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(int cpu)
 162 {
 163         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 164         ktime_t now, delta;
 165
 166         now = ktime_get();
 167         if (ts->idle_active) {
 168                 delta = ktime_sub(now, ts->idle_entrytime);
 169                 ts->idle_lastupdate = now;
 170                 ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
 171         }
 172         ts->idle_entrytime = now;
 173         ts->idle_active = 1;
 174         return now;
 175 }
 176
 177 u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
 178 {
 179         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 180
 181         *last_update_time = ktime_to_us(ts->idle_lastupdate);
 182         return ktime_to_us(ts->idle_sleeptime);
 183 }
 184
 185 /**
 186  * tick_nohz_stop_sched_tick - stop the idle tick from the idle task
 187  *
 188  * When the next event is more than a tick into the future, stop the idle tick
 189  * Called either from the idle loop or from irq_exit() when an idle period was
 190  * just interrupted by an interrupt which did not cause a reschedule.
 191  */
 192 void tick_nohz_stop_sched_tick(void)
 193 {
 194         unsigned long seq, last_jiffies, next_jiffies, delta_jiffies, flags;
 195         unsigned long rt_jiffies;
 196         struct tick_sched *ts;
 197         ktime_t last_update, expires, now;
 198         struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
 199         int cpu;
 200
 201         local_irq_save(flags);
 202
 203         cpu = smp_processor_id();
 204         now = tick_nohz_start_idle(cpu);
 205         ts = &per_cpu(tick_cpu_sched, cpu);
 206
 207         /*
 208          * If this cpu is offline and it is the one which updates
 209          * jiffies, then give up the assignment and let it be taken by
 210          * the cpu which runs the tick timer next. If we don't drop
 211          * this here the jiffies might be stale and do_timer() never
 212          * invoked.
 213          */
 214         if (unlikely(!cpu_online(cpu))) {
 215                 if (cpu == tick_do_timer_cpu)
 216                         tick_do_timer_cpu = -1;
 217         }
 218
 219         if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
 220                 goto end;
 221
 222         if (need_resched())
 223                 goto end;
 224
 225         cpu = smp_processor_id();
 226         if (unlikely(local_softirq_pending())) {
 227                 static int ratelimit;
 228
 229                 if (ratelimit < 10) {
 230                         printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n",
 231                                local_softirq_pending());
 232                         ratelimit++;
 233                 }
 234         }
 235
 236         ts->idle_calls++;
 237         /* Read jiffies and the time when jiffies were updated last */
 238         do {
 239                 seq = read_seqbegin(&xtime_lock);
 240                 last_update = last_jiffies_update;
 241                 last_jiffies = jiffies;
 242         } while (read_seqretry(&xtime_lock, seq));
 243
 244         /* Get the next timer wheel timer */
 245         next_jiffies = get_next_timer_interrupt(last_jiffies);
 246         delta_jiffies = next_jiffies - last_jiffies;
 247
 248         rt_jiffies = rt_needs_cpu(cpu);
 249         if (rt_jiffies && rt_jiffies < delta_jiffies)
 250                 delta_jiffies = rt_jiffies;
 251
 252         if (rcu_needs_cpu(cpu))
 253                 delta_jiffies = 1;
 254         /*
 255          * Do not stop the tick, if we are only one off
 256          * or if the cpu is required for rcu
 257          */
 258         if (!ts->tick_stopped && delta_jiffies == 1)
 259                 goto out;
 260
 261         /* Schedule the tick, if we are at least one jiffie off */
 262         if ((long)delta_jiffies >= 1) {
 263
 264                 if (delta_jiffies > 1)
 265                         cpu_set(cpu, nohz_cpu_mask);
 266                 /*
 267                  * nohz_stop_sched_tick can be called several times before
 268                  * the nohz_restart_sched_tick is called. This happens when
 269                  * interrupts arrive which do not cause a reschedule. In the
 270                  * first call we save the current tick time, so we can restart
 271                  * the scheduler tick in nohz_restart_sched_tick.
 272                  */
 273                 if (!ts->tick_stopped) {
 274                         if (select_nohz_load_balancer(1)) {
 275                                 /*
 276                                  * sched tick not stopped!
 277                                  */
 278                                 cpu_clear(cpu, nohz_cpu_mask);
 279                                 goto out;
 280                         }
 281
 282                         ts->idle_tick = ts->sched_timer.expires;
 283                         ts->tick_stopped = 1;
 284                         ts->idle_jiffies = last_jiffies;
 285                 }
 286
 287                 /*
 288                  * If this cpu is the one which updates jiffies, then
 289                  * give up the assignment and let it be taken by the
 290                  * cpu which runs the tick timer next, which might be
 291                  * this cpu as well. If we don't drop this here the
 292                  * jiffies might be stale and do_timer() never
 293                  * invoked.
