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         /*
  52          * Do a quick check without holding xtime_lock:
  53          */
  54         delta = ktime_sub(now, last_jiffies_update);
  55         if (delta.tv64 < tick_period.tv64)
  56                 return;
  57
  58         /* Reevalute with xtime_lock held */
  59         write_seqlock(&xtime_lock);
  60
  61         delta = ktime_sub(now, last_jiffies_update);
  62         if (delta.tv64 >= tick_period.tv64) {
  63
  64                 delta = ktime_sub(delta, tick_period);
  65                 last_jiffies_update = ktime_add(last_jiffies_update,
  66                                                 tick_period);
  67
  68                 /* Slow path for long timeouts */
  69                 if (unlikely(delta.tv64 >= tick_period.tv64)) {
  70                         s64 incr = ktime_to_ns(tick_period);
  71
  72                         ticks = ktime_divns(delta, incr);
  73
  74                         last_jiffies_update = ktime_add_ns(last_jiffies_update,
  75                                                            incr * ticks);
  76                 }
  77                 do_timer(++ticks);
  78         }
  79         write_sequnlock(&xtime_lock);
  80 }
  81
  82 /*
  83  * Initialize and return retrieve the jiffies update.
  84  */
  85 static ktime_t tick_init_jiffy_update(void)
  86 {
  87         ktime_t period;
  88
  89         write_seqlock(&xtime_lock);
  90         /* Did we start the jiffies update yet ? */
  91         if (last_jiffies_update.tv64 == 0)
  92                 last_jiffies_update = tick_next_period;
  93         period = last_jiffies_update;
  94         write_sequnlock(&xtime_lock);
  95         return period;
  96 }
  97
  98 /*
  99  * NOHZ - aka dynamic tick functionality
 100  */
 101 #ifdef CONFIG_NO_HZ
 102 /*
 103  * NO HZ enabled ?
 104  */
 105 static int tick_nohz_enabled __read_mostly  = 1;
 106
 107 /*
 108  * Enable / Disable tickless mode
 109  */
 110 static int __init setup_tick_nohz(char *str)
 111 {
 112         if (!strcmp(str, "off"))
 113                 tick_nohz_enabled = 0;
 114         else if (!strcmp(str, "on"))
 115                 tick_nohz_enabled = 1;
 116         else
 117                 return 0;
 118         return 1;
 119 }
 120
 121 __setup("nohz=", setup_tick_nohz);
 122
 123 /**
 124  * tick_nohz_update_jiffies - update jiffies when idle was interrupted
 125  *
 126  * Called from interrupt entry when the CPU was idle
 127  *
 128  * In case the sched_tick was stopped on this CPU, we have to check if jiffies
 129  * must be updated. Otherwise an interrupt handler could use a stale jiffy
 130  * value. We do this unconditionally on any cpu, as we don't know whether the
 131  * cpu, which has the update task assigned is in a long sleep.
 132  */
 133 void tick_nohz_update_jiffies(void)
 134 {
 135         int cpu = smp_processor_id();
 136         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 137         unsigned long flags;
 138         ktime_t now;
 139
 140         if (!ts->tick_stopped)
 141                 return;
 142
 143         touch_softlockup_watchdog();
 144
 145         cpu_clear(cpu, nohz_cpu_mask);
 146         now = ktime_get();
 147         ts->idle_waketime = now;
 148
 149         local_irq_save(flags);
 150         tick_do_update_jiffies64(now);
 151         local_irq_restore(flags);
 152 }
 153
 154 void tick_nohz_stop_idle(int cpu)
 155 {
 156         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 157
 158         if (ts->idle_active) {
 159                 ktime_t now, delta;
 160                 now = ktime_get();
 161                 delta = ktime_sub(now, ts->idle_entrytime);
 162                 ts->idle_lastupdate = now;
 163                 ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
 164                 ts->idle_active = 0;
 165         }
 166 }
 167
 168 static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
 169 {
 170         ktime_t now, delta;
 171
 172         now = ktime_get();
 173         if (ts->idle_active) {
 174                 delta = ktime_sub(now, ts->idle_entrytime);
 175                 ts->idle_lastupdate = now;
 176                 ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
 177         }
 178         ts->idle_entrytime = now;
 179         ts->idle_active = 1;
 180         return now;
 181 }
 182
 183 u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
 184 {
 185         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 186
 187         *last_update_time = ktime_to_us(ts->idle_lastupdate);
 188         return ktime_to_us(ts->idle_sleeptime);
 189 }
 190
 191 /**
 192  * tick_nohz_stop_sched_tick - stop the idle tick from the idle task
 193  *
 194  * When the next event is more than a tick into the future, stop the idle tick
 195  * Called either from the idle loop or from irq_exit() when an idle period was
 196  * just interrupted by an interrupt which did not cause a reschedule.
