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                 return;
 141         count = domain = 0;
 142         domoffsets[0] = 0;
 143         for (domain = 0; domain < vm_ndomains; domain++) {
 144                 domcount[domain] = CPU_COUNT(&cpuset_domain[domain]);
 145                 if (bootverbose)
 146                         printf("domcount[%d] %d\n", domain, domcount[domain]);
 147         }
 148         for (domain = 1; domain < vm_ndomains; domain++)
 149                 domoffsets[domain] = domoffsets[domain-1] + domcount[domain-1];
 150
 151         for (domain = 0; domain < vm_ndomains; domain++) {
 152                 if (domcount[domain] == 0) {
 153                         usedomains = false;
 154                         break;
 155                 }
 156         }
 157
 158         inited = 1;
 159 }
 160 SYSINIT(epoch, SI_SUB_CPU + 1, SI_ORDER_FIRST, epoch_init, NULL);
 161
 162 static void
 163 epoch_init_numa(epoch_t epoch)
 164 {
 165         int domain, cpu_offset;
 166         struct epoch_pcpu_state *eps;
 167         epoch_record_t er;
 168
 169         for (domain = 0; domain < vm_ndomains; domain++) {
 170                 eps = malloc_domain(sizeof(*eps)*domcount[domain], M_EPOCH,
 171                                                         domain, M_ZERO|M_WAITOK);
 172                 epoch->e_pcpu_dom[domain] = eps;
 173                 cpu_offset = domoffsets[domain];
 174                 for (int i = 0; i < domcount[domain]; i++, eps++) {
 175                         epoch->e_pcpu[cpu_offset + i] = eps;
 176                         er = &eps->eps_record;
 177                         STAILQ_INIT(&eps->eps_cblist);
 178                         ck_epoch_register(&epoch->e_epoch, &er->er_record, NULL);
 179                         TAILQ_INIT((struct threadlist *)(uintptr_t)&er->er_tdlist);
 180                         er->er_cpuid = cpu_offset + i;
 181                 }
 182         }
 183 }
 184
 185 static void
 186 epoch_init_legacy(epoch_t epoch)
 187 {
 188         struct epoch_pcpu_state *eps;
 189         epoch_record_t er;
 190
 191         eps = malloc(sizeof(*eps)*mp_ncpus, M_EPOCH, M_ZERO|M_WAITOK);
 192         epoch->e_pcpu_dom[0] = eps;
 193         for (int i = 0; i < mp_ncpus; i++, eps++) {
 194                 epoch->e_pcpu[i] = eps;
 195                 er = &eps->eps_record;
 196                 ck_epoch_register(&epoch->e_epoch, &er->er_record, NULL);
 197                 TAILQ_INIT((struct threadlist *)(uintptr_t)&er->er_tdlist);
 198                 STAILQ_INIT(&eps->eps_cblist);
 199                 er->er_cpuid = i;
 200         }
 201 }
 202
 203 static void
 204 epoch_callout(void *arg)
 205 {
 206         epoch_t epoch;
 207         uint64_t frees;
 208
 209         epoch = arg;
 210         frees = counter_u64_fetch(epoch->e_frees);
 211         /* pick some better value */
 212         if (frees - epoch->e_free_last > 10) {
 213                 GROUPTASK_ENQUEUE(&epoch->e_gtask);
 214                 epoch->e_free_last = frees;
 215         }
 216         if ((epoch->e_flags & EPOCH_EXITING) == 0)
 217                 callout_reset(&epoch->e_timer, poll_intvl, epoch_callout, epoch);
 218 }
 219
 220 epoch_t
 221 epoch_alloc(void)
 222 {
 223         epoch_t epoch;
 224
 225         if (__predict_false(!inited))
 226                 panic("%s called too early in boot", __func__);
 227         epoch = malloc(sizeof(struct epoch) + mp_ncpus*sizeof(void*),
 228                                    M_EPOCH, M_ZERO|M_WAITOK);
 229         ck_epoch_init(&epoch->e_epoch);
 230         epoch->e_frees = counter_u64_alloc(M_WAITOK);
 231         mtx_init(&epoch->e_lock, "epoch callout", NULL, MTX_DEF);
 232         callout_init_mtx(&epoch->e_timer, &epoch->e_lock, 0);
 233         taskqgroup_config_gtask_init(epoch, &epoch->e_gtask, epoch_call_task, "epoch call task");
 234         if (usedomains)
 235                 epoch_init_numa(epoch);
 236         else
 237                 epoch_init_legacy(epoch);
 238         callout_reset(&epoch->e_timer, poll_intvl, epoch_callout, epoch);
