu_clear(cpu, nohz_cpu_mask);
 384
 385         /*
 386          * We stopped the tick in idle. Update process times would miss the
 387          * time we slept as update_process_times does only a 1 tick
 388          * accounting. Enforce that this is accounted to idle !
 389          */
 390         ticks = jiffies - ts->idle_jiffies;
 391         /*
 392          * We might be one off. Do not randomly account a huge number of ticks!
 393          */
 394         if (ticks && ticks < LONG_MAX) {
 395                 add_preempt_count(HARDIRQ_OFFSET);
 396                 account_system_time(current, HARDIRQ_OFFSET,
 397                                     jiffies_to_cputime(ticks));
 398                 sub_preempt_count(HARDIRQ_OFFSET);
 399         }
 400
 401         /*
 402          * Cancel the scheduled timer and restore the tick
 403          */
 404         ts->tick_stopped  = 0;
 405         ts->idle_exittime = now;
 406         hrtimer_cancel(&ts->sched_timer);
 407         ts->sched_timer.expires = ts->idle_tick;
 408
 409         while (1) {
 410                 /* Forward the time to expire in the future */
 411                 hrtimer_forward(&ts->sched_timer, now, tick_period);
 412
 413                 if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
 414                         hrtimer_start(&ts->sched_timer,
 415                                       ts->sched_timer.expires,
 416                                       HRTIMER_MODE_ABS);
 417                         /* Check, if the timer was already in the past */
 418                         if (hrtimer_active(&ts->sched_timer))
 419                                 break;
 420                 } else {
 421                         if (!tick_program_event(ts->sched_timer.expires, 0))
 422                                 break;
 423                 }
 424                 /* Update jiffies and reread time */
 425                 tick_do_update_jiffies64(now);
 426                 now = ktime_get();
 427         }
 428         local_irq_enable();
 429 }
 430
 431 static int tick_nohz_reprogram(struct tick_sched *ts, ktime_t now)
 432 {
 433         hrtimer_forward(&ts->sched_timer, now, tick_period);
 434         return tick_program_event(ts->sched_timer.expires, 0);
 435 }
 436
 437 /*
 438  * The nohz low res interrupt handler
 439  */
 440 static void tick_nohz_handler(struct clock_event_device *dev)
 441 {
 442         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 443         struct pt_regs *regs = get_irq_regs();
 444         int cpu = smp_processor_id();
 445         ktime_t now = ktime_get();
 446
 447         dev->next_event.tv64 = KTIME_MAX;
 448
 449         /*
 450          * Check if the do_timer duty was dropped. We don't care about
 451          * concurrency: This happens only when the cpu in charge went
 452          * into a long sleep. If two cpus happen to assign themself to
 453          * this duty, then the jiffies update is still serialized by
 454          * xtime_lock.
 455          */
 456         if (unlikely(tick_do_timer_cpu == -1))
 457                 tick_do_timer_cpu = cpu;
 458
 459         /* Check, if the jiffies need an update */
 460         if (tick_do_timer_cpu == cpu)
 461                 tick_do_update_jiffies64(now);
 462
 463         /*
 464          * When we are idle and the tick is stopped, we have to touch
 465          * the watchdog as we might not schedule for a really long
 466          * time. This happens on complete idle SMP systems while
 467          * waiting on the login prompt. We also increment the "start
 468          * of idle" jiffy stamp so the idle accounting adjustment we
 469          * do when we go busy again does not account too much ticks.
 470          */
 471         if (ts->tick_stopped) {
 472                 touch_softlockup_watchdog();
 473                 ts->idle_jiffies++;
 474         }
 475
 476         update_process_times(user_mode(regs));
 477         profile_tick(CPU_PROFILING);
 478
 479         /* Do not restart, when we are in the idle loop */
 480         if (ts->tick_stopped)
 481                 return;
 482
 483         while (tick_nohz_reprogram(ts, now)) {
 484                 now = ktime_get();
 485                 tick_do_update_jiffies64(now);
 486         }
 487 }
 488
 489 /**
 490  * tick_nohz_switch_to_nohz - switch to nohz mode
 491  */
 492 static void tick_nohz_switch_to_nohz(void)
 493 {
 494         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 495         ktime_t next;
 496
 497         if (!tick_nohz_enabled)
 498                 return;
 499
 500         local_irq_disable();
 501         if (tick_switch_to_oneshot(tick_nohz_handler)) {
 502                 local_irq_enable();
 503                 return;
 504         }
 505
 506         ts->nohz_mode = NOHZ_MODE_LOWRES;
 507
 508         /*
 509          * Recycle the hrtimer in ts, so we can share the
 510          * hrtimer_forward with the highres code.
 511          */
 512         hrtimer_init(&ts->sched_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 513         /* Get the next period */
 514         next = tick_init_jiffy_update();
 515
 516         for (;;) {
 517                 ts->sched_timer.expires = next;
 518                 if (!tick_program_event(next, 0))
 519                         break;
 520                 next = ktime_add(next, tick_period);
 521         }
 522         local_irq_enable();
 523
 524         printk(KERN_INFO "Switched to NOHz mode on CPU #%d\n",
 525                smp_processor_id());
 526 }
 527
 528 #else
 529
 530 static inline void tick_nohz_switch_to_nohz(void) { }
 531
 532 #endif /* NO_HZ */
 533
 534 /*
 535  * High resolution timer specific code
 536  */
 537 #ifdef CONFIG_HIGH_RES_TIMERS
 538 /*
 539  * We rearm the timer until we get disabled by the idle code.