 197  */
 198 void tick_nohz_stop_sched_tick(int inidle)
 199 {
 200         unsigned long seq, last_jiffies, next_jiffies, delta_jiffies, flags;
 201         struct tick_sched *ts;
 202         ktime_t last_update, expires, now;
 203         struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
 204         int cpu;
 205
 206         local_irq_save(flags);
 207
 208         cpu = smp_processor_id();
 209         ts = &per_cpu(tick_cpu_sched, cpu);
 210         now = tick_nohz_start_idle(ts);
 211
 212         /*
 213          * If this cpu is offline and it is the one which updates
 214          * jiffies, then give up the assignment and let it be taken by
 215          * the cpu which runs the tick timer next. If we don't drop
 216          * this here the jiffies might be stale and do_timer() never
 217          * invoked.
 218          */
 219         if (unlikely(!cpu_online(cpu))) {
 220                 if (cpu == tick_do_timer_cpu)
 221                         tick_do_timer_cpu = -1;
 222         }
 223
 224         if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
 225                 goto end;
 226
 227         if (!inidle && !ts->inidle)
 228                 goto end;
 229
 230         ts->inidle = 1;
 231
 232         if (need_resched())
 233                 goto end;
 234
 235         if (unlikely(local_softirq_pending())) {
 236                 static int ratelimit;
 237
 238                 if (ratelimit < 10) {
 239                         printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n",
 240                                local_softirq_pending());
 241                         ratelimit++;
 242                 }
 243                 goto end;
 244         }
 245
 246         ts->idle_calls++;
 247         /* Read jiffies and the time when jiffies were updated last */
 248         do {
 249                 seq = read_seqbegin(&xtime_lock);
 250                 last_update = last_jiffies_update;
 251                 last_jiffies = jiffies;
 252         } while (read_seqretry(&xtime_lock, seq));
 253
 254         /* Get the next timer wheel timer */
 255         next_jiffies = get_next_timer_interrupt(last_jiffies);
 256         delta_jiffies = next_jiffies - last_jiffies;
 257
 258         if (rcu_needs_cpu(cpu))
 259                 delta_jiffies = 1;
 260         /*
 261          * Do not stop the tick, if we are only one off
 262          * or if the cpu is required for rcu
 263          */
 264         if (!ts->tick_stopped && delta_jiffies == 1)
 265                 goto out;
 266
 267         /* Schedule the tick, if we are at least one jiffie off */
 268         if ((long)delta_jiffies >= 1) {
 269
 270                 if (delta_jiffies > 1)
 271                         cpu_set(cpu, nohz_cpu_mask);
 272                 /*
 273                  * nohz_stop_sched_tick can be called several times before
 274                  * the nohz_restart_sched_tick is called. This happens when
 275                  * interrupts arrive which do not cause a reschedule. In the
 276                  * first call we save the current tick time, so we can restart
 277                  * the scheduler tick in nohz_restart_sched_tick.
 278                  */
 279                 if (!ts->tick_stopped) {
 280                         if (select_nohz_load_balancer(1)) {
 281                                 /*
 282                                  * sched tick not stopped!
 283                                  */
 284                                 cpu_clear(cpu, nohz_cpu_mask);
 285                                 goto out;
 286                         }
 287
 288                         ts->idle_tick = ts->sched_timer.expires;
 289                         ts->tick_stopped = 1;
 290                         ts->idle_jiffies = last_jiffies;
 291                         rcu_enter_nohz();
 292                 }
 293
 294                 /*
 295                  * If this cpu is the one which updates jiffies, then
 296                  * give up the assignment and let it be taken by the
 297                  * cpu which runs the tick timer next, which might be
 298                  * this cpu as well. If we don't drop this here the
 299                  * jiffies might be stale and do_timer() never
 300                  * invoked.
