kernel/smp.c

   1 /*
   2  * Generic helpers for smp ipi calls
   3  *
   4  * (C) Jens Axboe <jens.axboe@oracle.com> 2008
   5  */
   6 #include <linux/rcupdate.h>
   7 #include <linux/rculist.h>
   8 #include <linux/kernel.h>
   9 #include <linux/export.h>
  10 #include <linux/percpu.h>
  11 #include <linux/init.h>
  12 #include <linux/gfp.h>
  13 #include <linux/smp.h>
  14 #include <linux/cpu.h>
  15
  16 #include "smpboot.h"
  17
  18 #ifdef CONFIG_USE_GENERIC_SMP_HELPERS
  19 enum {
  20         CSD_FLAG_LOCK           = 0x01,
  21 };
  22
  23 struct call_function_data {
  24         struct call_single_data __percpu *csd;
  25         cpumask_var_t           cpumask;
  26         cpumask_var_t           cpumask_ipi;
  27 };
  28
  29 static DEFINE_PER_CPU_SHARED_ALIGNED(struct call_function_data, cfd_data);
  30
  31 struct call_single_queue {
  32         struct list_head        list;
  33         raw_spinlock_t          lock;
  34 };
  35
  36 static DEFINE_PER_CPU_SHARED_ALIGNED(struct call_single_queue, call_single_queue);
  37
  38 static int
  39 hotplug_cfd(struct notifier_block *nfb, unsigned long action, void *hcpu)
  40 {
  41         long cpu = (long)hcpu;
  42         struct call_function_data *cfd = &per_cpu(cfd_data, cpu);
  43
  44         switch (action) {
  45         case CPU_UP_PREPARE:
  46         case CPU_UP_PREPARE_FROZEN:
  47                 if (!zalloc_cpumask_var_node(&cfd->cpumask, GFP_KERNEL,
  48                                 cpu_to_node(cpu)))
  49                         return notifier_from_errno(-ENOMEM);
  50                 if (!zalloc_cpumask_var_node(&cfd->cpumask_ipi, GFP_KERNEL,
  51                                 cpu_to_node(cpu)))
  52                         return notifier_from_errno(-ENOMEM);
  53                 cfd->csd = alloc_percpu(struct call_single_data);
  54                 if (!cfd->csd) {
  55                         free_cpumask_var(cfd->cpumask);
  56                         return notifier_from_errno(-ENOMEM);
  57                 }
  58                 break;
  59
  60 #ifdef CONFIG_HOTPLUG_CPU
  61         case CPU_UP_CANCELED:
  62         case CPU_UP_CANCELED_FROZEN:
  63
  64         case CPU_DEAD:
  65         case CPU_DEAD_FROZEN:
  66                 free_cpumask_var(cfd->cpumask);
  67                 free_cpumask_var(cfd->cpumask_ipi);
  68                 free_percpu(cfd->csd);
  69                 break;
  70 #endif
  71         };
  72
  73         return NOTIFY_OK;
  74 }
  75
  76 static struct notifier_block hotplug_cfd_notifier = {
  77         .notifier_call          = hotplug_cfd,
  78 };
  79
  80 void __init call_function_init(void)
  81 {
  82         void *cpu = (void *)(long)smp_processor_id();
  83         int i;
  84
  85         for_each_possible_cpu(i) {
  86                 struct call_single_queue *q = &per_cpu(call_single_queue, i);
  87
  88                 raw_spin_lock_init(&q->lock);
  89                 INIT_LIST_HEAD(&q->list);
  90         }
  91
  92         hotplug_cfd(&hotplug_cfd_notifier, CPU_UP_PREPARE, cpu);
  93         register_cpu_notifier(&hotplug_cfd_notifier);
  94 }
  95
  96 /*
  97  * csd_lock/csd_unlock used to serialize access to per-cpu csd resources
  98  *
  99  * For non-synchronous ipi calls the csd can still be in use by the
 100  * previous function call. For multi-cpu calls its even more interesting
 101  * as we'll have to ensure no other cpu is observing our csd.
