sys/kern/subr_epoch.c

   1 /*-
   2  * Copyright (c) 2018, Matthew Macy <mmacy@freebsd.org>
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions are met:
   6  *
   7  *  1. Redistributions of source code must retain the above copyright notice,
   8  *     this list of conditions and the following disclaimer.
   9  *
  10  *  2. Neither the name of Matthew Macy nor the names of its
  11  *     contributors may be used to endorse or promote products derived from
  12  *     this software without specific prior written permission.
  13  *
  14  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  15  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  16  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  17  * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
  18  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  19  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  20  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  21  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  22  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  23  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  24  * POSSIBILITY OF SUCH DAMAGE.
  25  */
  26
  27 #include <sys/cdefs.h>
  28 __FBSDID("$FreeBSD$");
  29
  30 #include <sys/param.h>
  31 #include <sys/types.h>
  32 #include <sys/systm.h>
  33 #include <sys/counter.h>
  34 #include <sys/epoch.h>
  35 #include <sys/gtaskqueue.h>
  36 #include <sys/kernel.h>
  37 #include <sys/limits.h>
  38 #include <sys/lock.h>
  39 #include <sys/malloc.h>
  40 #include <sys/mutex.h>
  41 #include <sys/proc.h>
  42 #include <sys/sched.h>
  43 #include <sys/smp.h>
  44 #include <sys/sysctl.h>
  45 #include <sys/turnstile.h>
  46 #include <vm/vm.h>
  47 #include <vm/vm_extern.h>
  48 #include <vm/vm_kern.h>
  49
  50 #include <ck_epoch.h>
  51
  52 MALLOC_DEFINE(M_EPOCH, "epoch", "epoch based reclamation");
  53
  54 /* arbitrary --- needs benchmarking */
  55 #define MAX_ADAPTIVE_SPIN 5000
  56
  57 #define EPOCH_EXITING 0x1
  58 #ifdef __amd64__
  59 #define EPOCH_ALIGN CACHE_LINE_SIZE*2
  60 #else
  61 #define EPOCH_ALIGN CACHE_LINE_SIZE
  62 #endif
  63
  64 SYSCTL_NODE(_kern, OID_AUTO, epoch, CTLFLAG_RW, 0, "epoch information");
  65 SYSCTL_NODE(_kern_epoch, OID_AUTO, stats, CTLFLAG_RW, 0, "epoch stats");
  66
  67 static int poll_intvl;
  68 SYSCTL_INT(_kern_epoch, OID_AUTO, poll_intvl, CTLFLAG_RWTUN,
  69                    &poll_intvl, 0, "# of ticks to wait between garbage collecting deferred frees");
  70 /* Stats. */
  71 static counter_u64_t block_count;
  72 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, nblocked, CTLFLAG_RW,
  73                                    &block_count, "# of times a thread was in an epoch when epoch_wait was called");
  74 static counter_u64_t migrate_count;
  75 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, migrations, CTLFLAG_RW,
  76                                    &migrate_count, "# of times thread was migrated to another CPU in epoch_wait");
  77 static counter_u64_t turnstile_count;
  78 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, ncontended, CTLFLAG_RW,
  79                                    &turnstile_count, "# of times a thread was blocked on a lock in an epoch during an epoch_wait");
  80 static counter_u64_t switch_count;
  81 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, switches, CTLFLAG_RW,
  82                                    &switch_count, "# of times a thread voluntarily context switched in epoch_wait");
  83
  84 typedef struct epoch_cb {
  85         void (*ec_callback)(epoch_context_t);
  86         STAILQ_ENTRY(epoch_cb) ec_link;
  87 } *epoch_cb_t;
  88
  89 TAILQ_HEAD(threadlist, thread);
  90
  91 typedef struct epoch_record {
  92         ck_epoch_record_t er_record;
  93         volatile struct threadlist er_tdlist;
  94         volatile uint32_t er_gen;
  95         uint32_t er_cpuid;