 239         return (epoch);
 240 }
 241
 242 void
 243 epoch_free(epoch_t epoch)
 244 {
 245         int domain;
 246 #ifdef INVARIANTS
 247         struct epoch_pcpu_state *eps;
 248         int cpu;
 249
 250         CPU_FOREACH(cpu) {
 251                 eps = epoch->e_pcpu[cpu];
 252                 MPASS(TAILQ_EMPTY(&eps->eps_record.er_tdlist));
 253         }
 254 #endif
 255         mtx_lock(&epoch->e_lock);
 256         epoch->e_flags |= EPOCH_EXITING;
 257         mtx_unlock(&epoch->e_lock);
 258         /*
 259          * Execute any lingering callbacks
 260          */
 261         GROUPTASK_ENQUEUE(&epoch->e_gtask);
 262         gtaskqueue_drain(epoch->e_gtask.gt_taskqueue, &epoch->e_gtask.gt_task);
 263         callout_drain(&epoch->e_timer);
 264         mtx_destroy(&epoch->e_lock);
 265         counter_u64_free(epoch->e_frees);
 266         taskqgroup_config_gtask_deinit(&epoch->e_gtask);
 267         if (usedomains)
 268                 for (domain = 0; domain < vm_ndomains; domain++)
 269                         free_domain(epoch->e_pcpu_dom[domain], M_EPOCH);
 270         else
 271                 free(epoch->e_pcpu_dom[0], M_EPOCH);
 272         free(epoch, M_EPOCH);
 273 }
 274
 275 #define INIT_CHECK(epoch)                                                               \
 276         do {                                                                                    \
 277                 if (__predict_false((epoch) == NULL))           \
 278                         return;                                                                 \
 279         } while (0)
 280
 281 void
 282 epoch_enter(epoch_t epoch)
 283 {
 284         struct epoch_pcpu_state *eps;
 285         struct thread *td;
 286
 287         INIT_CHECK(epoch);
 288
 289         td = curthread;
 290         critical_enter();
 291         eps = epoch->e_pcpu[curcpu];
 292         td->td_epochnest++;
 293         MPASS(td->td_epochnest < UCHAR_MAX - 2);
 294         if (td->td_epochnest == 1)
 295                 TAILQ_INSERT_TAIL(&eps->eps_record.er_tdlist, td, td_epochq);
 296 #ifdef INVARIANTS
 297         if (td->td_epochnest > 1) {
 298                 struct thread *curtd;
 299                 int found = 0;
 300
 301                 TAILQ_FOREACH(curtd, &eps->eps_record.er_tdlist, td_epochq)
 302                         if (curtd == td)
 303                                 found = 1;
 304                 KASSERT(found, ("recursing on a second epoch"));
 305         }
 306 #endif
 307         sched_pin();
 308         ck_epoch_begin(&eps->eps_record.er_record, NULL);
 309         critical_exit();
 310 }
 311
 312 void
 313 epoch_enter_nopreempt(epoch_t epoch)
 314 {
 315         struct epoch_pcpu_state *eps;
 316
 317         INIT_CHECK(epoch);
 318         critical_enter();
 319         eps = epoch->e_pcpu[curcpu];
 320         curthread->td_epochnest++;
 321         MPASS(curthread->td_epochnest < UCHAR_MAX - 2);
 322         ck_epoch_begin(&eps->eps_record.er_record, NULL);
 323 }
 324
 325 void
 326 epoch_exit(epoch_t epoch)
 327 {
 328         struct epoch_pcpu_state *eps;
 329         struct thread *td;
 330
 331         td = curthread;
 332         INIT_CHECK(epoch);
 333         critical_enter();
 334         eps = epoch->e_pcpu[curcpu];
 335         sched_unpin();
 336         ck_epoch_end(&eps->eps_record.er_record, NULL);
 337         td->td_epochnest--;
 338         if (td->td_epochnest == 0)
 339                 TAILQ_REMOVE(&eps->eps_record.er_tdlist, td, td_epochq);
 340         eps->eps_record.er_gen++;
 341         critical_exit();
 342 }
 343
 344 void
 345 epoch_exit_nopreempt(epoch_t epoch)
 346 {
 347         struct epoch_pcpu_state *eps;
 348
 349         INIT_CHECK(epoch);
 350         MPASS(curthread->td_critnest);
 351         eps = epoch->e_pcpu[curcpu];
 352         ck_epoch_end(&eps->eps_record.er_record, NULL);
 353         curthread->td_epochnest--;
 354         critical_exit();
 355 }
 356
 357 /*
 358  * epoch_block_handler is a callback from the ck code when another thread is
 359  * currently in an epoch section.