 301                  */
 302                 if (cpu == tick_do_timer_cpu)
 303                         tick_do_timer_cpu = -1;
 304
 305                 ts->idle_sleeps++;
 306
 307                 /*
 308                  * delta_jiffies >= NEXT_TIMER_MAX_DELTA signals that
 309                  * there is no timer pending or at least extremly far
 310                  * into the future (12 days for HZ=1000). In this case
 311                  * we simply stop the tick timer:
 312                  */
 313                 if (unlikely(delta_jiffies >= NEXT_TIMER_MAX_DELTA)) {
 314                         ts->idle_expires.tv64 = KTIME_MAX;
 315                         if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
 316                                 hrtimer_cancel(&ts->sched_timer);
 317                         goto out;
 318                 }
 319
 320                 /*
 321                  * calculate the expiry time for the next timer wheel
 322                  * timer
 323                  */
 324                 expires = ktime_add_ns(last_update, tick_period.tv64 *
 325                                        delta_jiffies);
 326                 ts->idle_expires = expires;
 327
 328                 if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
 329                         hrtimer_start(&ts->sched_timer, expires,
 330                                       HRTIMER_MODE_ABS);
 331                         /* Check, if the timer was already in the past */
 332                         if (hrtimer_active(&ts->sched_timer))
 333                                 goto out;
 334                 } else if (!tick_program_event(expires, 0))
 335                                 goto out;
 336                 /*
 337                  * We are past the event already. So we crossed a
 338                  * jiffie boundary. Update jiffies and raise the
 339                  * softirq.
 340                  */
 341                 tick_do_update_jiffies64(ktime_get());
 342                 cpu_clear(cpu, nohz_cpu_mask);
 343         }
 344         raise_softirq_irqoff(TIMER_SOFTIRQ);
 345 out:
 346         ts->next_jiffies = next_jiffies;
 347         ts->last_jiffies = last_jiffies;
 348         ts->sleep_length = ktime_sub(dev->next_event, now);
 349 end:
 350         local_irq_restore(flags);
 351 }
 352
 353 /**
 354  * tick_nohz_get_sleep_length - return the length of the current sleep
 355  *
 356  * Called from power state control code with interrupts disabled
 357  */
 358 ktime_t tick_nohz_get_sleep_length(void)
 359 {
 360         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 361
 362         return ts->sleep_length;
 363 }
 364
 365 /**
 366  * tick_nohz_restart_sched_tick - restart the idle tick from the idle task
 367  *
 368  * Restart the idle tick when the CPU is woken up from idle
 369  */
 370 void tick_nohz_restart_sched_tick(void)
 371 {
 372         int cpu = smp_processor_id();
 373         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 374         unsigned long ticks;
 375         ktime_t now;
 376
 377         local_irq_disable();
 378         tick_nohz_stop_idle(cpu);
 379
 380         if (!ts->inidle || !ts->tick_stopped) {
 381                 ts->inidle = 0;
 382                 local_irq_enable();
 383                 return;
 384         }
 385
 386         ts->inidle = 0;
 387
 388         rcu_exit_nohz();
 389
 390         /* Update jiffies first */
 391         select_nohz_load_balancer(0);
 392         now = ktime_get();
 393         tick_do_update_jiffies64(now);
 394         cpu_clear(cpu, nohz_cpu_mask);
 395
 396         /*
 397          * We stopped the tick in idle. Update process times would miss the
 398          * time we slept as update_process_times does only a 1 tick
 399          * accounting. Enforce that this is accounted to idle !
 400          */
 401         ticks = jiffies - ts->idle_jiffies;
 402         /*
 403          * We might be one off. Do not randomly account a huge number of ticks!
 404          */
 405         if (ticks && ticks < LONG_MAX) {
 406                 add_preempt_count(HARDIRQ_OFFSET);
 407                 account_system_time(current, HARDIRQ_OFFSET,
 408                                     jiffies_to_cputime(ticks));
 409                 sub_preempt_count(HARDIRQ_OFFSET);
 410         }
 411
 412         touch_softlockup_watchdog();
 413         /*
 414          * Cancel the scheduled timer and restore the tick
 415          */
 416         ts->tick_stopped  = 0;
 417         ts->idle_exittime = now;
 418         hrtimer_cancel(&ts->sched_timer);
 419         ts->sched_timer.expires = ts->idle_tick;
 420
 421         while (1) {
 422                 /* Forward the time to expire in the future */
 423                 hrtimer_forward(&ts->sched_timer, now, tick_period);
 424
 425                 if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
 426                         hrtimer_start(&ts->sched_timer,
 427                                       ts->sched_timer.expires,
 428                                       HRTIMER_MODE_ABS);
 429                         /* Check, if the timer was already in the past */
 430                         if (hrtimer_active(&ts->sched_timer))
 431                                 break;
 432                 } else {
 433                         if (!tick_program_event(ts->sched_timer.expires, 0))
 434                                 break;
 435                 }
 436                 /* Update jiffies and reread time */
 437                 tick_do_update_jiffies64(now);
 438                 now = ktime_get();
 439         }
 440         local_irq_enable();
 441 }
 442
 443 static int tick_nohz_reprogram(struct tick_sched *ts, ktime_t now)
 444 {
 445         hrtimer_forward(&ts->sched_timer, now, tick_period);
 446         return tick_program_event(ts->sched_timer.expires, 0);
 447 }
 448
 449 /*
 450  * The nohz low res interrupt handler
 451  */
 452 static void tick_nohz_handler(struct clock_event_device *dev)
 453 {
 454         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 455         struct pt_regs *regs = get_irq_regs();
 456         int cpu = smp_processor_id();
 457         ktime_t now = ktime_get();
 458
 459         dev->next_event.tv64 = KTIME_MAX;
 460
 461         /*
 462          * Check if the do_timer duty was dropped. We don't care about
 463          * concurrency: This happens only when the cpu in charge went
 464          * into a long sleep. If two cpus happen to assign themself to
 465          * this duty, then the jiffies update is still serialized by
 466          * xtime_lock.