  96 } *epoch_record_t;
  97
  98 struct epoch_pcpu_state {
  99         struct epoch_record eps_record;
 100         STAILQ_HEAD(, epoch_cb) eps_cblist;
 101 } __aligned(EPOCH_ALIGN);
 102
 103 struct epoch {
 104         struct ck_epoch e_epoch __aligned(EPOCH_ALIGN);
 105         struct grouptask e_gtask;
 106         struct callout e_timer;
 107         struct mtx e_lock;
 108         int e_flags;
 109         /* make sure that immutable data doesn't overlap with the gtask, callout, and mutex*/
 110         struct epoch_pcpu_state *e_pcpu_dom[MAXMEMDOM] __aligned(EPOCH_ALIGN);
 111         counter_u64_t e_frees;
 112         uint64_t e_free_last;
 113         struct epoch_pcpu_state *e_pcpu[0];
 114 };
 115
 116 static __read_mostly int domcount[MAXMEMDOM];
 117 static __read_mostly int domoffsets[MAXMEMDOM];
 118 static __read_mostly int inited;
 119
 120 static void epoch_call_task(void *context);
 121
 122 #if defined(__powerpc64__) || defined(__powerpc__)
 123 static bool usedomains = false;
 124 #else
 125 static bool usedomains = true;
 126 #endif
 127 static void
 128 epoch_init(void *arg __unused)
 129 {
 130         int domain, count;
 131
 132         if (poll_intvl == 0)
 133                 poll_intvl = hz;
 134
 135         block_count = counter_u64_alloc(M_WAITOK);
 136         migrate_count = counter_u64_alloc(M_WAITOK);
 137         turnstile_count = counter_u64_alloc(M_WAITOK);
 138         switch_count = counter_u64_alloc(M_WAITOK);
 139         if (usedomains == false) {
 140                 inited = 1;
 141                 return;
 142         }
 143         count = domain = 0;
 144         domoffsets[0] = 0;
 145         for (domain = 0; domain < vm_ndomains; domain++) {
 146                 domcount[domain] = CPU_COUNT(&cpuset_domain[domain]);
 147                 if (bootverbose)
 148                         printf("domcount[%d] %d\n", domain, domcount[domain]);
 149         }
 150         for (domain = 1; domain < vm_ndomains; domain++)
 151                 domoffsets[domain] = domoffsets[domain-1] + domcount[domain-1];
 152
 153         for (domain = 0; domain < vm_ndomains; domain++) {
 154                 if (domcount[domain] == 0) {
 155                         usedomains = false;
 156                         break;
 157                 }
 158         }
 159         inited = 1;
 160 }
 161 SYSINIT(epoch, SI_SUB_CPU + 1, SI_ORDER_FIRST, epoch_init, NULL);
 162
 163 static void
 164 epoch_init_numa(epoch_t epoch)
 165 {
 166         int domain, cpu_offset;
 167         struct epoch_pcpu_state *eps;
 168         epoch_record_t er;
 169
 170         for (domain = 0; domain < vm_ndomains; domain++) {
 171                 eps = malloc_domain(sizeof(*eps)*domcount[domain], M_EPOCH,
 172                                                         domain, M_ZERO|M_WAITOK);
 173                 epoch->e_pcpu_dom[domain] = eps;
 174                 cpu_offset = domoffsets[domain];
 175                 for (int i = 0; i < domcount[domain]; i++, eps++) {
 176                         epoch->e_pcpu[cpu_offset + i] = eps;
 177                         er = &eps->eps_record;
 178                         STAILQ_INIT(&eps->eps_cblist);
 179                         ck_epoch_register(&epoch->e_epoch, &er->er_record, NULL);
 180                         TAILQ_INIT((struct threadlist *)(uintptr_t)&er->er_tdlist);
 181                         er->er_cpuid = cpu_offset + i;
 182                 }
 183         }
 184 }
 185
 186 static void
 187 epoch_init_legacy(epoch_t epoch)
 188 {
 189         struct epoch_pcpu_state *eps;
 190         epoch_record_t er;
 191
 192         eps = malloc(sizeof(*eps)*mp_ncpus, M_EPOCH, M_ZERO|M_WAITOK);
 193         epoch->e_pcpu_dom[0] = eps;
 194         for (int i = 0; i < mp_ncpus; i++, eps++) {
 195                 epoch->e_pcpu[i] = eps;
 196                 er = &eps->eps_record;
 197                 ck_epoch_register(&epoch->e_epoch, &er->er_record, NULL);
