sys/kern/subr_epoch.c

   1 /*-
   2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
   3  *
   4  * Copyright (c) 2018, Matthew Macy <mmacy@freebsd.org>
   5  *
   6  * Redistribution and use in source and binary forms, with or without
   7  * modification, are permitted provided that the following conditions
   8  * are met:
   9  * 1. Redistributions of source code must retain the above copyright
  10  *    notice, this list of conditions and the following disclaimer.
  11  * 2. Redistributions in binary form must reproduce the above copyright
  12  *    notice, this list of conditions and the following disclaimer in the
  13  *    documentation and/or other materials provided with the distribution.
  14  *
  15  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  16  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  17  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  18  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  19  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  20  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  21  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  22  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  23  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  24  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  25  * SUCH DAMAGE.
  26  *
  27  */
  28
  29 #include <sys/cdefs.h>
  30 __FBSDID("$FreeBSD$");
  31
  32 #include <sys/param.h>
  33 #include <sys/types.h>
  34 #include <sys/systm.h>
  35 #include <sys/counter.h>
  36 #include <sys/epoch.h>
  37 #include <sys/gtaskqueue.h>
  38 #include <sys/kernel.h>
  39 #include <sys/limits.h>
  40 #include <sys/lock.h>
  41 #include <sys/malloc.h>
  42 #include <sys/mutex.h>
  43 #include <sys/proc.h>
  44 #include <sys/sched.h>
  45 #include <sys/smp.h>
  46 #include <sys/sysctl.h>
  47 #include <sys/turnstile.h>
  48 #include <vm/vm.h>
  49 #include <vm/vm_extern.h>
  50 #include <vm/vm_kern.h>
  51
  52 #include <ck_epoch.h>
  53
  54 static MALLOC_DEFINE(M_EPOCH, "epoch", "epoch based reclamation");
  55
  56 /* arbitrary --- needs benchmarking */
  57 #define MAX_ADAPTIVE_SPIN 1000
  58
  59 #define EPOCH_EXITING 0x1
  60 #ifdef __amd64__
  61 #define EPOCH_ALIGN CACHE_LINE_SIZE*2
  62 #else
  63 #define EPOCH_ALIGN CACHE_LINE_SIZE
  64 #endif
  65
  66 CTASSERT(sizeof(epoch_section_t) == sizeof(ck_epoch_section_t));
  67 SYSCTL_NODE(_kern, OID_AUTO, epoch, CTLFLAG_RW, 0, "epoch information");
  68 SYSCTL_NODE(_kern_epoch, OID_AUTO, stats, CTLFLAG_RW, 0, "epoch stats");
  69
  70 static int poll_intvl;
  71 SYSCTL_INT(_kern_epoch, OID_AUTO, poll_intvl, CTLFLAG_RWTUN,
  72                    &poll_intvl, 0, "# of ticks to wait between garbage collecting deferred frees");
  73 /* Stats. */
  74 static counter_u64_t block_count;
  75 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, nblocked, CTLFLAG_RW,
  76                                    &block_count, "# of times a thread was in an epoch when epoch_wait was called");
  77 static counter_u64_t migrate_count;
  78 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, migrations, CTLFLAG_RW,
  79                                    &migrate_count, "# of times thread was migrated to another CPU in epoch_wait");
  80 static counter_u64_t turnstile_count;
  81 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, ncontended, CTLFLAG_RW,
  82                                    &turnstile_count, "# of times a thread was blocked on a lock in an epoch during an epoch_wait");
  83 static counter_u64_t switch_count;
  84 SYSCTL_COUNTER_U64(_kern_epoch_stats, OID_AUTO, switches, CTLFLAG_RW,
  85                                    &switch_count, "# of times a thread voluntarily context switched in epoch_wait");
  86
  87 typedef struct epoch_cb {
  88         void (*ec_callback)(epoch_context_t);
  89         STAILQ_ENTRY(epoch_cb) ec_link;
  90 } *epoch_cb_t;
  91
  92 TAILQ_HEAD(threadlist, thread);
  93
  94 typedef struct epoch_record {
  95         ck_epoch_record_t er_record;
  96         volatile struct threadlist er_tdlist;
  97         volatile uint32_t er_gen;
  98         uint32_t er_cpuid;
  99 } *epoch_record_t;
 100
 101 struct epoch_pcpu_state {
 102         struct epoch_record eps_record;
 103         STAILQ_HEAD(, epoch_cb) eps_cblist;
 104 } __aligned(EPOCH_ALIGN);
 105
 106 struct epoch {
 107         struct ck_epoch e_epoch __aligned(EPOCH_ALIGN);
 108         struct grouptask e_gtask;
 109         struct callout e_timer;
 110         struct mtx e_lock;
 111         int e_flags;