 467          */
 468         if (unlikely(tick_do_timer_cpu == -1))
 469                 tick_do_timer_cpu = cpu;
 470
 471         /* Check, if the jiffies need an update */
 472         if (tick_do_timer_cpu == cpu)
 473                 tick_do_update_jiffies64(now);
 474
 475         /*
 476          * When we are idle and the tick is stopped, we have to touch
 477          * the watchdog as we might not schedule for a really long
 478          * time. This happens on complete idle SMP systems while
 479          * waiting on the login prompt. We also increment the "start
 480          * of idle" jiffy stamp so the idle accounting adjustment we
 481          * do when we go busy again does not account too much ticks.
 482          */
 483         if (ts->tick_stopped) {
 484                 touch_softlockup_watchdog();
 485                 ts->idle_jiffies++;
 486         }
 487
 488         update_process_times(user_mode(regs));
 489         profile_tick(CPU_PROFILING);
 490
 491         /* Do not restart, when we are in the idle loop */
 492         if (ts->tick_stopped)
 493                 return;
 494
 495         while (tick_nohz_reprogram(ts, now)) {
 496                 now = ktime_get();
 497                 tick_do_update_jiffies64(now);
 498         }
 499 }
 500
 501 /**
 502  * tick_nohz_switch_to_nohz - switch to nohz mode
 503  */
 504 static void tick_nohz_switch_to_nohz(void)
 505 {
 506         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 507         ktime_t next;
 508
 509         if (!tick_nohz_enabled)
 510                 return;
 511
 512         local_irq_disable();
 513         if (tick_switch_to_oneshot(tick_nohz_handler)) {
 514                 local_irq_enable();
 515                 return;
 516         }
 517
 518         ts->nohz_mode = NOHZ_MODE_LOWRES;
 519
 520         /*
 521          * Recycle the hrtimer in ts, so we can share the
 522          * hrtimer_forward with the highres code.
 523          */
 524         hrtimer_init(&ts->sched_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 525         /* Get the next period */
 526         next = tick_init_jiffy_update();
 527
 528         for (;;) {
 529                 ts->sched_timer.expires = next;
 530                 if (!tick_program_event(next, 0))
 531                         break;
 532                 next = ktime_add(next, tick_period);
 533         }
 534         local_irq_enable();
 535
 536         printk(KERN_INFO "Switched to NOHz mode on CPU #%d\n",
 537                smp_processor_id());
 538 }
 539
 540 #else
 541
 542 static inline void tick_nohz_switch_to_nohz(void) { }
 543
 544 #endif /* NO_HZ */
 545
 546 /*
 547  * High resolution timer specific code
 548  */
 549 #ifdef CONFIG_HIGH_RES_TIMERS
 550 /*
 551  * We rearm the timer until we get disabled by the idle code.
 552  * Called with interrupts disabled and timer->base->cpu_base->lock held.
 553  */
 554 static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
 555 {
 556         struct tick_sched *ts =
 557                 container_of(timer, struct tick_sched, sched_timer);
 558         struct pt_regs *regs = get_irq_regs();
 559         ktime_t now = ktime_get();
 560         int cpu = smp_processor_id();
 561
 562 #ifdef CONFIG_NO_HZ
 563         /*
 564          * Check if the do_timer duty was dropped. We don't care about
 565          * concurrency: This happens only when the cpu in charge went
 566          * into a long sleep. If two cpus happen to assign themself to
 567          * this duty, then the jiffies update is still serialized by
 568          * xtime_lock.