 198                 TAILQ_INIT((struct threadlist *)(uintptr_t)&er->er_tdlist);
 199                 STAILQ_INIT(&eps->eps_cblist);
 200                 er->er_cpuid = i;
 201         }
 202 }
 203
 204 static void
 205 epoch_callout(void *arg)
 206 {
 207         epoch_t epoch;
 208         uint64_t frees;
 209
 210         epoch = arg;
 211         frees = counter_u64_fetch(epoch->e_frees);
 212         /* pick some better value */
 213         if (frees - epoch->e_free_last > 10) {
 214                 GROUPTASK_ENQUEUE(&epoch->e_gtask);
 215                 epoch->e_free_last = frees;
 216         }
 217         if ((epoch->e_flags & EPOCH_EXITING) == 0)
 218                 callout_reset(&epoch->e_timer, poll_intvl, epoch_callout, epoch);
 219 }
 220
 221 epoch_t
 222 epoch_alloc(void)
 223 {
 224         epoch_t epoch;
 225
 226         if (__predict_false(!inited))
 227                 panic("%s called too early in boot", __func__);
 228         epoch = malloc(sizeof(struct epoch) + mp_ncpus*sizeof(void*),
 229                                    M_EPOCH, M_ZERO|M_WAITOK);
 230         ck_epoch_init(&epoch->e_epoch);
 231         epoch->e_frees = counter_u64_alloc(M_WAITOK);
 232         mtx_init(&epoch->e_lock, "epoch callout", NULL, MTX_DEF);
 233         callout_init_mtx(&epoch->e_timer, &epoch->e_lock, 0);
 234         taskqgroup_config_gtask_init(epoch, &epoch->e_gtask, epoch_call_task, "epoch call task");
 235         if (usedomains)
 236                 epoch_init_numa(epoch);
 237         else
 238                 epoch_init_legacy(epoch);
 239         callout_reset(&epoch->e_timer, poll_intvl, epoch_callout, epoch);
 240         return (epoch);
 241 }
 242
 243 void
 244 epoch_free(epoch_t epoch)
 245 {
 246         int domain;
 247 #ifdef INVARIANTS
 248         struct epoch_pcpu_state *eps;
 249         int cpu;
 250
 251         CPU_FOREACH(cpu) {
 252                 eps = epoch->e_pcpu[cpu];
 253                 MPASS(TAILQ_EMPTY(&eps->eps_record.er_tdlist));
 254         }
 255 #endif
 256         mtx_lock(&epoch->e_lock);
 257         epoch->e_flags |= EPOCH_EXITING;
 258         mtx_unlock(&epoch->e_lock);
 259         /*
 260          * Execute any lingering callbacks
 261          */
 262         GROUPTASK_ENQUEUE(&epoch->e_gtask);
 263         gtaskqueue_drain(epoch->e_gtask.gt_taskqueue, &epoch->e_gtask.gt_task);
 264         callout_drain(&epoch->e_timer);
 265         mtx_destroy(&epoch->e_lock);
 266         counter_u64_free(epoch->e_frees);
 267         taskqgroup_config_gtask_deinit(&epoch->e_gtask);
 268         if (usedomains)
 269                 for (domain = 0; domain < vm_ndomains; domain++)
 270                         free_domain(epoch->e_pcpu_dom[domain], M_EPOCH);
 271         else
 272                 free(epoch->e_pcpu_dom[0], M_EPOCH);
 273         free(epoch, M_EPOCH);
 274 }
 275
 276 #define INIT_CHECK(epoch)                                                               \
 277         do {                                                                                    \
 278                 if (__predict_false((epoch) == NULL))           \
 279                         return;                                                                 \
 280         } while (0)
 281
 282 void
 283 epoch_enter(epoch_t epoch)
 284 {
 285         struct epoch_pcpu_state *eps;
 286         struct thread *td;
 287
 288         INIT_CHECK(epoch);
 289
 290         td = curthread;
 291         critical_enter();
 292         eps = epoch->e_pcpu[curcpu];
 293         td->td_epochnest++;
 294         MPASS(td->td_epochnest < UCHAR_MAX - 2);
 295         if (td->td_epochnest == 1)
 296                 TAILQ_INSERT_TAIL(&eps->eps_record.er_tdlist, td, td_epochq);
 297 #ifdef INVARIANTS
 298         if (td->td_epochnest > 1) {
 299                 struct thread *curtd;
 300                 int found = 0;