 540  * Called with interrupts disabled and timer->base->cpu_base->lock held.
 541  */
 542 static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
 543 {
 544         struct tick_sched *ts =
 545                 container_of(timer, struct tick_sched, sched_timer);
 546         struct pt_regs *regs = get_irq_regs();
 547         ktime_t now = ktime_get();
 548         int cpu = smp_processor_id();
 549
 550 #ifdef CONFIG_NO_HZ
 551         /*
 552          * Check if the do_timer duty was dropped. We don't care about
 553          * concurrency: This happens only when the cpu in charge went
 554          * into a long sleep. If two cpus happen to assign themself to
 555          * this duty, then the jiffies update is still serialized by
 556          * xtime_lock.
 557          */
 558         if (unlikely(tick_do_timer_cpu == -1))
 559                 tick_do_timer_cpu = cpu;
 560 #endif
 561
 562         /* Check, if the jiffies need an update */
 563         if (tick_do_timer_cpu == cpu)
 564                 tick_do_update_jiffies64(now);
 565
 566         /*
 567          * Do not call, when we are not in irq context and have
 568          * no valid regs pointer
 569          */
 570         if (regs) {
 571                 /*
 572                  * When we are idle and the tick is stopped, we have to touch
 573                  * the watchdog as we might not schedule for a really long
 574                  * time. This happens on complete idle SMP systems while
 575                  * waiting on the login prompt. We also increment the "start of
 576                  * idle" jiffy stamp so the idle accounting adjustment we do
 577                  * when we go busy again does not account too much ticks.
 578                  */
 579                 if (ts->tick_stopped) {
 580                         touch_softlockup_watchdog();
 581                         ts->idle_jiffies++;
 582                 }
 583                 update_process_times(user_mode(regs));
 584                 profile_tick(CPU_PROFILING);
 585         }
 586
 587         /* Do not restart, when we are in the idle loop */
 588         if (ts->tick_stopped)
 589                 return HRTIMER_NORESTART;
 590
 591         hrtimer_forward(timer, now, tick_period);
 592
 593         return HRTIMER_RESTART;
 594 }
 595
 596 /**
 597  * tick_setup_sched_timer - setup the tick emulation timer
 598  */
 599 void tick_setup_sched_timer(void)
 600 {
 601         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 602         ktime_t now = ktime_get();
 603         u64 offset;
 604
 605         /*
 606          * Emulate tick processing via per-CPU hrtimers:
 607          */
 608         hrtimer_init(&ts->sched_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 609         ts->sched_timer.function = tick_sched_timer;
 610         ts->sched_timer.cb_mode = HRTIMER_CB_IRQSAFE_NO_SOFTIRQ;
 611
 612         /* Get the next period (per cpu) */
 613         ts->sched_timer.expires = tick_init_jiffy_update();
 614         offset = ktime_to_ns(tick_period) >> 1;
 615         do_div(offset, num_possible_cpus());
 616         offset *= smp_processor_id();
 617         ts->sched_timer.expires = ktime_add_ns(ts->sched_timer.expires, offset);
 618
 619         for (;;) {
 620                 hrtimer_forward(&ts->sched_timer, now, tick_period);
 621                 hrtimer_start(&ts->sched_timer, ts->sched_timer.expires,
 622                               HRTIMER_MODE_ABS);
 623                 /* Check, if the timer was already in the past */
 624                 if (hrtimer_active(&ts->sched_timer))
 625                         break;
 626                 now = ktime_get();
 627         }
 628
 629 #ifdef CONFIG_NO_HZ
 630         if (tick_nohz_enabled)
 631                 ts->nohz_mode = NOHZ_MODE_HIGHRES;
 632 #endif
 633 }
 634
 635 void tick_cancel_sched_timer(int cpu)
 636 {
 637         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 638
 639         if (ts->sched_timer.base)
 640                 hrtimer_cancel(&ts->sched_timer);
 641
 642         ts->nohz_mode = NOHZ_MODE_INACTIVE;
 643 }
 644 #endif /* HIGH_RES_TIMERS */
 645
 646 /**
 647  * Async notification about clocksource changes
 648  */
 649 void tick_clock_notify(void)
 650 {
 651         int cpu;
 652
 653         for_each_possible_cpu(cpu)
 654                 set_bit(0, &per_cpu(tick_cpu_sched, cpu).check_clocks);
 655 }
 656
 657 /*
 658  * Async notification about clock event changes
 659  */
 660 void tick_oneshot_notify(void)
 661 {
 662         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 663
 664         set_bit(0, &ts->check_clocks);
 665 }
 666
 667 /**
 668  * Check, if a change happened, which makes oneshot possible.
 669  *
 670  * Called cyclic from the hrtimer softirq (driven by the timer
 671  * softirq) allow_nohz signals, that we can switch into low-res nohz
 672  * mode, because high resolution timers are disabled (either compile
 673  * or runtime).
 674  */
 675 int tick_check_oneshot_change(int allow_nohz)
 676 {
 677         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 678
 679         if (!test_and_clear_bit(0, &ts->check_clocks))
 680                 return 0;
 681
 682         if (ts->nohz_mode != NOHZ_MODE_INACTIVE)
 683                 return 0;
 684
 685         if (!timekeeping_valid_for_hres() || !tick_is_oneshot_available())
 686                 return 0;
 687
 688         if (!allow_nohz)
 689                 return 1;
 690
 691         tick_nohz_switch_to_nohz();
 692         return 0;
 693 }