 569          */
 570         if (unlikely(tick_do_timer_cpu == -1))
 571                 tick_do_timer_cpu = cpu;
 572 #endif
 573
 574         /* Check, if the jiffies need an update */
 575         if (tick_do_timer_cpu == cpu)
 576                 tick_do_update_jiffies64(now);
 577
 578         /*
 579          * Do not call, when we are not in irq context and have
 580          * no valid regs pointer
 581          */
 582         if (regs) {
 583                 /*
 584                  * When we are idle and the tick is stopped, we have to touch
 585                  * the watchdog as we might not schedule for a really long
 586                  * time. This happens on complete idle SMP systems while
 587                  * waiting on the login prompt. We also increment the "start of
 588                  * idle" jiffy stamp so the idle accounting adjustment we do
 589                  * when we go busy again does not account too much ticks.
 590                  */
 591                 if (ts->tick_stopped) {
 592                         touch_softlockup_watchdog();
 593                         ts->idle_jiffies++;
 594                 }
 595                 update_process_times(user_mode(regs));
 596                 profile_tick(CPU_PROFILING);
 597         }
 598
 599         /* Do not restart, when we are in the idle loop */
 600         if (ts->tick_stopped)
 601                 return HRTIMER_NORESTART;
 602
 603         hrtimer_forward(timer, now, tick_period);
 604
 605         return HRTIMER_RESTART;
 606 }
 607
 608 /**
 609  * tick_setup_sched_timer - setup the tick emulation timer
 610  */
 611 void tick_setup_sched_timer(void)
 612 {
 613         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 614         ktime_t now = ktime_get();
 615         u64 offset;
 616
 617         /*
 618          * Emulate tick processing via per-CPU hrtimers:
 619          */
 620         hrtimer_init(&ts->sched_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 621         ts->sched_timer.function = tick_sched_timer;
 622         ts->sched_timer.cb_mode = HRTIMER_CB_IRQSAFE_NO_SOFTIRQ;
 623
 624         /* Get the next period (per cpu) */
 625         ts->sched_timer.expires = tick_init_jiffy_update();
 626         offset = ktime_to_ns(tick_period) >> 1;
 627         do_div(offset, num_possible_cpus());
 628         offset *= smp_processor_id();
 629         ts->sched_timer.expires = ktime_add_ns(ts->sched_timer.expires, offset);
 630
 631         for (;;) {
 632                 hrtimer_forward(&ts->sched_timer, now, tick_period);
 633                 hrtimer_start(&ts->sched_timer, ts->sched_timer.expires,
 634                               HRTIMER_MODE_ABS);
 635                 /* Check, if the timer was already in the past */
 636                 if (hrtimer_active(&ts->sched_timer))
 637                         break;
 638                 now = ktime_get();
 639         }
 640
 641 #ifdef CONFIG_NO_HZ
 642         if (tick_nohz_enabled)
 643                 ts->nohz_mode = NOHZ_MODE_HIGHRES;
 644 #endif
 645 }
 646
 647 void tick_cancel_sched_timer(int cpu)
 648 {
 649         struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 650
 651         if (ts->sched_timer.base)
 652                 hrtimer_cancel(&ts->sched_timer);
 653
 654         ts->nohz_mode = NOHZ_MODE_INACTIVE;
 655 }
 656 #endif /* HIGH_RES_TIMERS */
 657
 658 /**
 659  * Async notification about clocksource changes
 660  */
 661 void tick_clock_notify(void)
 662 {
 663         int cpu;
 664
 665         for_each_possible_cpu(cpu)
 666                 set_bit(0, &per_cpu(tick_cpu_sched, cpu).check_clocks);
 667 }
 668
 669 /*
 670  * Async notification about clock event changes
 671  */
 672 void tick_oneshot_notify(void)
 673 {
 674         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 675
 676         set_bit(0, &ts->check_clocks);
 677 }
 678
 679 /**
 680  * Check, if a change happened, which makes oneshot possible.
 681  *
 682  * Called cyclic from the hrtimer softirq (driven by the timer
 683  * softirq) allow_nohz signals, that we can switch into low-res nohz
 684  * mode, because high resolution timers are disabled (either compile
 685  * or runtime).
 686  */
 687 int tick_check_oneshot_change(int allow_nohz)
 688 {
 689         struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
 690
 691         if (!test_and_clear_bit(0, &ts->check_clocks))
 692                 return 0;
 693
 694         if (ts->nohz_mode != NOHZ_MODE_INACTIVE)
 695                 return 0;
 696
 697         if (!timekeeping_valid_for_hres() || !tick_is_oneshot_available())
 698                 return 0;
 699
 700         if (!allow_nohz)
 701                 return 1;
 702
 703         tick_nohz_switch_to_nohz();
 704         return 0;
 705 }