 301
 302                 TAILQ_FOREACH(curtd, &eps->eps_record.er_tdlist, td_epochq)
 303                         if (curtd == td)
 304                                 found = 1;
 305                 KASSERT(found, ("recursing on a second epoch"));
 306         }
 307 #endif
 308         sched_pin();
 309         ck_epoch_begin(&eps->eps_record.er_record, NULL);
 310         critical_exit();
 311 }
 312
 313 void
 314 epoch_enter_nopreempt(epoch_t epoch)
 315 {
 316         struct epoch_pcpu_state *eps;
 317
 318         INIT_CHECK(epoch);
 319         critical_enter();
 320         eps = epoch->e_pcpu[curcpu];
 321         curthread->td_epochnest++;
 322         MPASS(curthread->td_epochnest < UCHAR_MAX - 2);
 323         ck_epoch_begin(&eps->eps_record.er_record, NULL);
 324 }
 325
 326 void
 327 epoch_exit(epoch_t epoch)
 328 {
 329         struct epoch_pcpu_state *eps;
 330         struct thread *td;
 331
 332         td = curthread;
 333         INIT_CHECK(epoch);
 334         critical_enter();
 335         eps = epoch->e_pcpu[curcpu];
 336         sched_unpin();
 337         ck_epoch_end(&eps->eps_record.er_record, NULL);
 338         td->td_epochnest--;
 339         if (td->td_epochnest == 0)
 340                 TAILQ_REMOVE(&eps->eps_record.er_tdlist, td, td_epochq);
 341         eps->eps_record.er_gen++;
 342         critical_exit();
 343 }
 344
 345 void
 346 epoch_exit_nopreempt(epoch_t epoch)
 347 {
 348         struct epoch_pcpu_state *eps;
 349
 350         INIT_CHECK(epoch);
 351         MPASS(curthread->td_critnest);
 352         eps = epoch->e_pcpu[curcpu];
 353         ck_epoch_end(&eps->eps_record.er_record, NULL);
 354         curthread->td_epochnest--;
 355         critical_exit();
 356 }
 357
 358 /*
 359  * epoch_block_handler is a callback from the ck code when another thread is
 360  * currently in an epoch section.
 361  */
 362 static void
 363 epoch_block_handler(struct ck_epoch *global __unused, ck_epoch_record_t *cr,
 364                                         void *arg __unused)
 365 {
 366         epoch_record_t record;
 367         struct epoch_pcpu_state *eps;
 368         struct thread *td, *tdwait, *owner;
 369         struct turnstile *ts;
 370         struct lock_object *lock;
 371         int spincount, gen;
 372
 373         eps = arg;
 374         record = __containerof(cr, struct epoch_record, er_record);
 375         td = curthread;
 376         spincount = 0;
 377         counter_u64_add(block_count, 1);
 378         if (record->er_cpuid != curcpu) {
 379                 /*
 380                  * If the head of the list is running, we can wait for it
 381                  * to remove itself from the list and thus save us the
 382                  * overhead of a migration
 383                  */
 384                 if ((tdwait = TAILQ_FIRST(&record->er_tdlist)) != NULL &&
 385                         TD_IS_RUNNING(tdwait)) {
 386                         gen = record->er_gen;
 387                         thread_unlock(td);
 388                         do {
 389                                 cpu_spinwait();
 390                         } while (tdwait == TAILQ_FIRST(&record->er_tdlist) &&
 391                                          gen == record->er_gen && TD_IS_RUNNING(tdwait) &&
 392                                          spincount++ < MAX_ADAPTIVE_SPIN);
 393                         thread_lock(td);
 394                         return;
 395                 }
 396
 397                 /*
 398                  * Being on the same CPU as that of the record on which
 399                  * we need to wait allows us access to the thread
 400                  * list associated with that CPU. We can then examine the
 401                  * oldest thread in the queue and wait on its turnstile
 402                  * until it resumes and so on until a grace period
 403                  * elapses.