 102  */
 103 static void csd_lock_wait(struct call_single_data *csd)
 104 {
 105         while (csd->flags & CSD_FLAG_LOCK)
 106                 cpu_relax();
 107 }
 108
 109 static void csd_lock(struct call_single_data *csd)
 110 {
 111         csd_lock_wait(csd);
 112         csd->flags |= CSD_FLAG_LOCK;
 113
 114         /*
 115          * prevent CPU from reordering the above assignment
 116          * to ->flags with any subsequent assignments to other
 117          * fields of the specified call_single_data structure:
 118          */
 119         smp_mb();
 120 }
 121
 122 static void csd_unlock(struct call_single_data *csd)
 123 {
 124         WARN_ON(!(csd->flags & CSD_FLAG_LOCK));
 125
 126         /*
 127          * ensure we're all done before releasing data:
 128          */
 129         smp_mb();
 130
 131         csd->flags &= ~CSD_FLAG_LOCK;
 132 }
 133
 134 /*
 135  * Insert a previously allocated call_single_data element
 136  * for execution on the given CPU. data must already have
 137  * ->func, ->info, and ->flags set.
 138  */
 139 static
 140 void generic_exec_single(int cpu, struct call_single_data *csd, int wait)
 141 {
 142         struct call_single_queue *dst = &per_cpu(call_single_queue, cpu);
 143         unsigned long flags;
 144         int ipi;
 145
 146         raw_spin_lock_irqsave(&dst->lock, flags);
 147         ipi = list_empty(&dst->list);
 148         list_add_tail(&csd->list, &dst->list);
 149         raw_spin_unlock_irqrestore(&dst->lock, flags);
 150
 151         /*
 152          * The list addition should be visible before sending the IPI
 153          * handler locks the list to pull the entry off it because of
 154          * normal cache coherency rules implied by spinlocks.
 155          *
 156          * If IPIs can go out of order to the cache coherency protocol
 157          * in an architecture, sufficient synchronisation should be added
 158          * to arch code to make it appear to obey cache coherency WRT
 159          * locking and barrier primitives. Generic code isn't really
 160          * equipped to do the right thing...
 161          */
 162         if (ipi)
 163                 arch_send_call_function_single_ipi(cpu);
 164
 165         if (wait)
 166                 csd_lock_wait(csd);
 167 }
 168
 169 /*
 170  * Invoked by arch to handle an IPI for call function single. Must be
 171  * called from the arch with interrupts disabled.
 172  */
 173 void generic_smp_call_function_single_interrupt(void)
 174 {
 175         struct call_single_queue *q = &__get_cpu_var(call_single_queue);
 176         LIST_HEAD(list);
 177
 178         /*
 179          * Shouldn't receive this interrupt on a cpu that is not yet online.
 180          */
 181         WARN_ON_ONCE(!cpu_online(smp_processor_id()));
 182
 183         raw_spin_lock(&q->lock);
 184         list_replace_init(&q->list, &list);
 185         raw_spin_unlock(&q->lock);
 186
 187         while (!list_empty(&list)) {
 188                 struct call_single_data *csd;
 189
 190                 csd = list_entry(list.next, struct call_single_data, list);
 191                 list_del(&csd->list);
 192
 193                 csd->func(csd->info);
 194
 195                 csd_unlock(csd);
 196         }
 197 }
 198
 199 static DEFINE_PER_CPU_SHARED_ALIGNED(struct call_single_data, csd_data);
 200
 201 /*
 202  * smp_call_function_single - Run a function on a specific CPU
 203  * @func: The function to run. This must be fast and non-blocking.
 204  * @info: An arbitrary pointer to pass to the function.
 205  * @wait: If true, wait until function has completed on other CPUs.
 206  *
 207  * Returns 0 on success, else a negative status code.
 208  */
 209 int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
 210                              int wait)
 211 {
 212         struct call_single_data d = {
 213                 .flags = 0,
 214         };
 215         unsigned long flags;
 216         int this_cpu;
 217         int err = 0;
 218
 219         /*
 220          * prevent preemption and reschedule on another processor,
 221          * as well as CPU removal
 222          */
 223         this_cpu = get_cpu();
 224
 225         /*
 226          * Can deadlock when called with interrupts disabled.
 227          * We allow cpu's that are not yet online though, as no one else can
 228          * send smp call function interrupt to this cpu and as such deadlocks
 229          * can't happen.