 112         /* make sure that immutable data doesn't overlap with the gtask, callout, and mutex*/
 113         struct epoch_pcpu_state *e_pcpu_dom[MAXMEMDOM] __aligned(EPOCH_ALIGN);
 114         counter_u64_t e_frees;
 115         uint64_t e_free_last;
 116         struct epoch_pcpu_state *e_pcpu[0];
 117 };
 118
 119 static __read_mostly int domcount[MAXMEMDOM];
 120 static __read_mostly int domoffsets[MAXMEMDOM];
 121 static __read_mostly int inited;
 122 __read_mostly epoch_t global_epoch;
 123
 124 static void epoch_call_task(void *context);
 125
 126 #if defined(__powerpc64__) || defined(__powerpc__) || !defined(NUMA)
 127 static bool usedomains = false;
 128 #else
 129 static bool usedomains = true;
 130 #endif
 131 static void
 132 epoch_init(void *arg __unused)
 133 {
 134         int domain, count;
 135
 136         if (poll_intvl == 0)
 137                 poll_intvl = hz;
 138
 139         block_count = counter_u64_alloc(M_WAITOK);
 140         migrate_count = counter_u64_alloc(M_WAITOK);
 141         turnstile_count = counter_u64_alloc(M_WAITOK);
 142         switch_count = counter_u64_alloc(M_WAITOK);
 143         if (usedomains == false)
 144                 goto done;
 145         count = domain = 0;
 146         domoffsets[0] = 0;
 147         for (domain = 0; domain < vm_ndomains; domain++) {
 148                 domcount[domain] = CPU_COUNT(&cpuset_domain[domain]);
 149                 if (bootverbose)
 150                         printf("domcount[%d] %d\n", domain, domcount[domain]);
 151         }
 152         for (domain = 1; domain < vm_ndomains; domain++)
 153                 domoffsets[domain] = domoffsets[domain-1] + domcount[domain-1];
 154
 155         for (domain = 0; domain < vm_ndomains; domain++) {
 156                 if (domcount[domain] == 0) {
 157                         usedomains = false;
 158                         break;
 159                 }
 160         }
 161  done:
 162         inited = 1;
 163         global_epoch = epoch_alloc();
 164 }
 165 SYSINIT(epoch, SI_SUB_TASKQ + 1, SI_ORDER_FIRST, epoch_init, NULL);
 166
 167 static void
 168 epoch_init_numa(epoch_t epoch)
 169 {
 170         int domain, cpu_offset;
 171         struct epoch_pcpu_state *eps;
 172         epoch_record_t er;
 173
 174         for (domain = 0; domain < vm_ndomains; domain++) {
 175                 eps = malloc_domain(sizeof(*eps)*domcount[domain], M_EPOCH,
 176                                                         domain, M_ZERO|M_WAITOK);
 177                 epoch->e_pcpu_dom[domain] = eps;
 178                 cpu_offset = domoffsets[domain];
 179                 for (int i = 0; i < domcount[domain]; i++, eps++) {
 180                         epoch->e_pcpu[cpu_offset + i] = eps;
 181                         er = &eps->eps_record;
 182                         STAILQ_INIT(&eps->eps_cblist);
 183                         ck_epoch_register(&epoch->e_epoch, &er->er_record, NULL);
 184                         TAILQ_INIT((struct threadlist *)(uintptr_t)&er->er_tdlist);
 185                         er->er_cpuid = cpu_offset + i;
 186                 }
 187         }
 188 }
 189
 190 static void
 191 epoch_init_legacy(epoch_t epoch)
 192 {
 193         struct epoch_pcpu_state *eps;
 194         epoch_record_t er;
 195
 196         eps = malloc(sizeof(*eps)*mp_ncpus, M_EPOCH, M_ZERO|M_WAITOK);
 197         epoch->e_pcpu_dom[0] = eps;
 198         for (int i = 0; i < mp_ncpus; i++, eps++) {
 199                 epoch->e_pcpu[i] = eps;
 200                 er = &eps->eps_record;
 201                 ck_epoch_register(&epoch->e_epoch, &er->er_record, NULL);
 202                 TAILQ_INIT((struct threadlist *)(uintptr_t)&er->er_tdlist);
 203                 STAILQ_INIT(&eps->eps_cblist);
 204                 er->er_cpuid = i;
 205         }
 206 }
 207
 208 static void
 209 epoch_callout(void *arg)
 210 {
 211         epoch_t epoch;
 212         uint64_t frees;
 213
 214         epoch = arg;
 215         frees = counter_u64_fetch(epoch->e_frees);
 216         /* pick some better value */
 217         if (frees - epoch->e_free_last > 10) {
 218                 GROUPTASK_ENQUEUE(&epoch->e_gtask);
 219                 epoch->e_free_last = frees;
 220         }
 221         if ((epoch->e_flags & EPOCH_EXITING) == 0)
 222                 callout_reset(&epoch->e_timer, poll_intvl, epoch_callout, epoch);
 223 }
 224
 225 epoch_t
 226 epoch_alloc(void)
 227 {
 228         epoch_t epoch;
 229
 230         if (__predict_false(!inited))