 404                  *
 405                  */
 406                 counter_u64_add(migrate_count, 1);
 407                 sched_bind(td, record->er_cpuid);
 408                 /*
 409                  * At this point we need to return to the ck code
 410                  * to scan to see if a grace period has elapsed.
 411                  * We can't move on to check the thread list, because
 412                  * in the meantime new threads may have arrived that
 413                  * in fact belong to a different epoch.
 414                  */
 415                 return;
 416         }
 417         /*
 418          * Try to find a thread in an epoch section on this CPU
 419          * waiting on a turnstile. Otherwise find the lowest
 420          * priority thread (highest prio value) and drop our priority
 421          * to match to allow it to run.
 422          */
 423         TAILQ_FOREACH(tdwait, &record->er_tdlist, td_epochq) {
 424                 /*
 425                  * Propagate our priority to any other waiters to prevent us
 426                  * from starving them. They will have their original priority
 427                  * restore on exit from epoch_wait().
 428                  */
 429                 if (!TD_IS_INHIBITED(tdwait) && tdwait->td_priority > td->td_priority) {
 430                         thread_lock(tdwait);
 431                         sched_prio(tdwait, td->td_priority);
 432                         thread_unlock(tdwait);
 433                 }
 434                 if (TD_IS_INHIBITED(tdwait) && TD_ON_LOCK(tdwait) &&
 435                         ((ts = tdwait->td_blocked) != NULL)) {
 436                         /*
 437                          * We unlock td to allow turnstile_wait to reacquire the
 438                          * the thread lock. Before unlocking it we enter a critical
 439                          * section to prevent preemption after we reenable interrupts
 440                          * by dropping the thread lock in order to prevent tdwait
 441                          * from getting to run.
 442                          */
 443                         critical_enter();
 444                         thread_unlock(td);
 445                         owner = turnstile_lock(ts, &lock);
 446                         /*
 447                          * The owner pointer indicates that the lock succeeded. Only
 448                          * in case we hold the lock and the turnstile we locked is still
 449                          * the one that tdwait is blocked on can we continue. Otherwise
 450                          * The turnstile pointer has been changed out from underneath
 451                          * us, as in the case where the lock holder has signalled tdwait,
 452                          * and we need to continue.
 453                          */
 454                         if (owner != NULL && ts == tdwait->td_blocked) {
 455                                 MPASS(TD_IS_INHIBITED(tdwait) && TD_ON_LOCK(tdwait));
 456                                 critical_exit();
 457                                 turnstile_wait(ts, owner, tdwait->td_tsqueue);
 458                                 counter_u64_add(turnstile_count, 1);
 459                                 thread_lock(td);
 460                                 return;
 461                         } else if (owner != NULL)
 462                                 turnstile_unlock(ts, lock);
 463                         thread_lock(td);
 464                         critical_exit();
 465                         KASSERT(td->td_locks == 0,
 466                                         ("%d locks held", td->td_locks));
 467                 }
 468         }
 469         /*
 470          * We didn't find any threads actually blocked on a lock
 471          * so we have nothing to do except context switch away.