 230          */
 231         WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
 232                      && !oops_in_progress);
 233
 234         if (cpu == this_cpu) {
 235                 local_irq_save(flags);
 236                 func(info);
 237                 local_irq_restore(flags);
 238         } else {
 239                 if ((unsigned)cpu < nr_cpu_ids && cpu_online(cpu)) {
 240                         struct call_single_data *csd = &d;
 241
 242                         if (!wait)
 243                                 csd = &__get_cpu_var(csd_data);
 244
 245                         csd_lock(csd);
 246
 247                         csd->func = func;
 248                         csd->info = info;
 249                         generic_exec_single(cpu, csd, wait);
 250                 } else {
 251                         err = -ENXIO;   /* CPU not online */
 252                 }
 253         }
 254
 255         put_cpu();
 256
 257         return err;
 258 }
 259 EXPORT_SYMBOL(smp_call_function_single);
 260
 261 /*
 262  * smp_call_function_any - Run a function on any of the given cpus
 263  * @mask: The mask of cpus it can run on.
 264  * @func: The function to run. This must be fast and non-blocking.
 265  * @info: An arbitrary pointer to pass to the function.
 266  * @wait: If true, wait until function has completed.
 267  *
 268  * Returns 0 on success, else a negative status code (if no cpus were online).
 269  *
 270  * Selection preference:
 271  *      1) current cpu if in @mask
 272  *      2) any cpu of current node if in @mask
 273  *      3) any other online cpu in @mask
 274  */
 275 int smp_call_function_any(const struct cpumask *mask,
 276                           smp_call_func_t func, void *info, int wait)
 277 {
 278         unsigned int cpu;
 279         const struct cpumask *nodemask;
 280         int ret;
 281
 282         /* Try for same CPU (cheapest) */
 283         cpu = get_cpu();
 284         if (cpumask_test_cpu(cpu, mask))
 285                 goto call;
 286
 287         /* Try for same node. */
 288         nodemask = cpumask_of_node(cpu_to_node(cpu));
 289         for (cpu = cpumask_first_and(nodemask, mask); cpu < nr_cpu_ids;
 290              cpu = cpumask_next_and(cpu, nodemask, mask)) {
 291                 if (cpu_online(cpu))
 292                         goto call;
 293         }
 294
 295         /* Any online will do: smp_call_function_single handles nr_cpu_ids. */
 296         cpu = cpumask_any_and(mask, cpu_online_mask);
 297 call:
 298         ret = smp_call_function_single(cpu, func, info, wait);
 299         put_cpu();
 300         return ret;
 301 }
 302 EXPORT_SYMBOL_GPL(smp_call_function_any);
 303
 304 /**
 305  * __smp_call_function_single(): Run a function on a specific CPU
 306  * @cpu: The CPU to run on.
 307  * @data: Pre-allocated and setup data structure
 308  * @wait: If true, wait until function has completed on specified CPU.
 309  *
 310  * Like smp_call_function_single(), but allow caller to pass in a
 311  * pre-allocated data structure. Useful for embedding @data inside
 312  * other structures, for instance.
 313  */
 314 void __smp_call_function_single(int cpu, struct call_single_data *csd,
 315                                 int wait)
 316 {
 317         unsigned int this_cpu;
 318         unsigned long flags;
 319
 320         this_cpu = get_cpu();
 321         /*
 322          * Can deadlock when called with interrupts disabled.
 323          * We allow cpu's that are not yet online though, as no one else can
 324          * send smp call function interrupt to this cpu and as such deadlocks
 325          * can't happen.
 326          */
 327         WARN_ON_ONCE(cpu_online(smp_processor_id()) && wait && irqs_disabled()
 328                      && !oops_in_progress);
 329
 330         if (cpu == this_cpu) {
 331                 local_irq_save(flags);
 332                 csd->func(csd->info);
 333                 local_irq_restore(flags);
 334         } else {
 335                 csd_lock(csd);
 336                 generic_exec_single(cpu, csd, wait);
 337         }
 338         put_cpu();
 339 }
 340
 341 /**
 342  * smp_call_function_many(): Run a function on a set of other CPUs.
 343  * @mask: The set of cpus to run on (only runs on online subset).
 344  * @func: The function to run. This must be fast and non-blocking.