 294                  */
 295                 if (cpu == tick_do_timer_cpu)
 296                         tick_do_timer_cpu = -1;
 297
 298                 ts->idle_sleeps++;
 299
 300                 /*
 301                  * delta_jiffies >= NEXT_TIMER_MAX_DELTA signals that
 302                  * there is no timer pending or at least extremly far
 303                  * into the future (12 days for HZ=1000). In this case
 304                  * we simply stop the tick timer:
 305                  */
 306                 if (unlikely(delta_jiffies >= NEXT_TIMER_MAX_DELTA)) {
 307                         ts->idle_expires.tv64 = KTIME_MAX;
 308                         if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
 309                                 hrtimer_cancel(&ts->sched_timer);
 310                         goto out;
 311                 }
 312
 313                 /*
 314                  * calculate the expiry time for the next timer wheel
 315                  * timer
 316                  */
 317                 expires = ktime_add_ns(last_update, tick_period.tv64 *
 318                                        delta_jiffies);
 319                 ts->idle_expires = expires;
 320
 321                 if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
 322                         hrtimer_start(&ts->sched_timer, expires,
 323                                       HRTIMER_MODE_ABS);
 324                         /* Check, if the timer was already in the past */
 325                         if (hrtimer_active(&ts->sched_timer))
 326                                 goto out;
 327                 } else if (!tick_program_event(expires, 0))
 328                                 goto out;
 329                 /*
 330                  * We are past the event already. So we crossed a
 331                  * jiffie boundary. Update jiffies and raise the
 332                  * softirq.
 333                  */
 334                 tick_do_update_jiffies64(ktime_get());
 335                 cpu_clear(cpu, nohz_cpu_mask);
 336         }
 337         raise_softirq_irqoff(TIMER_SOFTIRQ);
 338 out:
 339         ts->next_jiffies = next_jiffies;
 340         ts->last_jiffies = last_jiffies;
 341         ts->sleep_length = ktime_sub(dev->next_event, now);
 342 end:
 343         local_irq_restore(flags);
 344 }
 345
 346 /**
 347  * tick_nohz_get_sleep_length - return the length of the current sleep
 348  *
 349  * Called from power state control code with interrupts disabled
 350  */
 351 ktime_t tick_nohz_get_sleep_length(void)
 352 {
 353         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 354
 355         return ts->sleep_length;
 356 }
 357
 358 /**
 359  * tick_nohz_restart_sched_tick - restart the idle tick from the idle task
 360  *
 361  * Restart the idle tick when the CPU is woken up from idle
 362  */
 363 void tick_nohz_restart_sched_tick(void)
 364 {
 365         int cpu = smp_processor_id();
 366         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 367         unsigned long ticks;
 368         ktime_t now;
 369
 370         local_irq_disable();
 371         tick_nohz_stop_idle(cpu);
 372
 373         if (!ts->tick_stopped) {
 374                 local_irq_enable();
 375                 return;
 376         }
 377
 378         /* Update jiffies first */
 379         select_nohz_load_balancer(0);
 380         now = ktime_get();
 381         tick_do_update_jiffies64(now);
 382         cpu_clear(cpu, nohz_cpu_mask);
 383
 384         /*
 385          * We stopped the tick in idle. Update process times would miss the
 386          * time we slept as update_process_times does only a 1 tick
 387          * accounting. Enforce that this is accounted to idle !
 388          */
 389         ticks = jiffies - ts->idle_jiffies;
 390         /*
 391          * We might be one off. Do not randomly account a huge number of ticks!