 360  */
 361 static void
 362 epoch_block_handler(struct ck_epoch *global __unused, ck_epoch_record_t *cr,
 363                                         void *arg __unused)
 364 {
 365         epoch_record_t record;
 366         struct epoch_pcpu_state *eps;
 367         struct thread *td, *tdwait, *owner;
 368         struct turnstile *ts;
 369         struct lock_object *lock;
 370         int spincount, gen;
 371
 372         eps = arg;
 373         record = __containerof(cr, struct epoch_record, er_record);
 374         td = curthread;
 375         spincount = 0;
 376         counter_u64_add(block_count, 1);
 377         if (record->er_cpuid != curcpu) {
 378                 /*
 379                  * If the head of the list is running, we can wait for it
 380                  * to remove itself from the list and thus save us the
 381                  * overhead of a migration
 382                  */
 383                 if ((tdwait = TAILQ_FIRST(&record->er_tdlist)) != NULL &&
 384                         TD_IS_RUNNING(tdwait)) {
 385                         gen = record->er_gen;
 386                         thread_unlock(td);
 387                         do {
 388                                 cpu_spinwait();
 389                         } while (tdwait == TAILQ_FIRST(&record->er_tdlist) &&
 390                                          gen == record->er_gen && TD_IS_RUNNING(tdwait) &&
 391                                          spincount++ < MAX_ADAPTIVE_SPIN);
 392                         thread_lock(td);
 393                         return;
 394                 }
 395
 396                 /*
 397                  * Being on the same CPU as that of the record on which
 398                  * we need to wait allows us access to the thread
 399                  * list associated with that CPU. We can then examine the
 400                  * oldest thread in the queue and wait on its turnstile
 401                  * until it resumes and so on until a grace period
 402                  * elapses.
 403                  *
 404                  */
 405                 counter_u64_add(migrate_count, 1);
 406                 sched_bind(td, record->er_cpuid);
 407                 /*
 408                  * At this point we need to return to the ck code
 409                  * to scan to see if a grace period has elapsed.
 410                  * We can't move on to check the thread list, because
 411                  * in the meantime new threads may have arrived that
 412                  * in fact belong to a different epoch.
 413                  */
 414                 return;
 415         }
 416         /*
 417          * Try to find a thread in an epoch section on this CPU
 418          * waiting on a turnstile. Otherwise find the lowest
 419          * priority thread (highest prio value) and drop our priority
 420          * to match to allow it to run.
 421          */
 422         TAILQ_FOREACH(tdwait, &record->er_tdlist, td_epochq) {
 423                 /*
 424                  * Propagate our priority to any other waiters to prevent us
 425                  * from starving them. They will have their original priority
 426                  * restore on exit from epoch_wait().
 427                  */
 428                 if (!TD_IS_INHIBITED(tdwait) && tdwait->td_priority > td->td_priority) {
 429                         thread_lock(tdwait);
 430                         sched_prio(tdwait, td->td_priority);
 431                         thread_unlock(tdwait);
 432                 }
 433                 if (TD_IS_INHIBITED(tdwait) && TD_ON_LOCK(tdwait) &&
 434                         ((ts = tdwait->td_blocked) != NULL)) {
 435                         /*
 436                          * We unlock td to allow turnstile_wait to reacquire the
 437                          * the thread lock. Before unlocking it we enter a critical
 438                          * section to prevent preemption after we reenable interrupts
 439                          * by dropping the thread lock in order to prevent tdwait
 440                          * from getting to run.
 441                          */
 442                         critical_enter();
 443                         thread_unlock(td);
 444                         owner = turnstile_lock(ts, &lock);
 445                         /*
 446                          * The owner pointer indicates that the lock succeeded. Only
 447                          * in case we hold the lock and the turnstile we locked is still
 448                          * the one that tdwait is blocked on can we continue. Otherwise
 449                          * The turnstile pointer has been changed out from underneath
 450                          * us, as in the case where the lock holder has signalled tdwait,
 451                          * and we need to continue.
 452                          */
 453                         if (owner != NULL && ts == tdwait->td_blocked) {
 454                                 MPASS(TD_IS_INHIBITED(tdwait) && TD_ON_LOCK(tdwait));
 455                                 critical_exit();
 456                                 turnstile_wait(ts, owner, tdwait->td_tsqueue);
 457                                 counter_u64_add(turnstile_count, 1);
 458                                 thread_lock(td);
 459                                 return;
 460                         } else if (owner != NULL)
 461                                 turnstile_unlock(ts, lock);
 462                         thread_lock(td);
 463                         critical_exit();
 464                         KASSERT(td->td_locks == 0,
 465                                         ("%d locks held", td->td_locks));
 466                 }
 467         }
 468         /*
 469          * We didn't find any threads actually blocked on a lock
 470          * so we have nothing to do except context switch away.