 345  * @info: An arbitrary pointer to pass to the function.
 346  * @wait: If true, wait (atomically) until function has completed
 347  *        on other CPUs.
 348  *
 349  * If @wait is true, then returns once @func has returned.
 350  *
 351  * You must not call this function with disabled interrupts or from a
 352  * hardware interrupt handler or from a bottom half handler. Preemption
 353  * must be disabled when calling this function.
 354  */
 355 void smp_call_function_many(const struct cpumask *mask,
 356                             smp_call_func_t func, void *info, bool wait)
 357 {
 358         struct call_function_data *cfd;
 359         int cpu, next_cpu, this_cpu = smp_processor_id();
 360
 361         /*
 362          * Can deadlock when called with interrupts disabled.
 363          * We allow cpu's that are not yet online though, as no one else can
 364          * send smp call function interrupt to this cpu and as such deadlocks
 365          * can't happen.
 366          */
 367         WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
 368                      && !oops_in_progress && !early_boot_irqs_disabled);
 369
 370         /* Try to fastpath.  So, what's a CPU they want? Ignoring this one. */
 371         cpu = cpumask_first_and(mask, cpu_online_mask);
 372         if (cpu == this_cpu)
 373                 cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
 374
 375         /* No online cpus?  We're done. */
 376         if (cpu >= nr_cpu_ids)
 377                 return;
 378
 379         /* Do we have another CPU which isn't us? */
 380         next_cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
 381         if (next_cpu == this_cpu)
 382                 next_cpu = cpumask_next_and(next_cpu, mask, cpu_online_mask);
 383
 384         /* Fastpath: do that cpu by itself. */
 385         if (next_cpu >= nr_cpu_ids) {
 386                 smp_call_function_single(cpu, func, info, wait);
 387                 return;
 388         }
 389
 390         cfd = &__get_cpu_var(cfd_data);
 391
 392         cpumask_and(cfd->cpumask, mask, cpu_online_mask);
 393         cpumask_clear_cpu(this_cpu, cfd->cpumask);
 394
 395         /* Some callers race with other cpus changing the passed mask */
 396         if (unlikely(!cpumask_weight(cfd->cpumask)))
 397                 return;
 398
 399         /*
 400          * After we put an entry into the list, cfd->cpumask may be cleared
 401          * again when another CPU sends another IPI for a SMP function call, so
 402          * cfd->cpumask will be zero.
 403          */
 404         cpumask_copy(cfd->cpumask_ipi, cfd->cpumask);
 405
 406         for_each_cpu(cpu, cfd->cpumask) {
 407                 struct call_single_data *csd = per_cpu_ptr(cfd->csd, cpu);
 408                 struct call_single_queue *dst =
 409                                         &per_cpu(call_single_queue, cpu);
 410                 unsigned long flags;
 411
 412                 csd_lock(csd);
 413                 csd->func = func;
 414                 csd->info = info;
 415
 416                 raw_spin_lock_irqsave(&dst->lock, flags);
 417                 list_add_tail(&csd->list, &dst->list);
 418                 raw_spin_unlock_irqrestore(&dst->lock, flags);
 419         }
 420
 421         /* Send a message to all CPUs in the map */
 422         arch_send_call_function_ipi_mask(cfd->cpumask_ipi);
 423
 424         if (wait) {
 425                 for_each_cpu(cpu, cfd->cpumask) {
 426                         struct call_single_data *csd;
 427
 428                         csd = per_cpu_ptr(cfd->csd, cpu);
 429                         csd_lock_wait(csd);
 430                 }
 431         }
 432 }
 433 EXPORT_SYMBOL(smp_call_function_many);
 434
 435 /**
 436  * smp_call_function(): Run a function on all other CPUs.
 437  * @func: The function to run. This must be fast and non-blocking.
 438  * @info: An arbitrary pointer to pass to the function.
 439  * @wait: If true, wait (atomically) until function has completed
 440  *        on other CPUs.
 441  *
 442  * Returns 0.
 443  *
 444  * If @wait is true, then returns once @func has returned; otherwise
 445  * it returns just before the target cpu calls @func.
 446  *
 447  * You must not call this function with disabled interrupts or from a
 448  * hardware interrupt handler or from a bottom half handler.