 472          */
 473         counter_u64_add(switch_count, 1);
 474         mi_switch(SW_VOL | SWT_RELINQUISH, NULL);
 475
 476         /*
 477          * Release the thread lock while yielding to
 478          * allow other threads to acquire the lock
 479          * pointed to by TDQ_LOCKPTR(td). Else a
 480          * deadlock like situation might happen. (HPS)
 481          */
 482         thread_unlock(td);
 483         thread_lock(td);
 484 }
 485
 486 void
 487 epoch_wait(epoch_t epoch)
 488 {
 489         struct thread *td;
 490         int was_bound;
 491         int old_cpu;
 492         int old_pinned;
 493         u_char old_prio;
 494
 495         INIT_CHECK(epoch);
 496
 497         WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL,
 498             "epoch_wait() can sleep");
 499
 500         td = curthread;
 501         KASSERT(td->td_epochnest == 0, ("epoch_wait() in the middle of an epoch section"));
 502         thread_lock(td);
 503
 504         DROP_GIANT();
 505
 506         old_cpu = PCPU_GET(cpuid);
 507         old_pinned = td->td_pinned;
 508         old_prio = td->td_priority;
 509         was_bound = sched_is_bound(td);
 510         sched_unbind(td);
 511         td->td_pinned = 0;
 512         sched_bind(td, old_cpu);
 513
 514         ck_epoch_synchronize_wait(&epoch->e_epoch, epoch_block_handler, NULL);
 515
 516         /* restore CPU binding, if any */
 517         if (was_bound != 0) {
 518                 sched_bind(td, old_cpu);
 519         } else {
 520                 /* get thread back to initial CPU, if any */
 521                 if (old_pinned != 0)
 522                         sched_bind(td, old_cpu);
 523                 sched_unbind(td);
 524         }
 525         /* restore pinned after bind */
 526         td->td_pinned = old_pinned;
 527
 528         /* restore thread priority */
 529         sched_prio(td, old_prio);
 530         thread_unlock(td);
 531         KASSERT(td->td_locks == 0,
 532                         ("%d locks held", td->td_locks));
 533         PICKUP_GIANT();
 534 }
 535
 536 void
 537 epoch_call(epoch_t epoch, epoch_context_t ctx, void (*callback) (epoch_context_t))
 538 {
 539         struct epoch_pcpu_state *eps;
 540         epoch_cb_t cb;
 541
 542         cb = (void *)ctx;
 543
 544         MPASS(cb->ec_callback == NULL);
 545         MPASS(cb->ec_link.stqe_next == NULL);
 546         MPASS(epoch);
 547         MPASS(callback);
 548         cb->ec_callback = callback;
 549         counter_u64_add(epoch->e_frees, 1);
 550         critical_enter();
 551         eps = epoch->e_pcpu[curcpu];
 552         STAILQ_INSERT_HEAD(&eps->eps_cblist, cb, ec_link);
 553         critical_exit();
 554 }
 555
 556 static void
 557 epoch_call_task(void *context)
 558 {
 559         struct epoch_pcpu_state *eps;
 560         epoch_t epoch;
 561         epoch_cb_t cb;
 562         struct thread *td;
 563         int cpu;
 564         STAILQ_HEAD(, epoch_cb) tmp_head;
 565
 566         epoch = context;
 567         STAILQ_INIT(&tmp_head);
 568         td = curthread;
 569         thread_lock(td);
 570         CPU_FOREACH(cpu) {
 571                 sched_bind(td, cpu);
 572                 eps = epoch->e_pcpu[cpu];
 573                 if (!STAILQ_EMPTY(&eps->eps_cblist))
 574                         STAILQ_CONCAT(&tmp_head, &eps->eps_cblist);
 575         }
 576         sched_unbind(td);
 577         thread_unlock(td);
 578         epoch_wait(epoch);
 579
 580         while ((cb = STAILQ_FIRST(&tmp_head)) != NULL) {
 581                 STAILQ_REMOVE_HEAD(&tmp_head, ec_link);
 582                 cb->ec_callback((void*)cb);
 583         }
 584 }
 585
 586 int
 587 in_epoch(void)
 588 {
 589         return (curthread->td_epochnest != 0);
 590 }