 471          */
 472         counter_u64_add(switch_count, 1);
 473         mi_switch(SW_VOL | SWT_RELINQUISH, NULL);
 474
 475         /*
 476          * Release the thread lock while yielding to
 477          * allow other threads to acquire the lock
 478          * pointed to by TDQ_LOCKPTR(td). Else a
 479          * deadlock like situation might happen. (HPS)
 480          */
 481         thread_unlock(td);
 482         thread_lock(td);
 483 }
 484
 485 void
 486 epoch_wait(epoch_t epoch)
 487 {
 488         struct thread *td;
 489         int was_bound;
 490         int old_cpu;
 491         int old_pinned;
 492         u_char old_prio;
 493
 494         INIT_CHECK(epoch);
 495
 496         WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL,
 497             "epoch_wait() can sleep");
 498
 499         td = curthread;
 500         KASSERT(td->td_epochnest == 0, ("epoch_wait() in the middle of an epoch section"));
 501         thread_lock(td);
 502
 503         DROP_GIANT();
 504
 505         old_cpu = PCPU_GET(cpuid);
 506         old_pinned = td->td_pinned;
 507         old_prio = td->td_priority;
 508         was_bound = sched_is_bound(td);
 509         sched_unbind(td);
 510         td->td_pinned = 0;
 511         sched_bind(td, old_cpu);
 512
 513         ck_epoch_synchronize_wait(&epoch->e_epoch, epoch_block_handler, NULL);
 514
 515         /* restore CPU binding, if any */
 516         if (was_bound != 0) {
 517                 sched_bind(td, old_cpu);
 518         } else {
 519                 /* get thread back to initial CPU, if any */
 520                 if (old_pinned != 0)
 521                         sched_bind(td, old_cpu);
 522                 sched_unbind(td);
 523         }
 524         /* restore pinned after bind */
 525         td->td_pinned = old_pinned;
 526
 527         /* restore thread priority */
 528         sched_prio(td, old_prio);
 529         thread_unlock(td);
 530         KASSERT(td->td_locks == 0,
 531                         ("%d locks held", td->td_locks));
 532         PICKUP_GIANT();
 533 }
 534
 535 void
 536 epoch_call(epoch_t epoch, epoch_context_t ctx, void (*callback) (epoch_context_t))
 537 {
 538         struct epoch_pcpu_state *eps;
 539         epoch_cb_t cb;
 540
 541         cb = (void *)ctx;
 542
 543         MPASS(cb->ec_callback == NULL);
 544         MPASS(cb->ec_link.stqe_next == NULL);
 545         MPASS(epoch);
 546         MPASS(callback);
 547         cb->ec_callback = callback;
 548         counter_u64_add(epoch->e_frees, 1);
 549         critical_enter();
 550         eps = epoch->e_pcpu[curcpu];
 551         STAILQ_INSERT_HEAD(&eps->eps_cblist, cb, ec_link);
 552         critical_exit();
 553 }
 554
 555 static void
 556 epoch_call_task(void *context)
 557 {
 558         struct epoch_pcpu_state *eps;
 559         epoch_t epoch;
 560         epoch_cb_t cb;
 561         struct thread *td;
 562         int cpu;
 563         STAILQ_HEAD(, epoch_cb) tmp_head;
 564
 565         epoch = context;
 566         STAILQ_INIT(&tmp_head);
 567         td = curthread;
 568         thread_lock(td);
 569         CPU_FOREACH(cpu) {
 570                 sched_bind(td, cpu);
 571                 eps = epoch->e_pcpu[cpu];
 572                 if (!STAILQ_EMPTY(&eps->eps_cblist))
 573                         STAILQ_CONCAT(&tmp_head, &eps->eps_cblist);
 574         }
 575         sched_unbind(td);
 576         thread_unlock(td);
 577         epoch_wait(epoch);
 578
 579         while ((cb = STAILQ_FIRST(&tmp_head)) != NULL) {
 580                 STAILQ_REMOVE_HEAD(&tmp_head, ec_link);
 581                 cb->ec_callback((void*)cb);
 582         }
 583 }
 584
 585 int
 586 in_epoch(void)
 587 {
 588         return (curthread->td_epochnest != 0);
 589 }