 449  */
 450 int smp_call_function(smp_call_func_t func, void *info, int wait)
 451 {
 452         preempt_disable();
 453         smp_call_function_many(cpu_online_mask, func, info, wait);
 454         preempt_enable();
 455
 456         return 0;
 457 }
 458 EXPORT_SYMBOL(smp_call_function);
 459 #endif /* USE_GENERIC_SMP_HELPERS */
 460
 461 /* Setup configured maximum number of CPUs to activate */
 462 unsigned int setup_max_cpus = NR_CPUS;
 463 EXPORT_SYMBOL(setup_max_cpus);
 464
 465
 466 /*
 467  * Setup routine for controlling SMP activation
 468  *
 469  * Command-line option of "nosmp" or "maxcpus=0" will disable SMP
 470  * activation entirely (the MPS table probe still happens, though).
 471  *
 472  * Command-line option of "maxcpus=<NUM>", where <NUM> is an integer
 473  * greater than 0, limits the maximum number of CPUs activated in
 474  * SMP mode to <NUM>.
 475  */
 476
 477 void __weak arch_disable_smp_support(void) { }
 478
 479 static int __init nosmp(char *str)
 480 {
 481         setup_max_cpus = 0;
 482         arch_disable_smp_support();
 483
 484         return 0;
 485 }
 486
 487 early_param("nosmp", nosmp);
 488
 489 /* this is hard limit */
 490 static int __init nrcpus(char *str)
 491 {
 492         int nr_cpus;
 493
 494         get_option(&str, &nr_cpus);
 495         if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
 496                 nr_cpu_ids = nr_cpus;
 497
 498         return 0;
 499 }
 500
 501 early_param("nr_cpus", nrcpus);
 502
 503 static int __init maxcpus(char *str)
 504 {
 505         get_option(&str, &setup_max_cpus);
 506         if (setup_max_cpus == 0)
 507                 arch_disable_smp_support();
 508
 509         return 0;
 510 }
 511
 512 early_param("maxcpus", maxcpus);
 513
 514 /* Setup number of possible processor ids */
 515 int nr_cpu_ids __read_mostly = NR_CPUS;
 516 EXPORT_SYMBOL(nr_cpu_ids);
 517
 518 /* An arch may set nr_cpu_ids earlier if needed, so this would be redundant */
 519 void __init setup_nr_cpu_ids(void)
 520 {
 521         nr_cpu_ids = find_last_bit(cpumask_bits(cpu_possible_mask),NR_CPUS) + 1;
 522 }
 523
 524 /* Called by boot processor to activate the rest. */
 525 void __init smp_init(void)
 526 {
 527         unsigned int cpu;
 528
 529         idle_threads_init();
 530
 531         /* FIXME: This should be done in userspace --RR */
 532         for_each_present_cpu(cpu) {
 533                 if (num_online_cpus() >= setup_max_cpus)
 534                         break;
 535                 if (!cpu_online(cpu))
 536                         cpu_up(cpu);
 537         }
 538
 539         /* Any cleanup work */
 540         printk(KERN_INFO "Brought up %ld CPUs\n", (long)num_online_cpus());
 541         smp_cpus_done(setup_max_cpus);
 542 }
 543
 544 /*
 545  * Call a function on all processors.  May be used during early boot while
 546  * early_boot_irqs_disabled is set.  Use local_irq_save/restore() instead
 547  * of local_irq_disable/enable().
 548  */
 549 int on_each_cpu(void (*func) (void *info), void *info, int wait)
 550 {
 551         unsigned long flags;
 552         int ret = 0;
 553
 554         preempt_disable();
 555         ret = smp_call_function(func, info, wait);
 556         local_irq_save(flags);
 557         func(info);
 558         local_irq_restore(flags);
 559         preempt_enable();
 560         return ret;
 561 }
 562 EXPORT_SYMBOL(on_each_cpu);
 563
 564 /**
 565  * on_each_cpu_mask(): Run a function on processors specified by
 566  * cpumask, which may include the local processor.
 567  * @mask: The set of cpus to run on (only runs on online subset).
 568  * @func: The function to run. This must be fast and non-blocking.
 569  * @info: An arbitrary pointer to pass to the function.
 570  * @wait: If true, wait (atomically) until function has completed
 571  *        on other CPUs.