 392          */
 393         if (ticks && ticks < LONG_MAX) {
 394                 add_preempt_count(HARDIRQ_OFFSET);
 395                 account_system_time(current, HARDIRQ_OFFSET,
 396                                     jiffies_to_cputime(ticks));
 397                 sub_preempt_count(HARDIRQ_OFFSET);
 398         }
 399
 400         /*
 401          * Cancel the scheduled timer and restore the tick
 402          */
 403         ts->tick_stopped  = 0;
 404         ts->idle_exittime = now;
 405         hrtimer_cancel(&ts->sched_timer);
 406         ts->sched_timer.expires = ts->idle_tick;
 407
 408         while (1) {
 409                 /* Forward the time to expire in the future */
 410                 hrtimer_forward(&ts->sched_timer, now, tick_period);
 411
 412                 if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
 413                         hrtimer_start(&ts->sched_timer,
 414                                       ts->sched_timer.expires,
 415                                       HRTIMER_MODE_ABS);
 416                         /* Check, if the timer was already in the past */
 417                         if (hrtimer_active(&ts->sched_timer))
 418                                 break;
 419                 } else {
 420                         if (!tick_program_event(ts->sched_timer.expires, 0))
 421                                 break;
 422                 }
 423                 /* Update jiffies and reread time */
 424                 tick_do_update_jiffies64(now);
 425                 now = ktime_get();
 426         }
 427         local_irq_enable();
 428 }
 429
 430 static int tick_nohz_reprogram(struct tick_sched *ts, ktime_t now)
 431 {
 432         hrtimer_forward(&ts->sched_timer, now, tick_period);
 433         return tick_program_event(ts->sched_timer.expires, 0);
 434 }
 435
 436 /*
 437  * The nohz low res interrupt handler
 438  */
 439 static void tick_nohz_handler(struct clock_event_device *dev)
 440 {
 441         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 442         struct pt_regs *regs = get_irq_regs();
 443         int cpu = smp_processor_id();
 444         ktime_t now = ktime_get();
 445
 446         dev->next_event.tv64 = KTIME_MAX;
 447
 448         /*
 449          * Check if the do_timer duty was dropped. We don't care about
 450          * concurrency: This happens only when the cpu in charge went
 451          * into a long sleep. If two cpus happen to assign themself to
 452          * this duty, then the jiffies update is still serialized by
 453          * xtime_lock.
 454          */
 455         if (unlikely(tick_do_timer_cpu == -1))
 456                 tick_do_timer_cpu = cpu;
 457
 458         /* Check, if the jiffies need an update */
 459         if (tick_do_timer_cpu == cpu)
 460                 tick_do_update_jiffies64(now);
 461
 462         /*
 463          * When we are idle and the tick is stopped, we have to touch
 464          * the watchdog as we might not schedule for a really long
 465          * time. This happens on complete idle SMP systems while
 466          * waiting on the login prompt. We also increment the "start
 467          * of idle" jiffy stamp so the idle accounting adjustment we
 468          * do when we go busy again does not account too much ticks.
 469          */
 470         if (ts->tick_stopped) {
 471                 touch_softlockup_watchdog();
 472                 ts->idle_jiffies++;
 473         }
 474
 475         update_process_times(user_mode(regs));
 476         profile_tick(CPU_PROFILING);
 477
 478         /* Do not restart, when we are in the idle loop */
 479         if (ts->tick_stopped)
 480                 return;
 481
 482         while (tick_nohz_reprogram(ts, now)) {
 483                 now = ktime_get();
 484                 tick_do_update_jiffies64(now);
 485         }
 486 }
 487
 488 /**
 489  * tick_nohz_switch_to_nohz - switch to nohz mode
 490  */
 491 static void tick_nohz_switch_to_nohz(void)
 492 {
 493         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 494         ktime_t next;
 495
 496         if (!tick_nohz_enabled)
 497                 return;
 498
 499         local_irq_disable();
 500         if (tick_switch_to_oneshot(tick_nohz_handler)) {
 501                 local_irq_enable();
 502                 return;
 503         }
 504
 505         ts->nohz_mode = NOHZ_MODE_LOWRES;
 506
 507         /*
 508          * Recycle the hrtimer in ts, so we can share the
 509          * hrtimer_forward with the highres code.
 510          */
 511         hrtimer_init(&ts->sched_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 512         /* Get the next period */
 513         next = tick_init_jiffy_update();
 514
 515         for (;;) {
 516                 ts->sched_timer.expires = next;
 517                 if (!tick_program_event(next, 0))
 518                         break;
 519                 next = ktime_add(next, tick_period);
 520         }
 521         local_irq_enable();
 522
 523         printk(KERN_INFO "Switched to NOHz mode on CPU #%d\n",
 524                smp_processor_id());
 525 }
 526
 527 #else
 528
 529 static inline void tick_nohz_switch_to_nohz(void) { }
 530
 531 #endif /* NO_HZ */
 532
 533 /*
 534  * High resolution timer specific code
 535  */
 536 #ifdef CONFIG_HIGH_RES_TIMERS
 537 /*
 538  * We rearm the timer until we get disabled by the idle code.
 539  * Called with interrupts disabled and timer->base->cpu_base->lock held.
 540  */
 541 static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
 542 {
 543         struct tick_sched *ts =
 544                 container_of(timer, struct tick_sched, sched_timer);
 545         struct pt_regs *regs = get_irq_regs();
 546         ktime_t now = ktime_get();
 547         int cpu = smp_processor_id();
 548
 549 #ifdef CONFIG_NO_HZ
 550         /*
 551          * Check if the do_timer duty was dropped. We don't care about
 552          * concurrency: This happens only when the cpu in charge went
 553          * into a long sleep. If two cpus happen to assign themself to
 554          * this duty, then the jiffies update is still serialized by
 555          * xtime_lock.
 556          */
 557         if (unlikely(tick_do_timer_cpu == -1))
 558                 tick_do_timer_cpu = cpu;
 559 #endif
 560
 561         /* Check, if the jiffies need an update */
 562         if (tick_do_timer_cpu == cpu)
 563                 tick_do_update_jiffies64(now);
 564
 565         /*
 566          * Do not call, when we are not in irq context and have
 567          * no valid regs pointer
 568          */
 569         if (regs) {
 570                 /*
 571                  * When we are idle and the tick is stopped, we have to touch
 572                  * the watchdog as we might not schedule for a really long
 573                  * time. This happens on complete idle SMP systems while
 574                  * waiting on the login prompt. We also increment the "start of
 575                  * idle" jiffy stamp so the idle accounting adjustment we do
 576                  * when we go busy again does not account too much ticks.
 577                  */
 578                 if (ts->tick_stopped) {
 579                         touch_softlockup_watchdog();
 580                         ts->idle_jiffies++;
 581                 }
 582                 update_process_times(user_mode(regs));
 583                 profile_tick(CPU_PROFILING);
 584         }
 585
 586         /* Do not restart, when we are in the idle loop */
 587         if (ts->tick_stopped)
 588                 return HRTIMER_NORESTART;
 589
 590         hrtimer_forward(timer, now, tick_period);
 591
 592         return HRTIMER_RESTART;
 593 }
 594
 595 /**
 596  * tick_setup_sched_timer - setup the tick emulation timer
 597  */
 598 void tick_setup_sched_timer(void)
 599 {
 600         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 601         ktime_t now = ktime_get();
 602         u64 offset;
 603
 604         /*
 605          * Emulate tick processing via per-CPU hrtimers:
 606          */
 607         hrtimer_init(&ts->sched_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 608         ts->sched_timer.function = tick_sched_timer;
 609         ts->sched_timer.cb_mode = HRTIMER_CB_IRQSAFE_NO_SOFTIRQ;
 610
 611         /* Get the next period (per cpu) */
 612         ts->sched_timer.expires = tick_init_jiffy_update();
 613         offset = ktime_to_ns(tick_period) >> 1;
 614         do_div(offset, num_possible_cpus());
 615         offset *= smp_processor_id();
 616         ts->sched_timer.expires = ktime_add_ns(ts->sched_timer.expires, offset);
 617
 618         for (;;) {
 619                 hrtimer_forward(&ts->sched_timer, now, tick_period);
 620                 hrtimer_start(&ts->sched_timer, ts->sched_timer.expires,
 621                               HRTIMER_MODE_ABS);
 622                 /* Check, if the timer was already in the past */
 623                 if (hrtimer_active(&ts->sched_timer))
 624                         break;
 625                 now = ktime_get();
 626         }
 627
 628 #ifdef CONFIG_NO_HZ
 629         if (tick_nohz_enabled)
 630                 ts->nohz_mode = NOHZ_MODE_HIGHRES;
 631 #endif
 632 }
 633
 634 void tick_cancel_sched_timer(int cpu)
 635 {
 636         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 637
 638         if (ts->sched_timer.base)
 639                 hrtimer_cancel(&ts->sched_timer);
 640         ts->tick_stopped = 0;
 641         ts->nohz_mode = NOHZ_MODE_INACTIVE;
 642 }
 643 #endif /* HIGH_RES_TIMERS */
 644
 645 /**
 646  * Async notification about clocksource changes
 647  */
 648 void tick_clock_notify(void)
 649 {
 650         int cpu;
 651
 652         for_each_possible_cpu(cpu)
 653                 set_bit(0, &per_cpu(tick_cpu_sched, cpu).check_clocks);
 654 }
 655
 656 /*
 657  * Async notification about clock event changes
 658  */
 659 void tick_oneshot_notify(void)
 660 {
 661         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 662
 663         set_bit(0, &ts->check_clocks);
 664 }
 665
 666 /**
 667  * Check, if a change happened, which makes oneshot possible.
 668  *
 669  * Called cyclic from the hrtimer softirq (driven by the timer
 670  * softirq) allow_nohz signals, that we can switch into low-res nohz
 671  * mode, because high resolution timers are disabled (either compile
 672  * or runtime).
 673  */
 674 int tick_check_oneshot_change(int allow_nohz)
 675 {
 676         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 677
 678         if (!test_and_clear_bit(0, &ts->check_clocks))
 679                 return 0;
 680
 681         if (ts->nohz_mode != NOHZ_MODE_INACTIVE)
 682                 return 0;
 683
 684         if (!timekeeping_is_continuous() || !tick_is_oneshot_available())
 685                 return 0;
 686
 687         if (!allow_nohz)
 688                 return 1;
 689
 690         tick_nohz_switch_to_nohz();
 691         return 0;
 692 }