 572  *
 573  * If @wait is true, then returns once @func has returned.
 574  *
 575  * You must not call this function with disabled interrupts or
 576  * from a hardware interrupt handler or from a bottom half handler.
 577  */
 578 void on_each_cpu_mask(const struct cpumask *mask, smp_call_func_t func,
 579                         void *info, bool wait)
 580 {
 581         int cpu = get_cpu();
 582
 583         smp_call_function_many(mask, func, info, wait);
 584         if (cpumask_test_cpu(cpu, mask)) {
 585                 local_irq_disable();
 586                 func(info);
 587                 local_irq_enable();
 588         }
 589         put_cpu();
 590 }
 591 EXPORT_SYMBOL(on_each_cpu_mask);
 592
 593 /*
 594  * on_each_cpu_cond(): Call a function on each processor for which
 595  * the supplied function cond_func returns true, optionally waiting
 596  * for all the required CPUs to finish. This may include the local
 597  * processor.
 598  * @cond_func:  A callback function that is passed a cpu id and
 599  *              the the info parameter. The function is called
 600  *              with preemption disabled. The function should
 601  *              return a blooean value indicating whether to IPI
 602  *              the specified CPU.
 603  * @func:       The function to run on all applicable CPUs.
 604  *              This must be fast and non-blocking.
 605  * @info:       An arbitrary pointer to pass to both functions.
 606  * @wait:       If true, wait (atomically) until function has
 607  *              completed on other CPUs.
 608  * @gfp_flags:  GFP flags to use when allocating the cpumask
 609  *              used internally by the function.
 610  *
 611  * The function might sleep if the GFP flags indicates a non
 612  * atomic allocation is allowed.
 613  *
 614  * Preemption is disabled to protect against CPUs going offline but not online.
 615  * CPUs going online during the call will not be seen or sent an IPI.
 616  *
 617  * You must not call this function with disabled interrupts or
 618  * from a hardware interrupt handler or from a bottom half handler.
 619  */
 620 void on_each_cpu_cond(bool (*cond_func)(int cpu, void *info),
 621                         smp_call_func_t func, void *info, bool wait,
 622                         gfp_t gfp_flags)
 623 {
 624         cpumask_var_t cpus;
 625         int cpu, ret;
 626
 627         might_sleep_if(gfp_flags & __GFP_WAIT);
 628
 629         if (likely(zalloc_cpumask_var(&cpus, (gfp_flags|__GFP_NOWARN)))) {
 630                 preempt_disable();
 631                 for_each_online_cpu(cpu)
 632                         if (cond_func(cpu, info))
 633                                 cpumask_set_cpu(cpu, cpus);
 634                 on_each_cpu_mask(cpus, func, info, wait);
 635                 preempt_enable();
 636                 free_cpumask_var(cpus);
 637         } else {
 638                 /*
 639                  * No free cpumask, bother. No matter, we'll
 640                  * just have to IPI them one by one.
 641                  */
 642                 preempt_disable();
 643                 for_each_online_cpu(cpu)
 644                         if (cond_func(cpu, info)) {
 645                                 ret = smp_call_function_single(cpu, func,
 646                                                                 info, wait);
 647                                 WARN_ON_ONCE(!ret);
 648                         }
 649                 preempt_enable();
 650         }
 651 }
 652 EXPORT_SYMBOL(on_each_cpu_cond);
 653
 654 static void do_nothing(void *unused)
 655 {
 656 }
 657
 658 /**
 659  * kick_all_cpus_sync - Force all cpus out of idle
 660  *
 661  * Used to synchronize the update of pm_idle function pointer. It's
 662  * called after the pointer is updated and returns after the dummy
 663  * callback function has been executed on all cpus. The execution of
 664  * the function can only happen on the remote cpus after they have
 665  * left the idle function which had been called via pm_idle function
 666  * pointer. So it's guaranteed that nothing uses the previous pointer
 667  * anymore.
 668  */
 669 void kick_all_cpus_sync(void)
 670 {
 671         /* Make sure the change is visible before we kick the cpus */
 672         smp_mb();
 673         smp_call_function(do_nothing, NULL, 1);
 674 }
 675 EXPORT_SYMBOL_GPL(kick_all_cpus_sync);