 231                 panic("%s called too early in boot", __func__);
 232         epoch = malloc(sizeof(struct epoch) + mp_ncpus*sizeof(void*),
 233                                    M_EPOCH, M_ZERO|M_WAITOK);
 234         ck_epoch_init(&epoch->e_epoch);
 235         epoch->e_frees = counter_u64_alloc(M_WAITOK);
 236         mtx_init(&epoch->e_lock, "epoch callout", NULL, MTX_DEF);
 237         callout_init_mtx(&epoch->e_timer, &epoch->e_lock, 0);
 238         taskqgroup_config_gtask_init(epoch, &epoch->e_gtask, epoch_call_task, "epoch call task");
 239         if (usedomains)
 240                 epoch_init_numa(epoch);
 241         else
 242                 epoch_init_legacy(epoch);
 243         callout_reset(&epoch->e_timer, poll_intvl, epoch_callout, epoch);
 244         return (epoch);
 245 }
 246
 247 void
 248 epoch_free(epoch_t epoch)
 249 {
 250         int domain;
 251 #ifdef INVARIANTS
 252         struct epoch_pcpu_state *eps;
 253         int cpu;
 254
 255         CPU_FOREACH(cpu) {
 256                 eps = epoch->e_pcpu[cpu];
 257                 MPASS(TAILQ_EMPTY(&eps->eps_record.er_tdlist));
 258         }
 259 #endif
 260         mtx_lock(&epoch->e_lock);
 261         epoch->e_flags |= EPOCH_EXITING;
 262         mtx_unlock(&epoch->e_lock);
 263         /*
 264          * Execute any lingering callbacks
 265          */
 266         GROUPTASK_ENQUEUE(&epoch->e_gtask);
 267         gtaskqueue_drain(epoch->e_gtask.gt_taskqueue, &epoch->e_gtask.gt_task);
 268         callout_drain(&epoch->e_timer);
 269         mtx_destroy(&epoch->e_lock);
 270         counter_u64_free(epoch->e_frees);
 271         taskqgroup_config_gtask_deinit(&epoch->e_gtask);
 272         if (usedomains)
 273                 for (domain = 0; domain < vm_ndomains; domain++)
 274                         free_domain(epoch->e_pcpu_dom[domain], M_EPOCH);
 275         else
 276                 free(epoch->e_pcpu_dom[0], M_EPOCH);
 277         free(epoch, M_EPOCH);
 278 }
 279
 280 #define INIT_CHECK(epoch)                                                               \
 281         do {                                                                                    \
 282                 if (__predict_false((epoch) == NULL))           \
 283                         return;                                                                 \
 284         } while (0)
 285
 286 void
 287 epoch_enter(epoch_t epoch)
 288 {
 289         struct epoch_pcpu_state *eps;
 290         struct thread *td;
 291
 292         INIT_CHECK(epoch);
 293
 294         td = curthread;
 295         critical_enter();
 296         eps = epoch->e_pcpu[curcpu];
 297         td->td_epochnest++;
 298         MPASS(td->td_epochnest < UCHAR_MAX - 2);
 299         if (td->td_epochnest == 1)
 300                 TAILQ_INSERT_TAIL(&eps->eps_record.er_tdlist, td, td_epochq);
 301 #ifdef INVARIANTS
 302         if (td->td_epochnest > 1) {
 303                 struct thread *curtd;
 304                 int found = 0;
 305
 306                 TAILQ_FOREACH(curtd, &eps->eps_record.er_tdlist, td_epochq)
 307                         if (curtd == td)
 308                                 found = 1;
 309                 KASSERT(found, ("recursing on a second epoch"));
 310         }
 311 #endif
 312         if (td->td_epochnest > 1) {
 313                 critical_exit();
 314                 return;
 315         }
 316         sched_pin();
 317         ck_epoch_begin(&eps->eps_record.er_record, (ck_epoch_section_t*)&td->td_epoch_section);
 318         critical_exit();
 319 }
 320
 321 void
 322 epoch_exit(epoch_t epoch)
 323 {
 324         struct epoch_pcpu_state *eps;
 325         struct thread *td;
 326
 327         td = curthread;
 328         INIT_CHECK(epoch);
 329         MPASS(td->td_epochnest);
 330         critical_enter();
 331         eps = epoch->e_pcpu[curcpu];
 332         td->td_epochnest--;
 333         if (td->td_epochnest == 0)
 334                 TAILQ_REMOVE(&eps->eps_record.er_tdlist, td, td_epochq);
 335         else {
 336                 critical_exit();
 337                 return;
 338         }
 339         sched_unpin();
 340         ck_epoch_end(&eps->eps_record.er_record, (ck_epoch_section_t*)&td->td_epoch_section);
 341         eps->eps_record.er_gen++;
 342         critical_exit();
 343 }
 344
 345 /*
 346  * epoch_block_handler is a callback from the ck code when another thread is
 347  * currently in an epoch section.
 348  */
 349 static void
 350 epoch_block_handler(struct ck_epoch *global __unused, ck_epoch_record_t *cr,
 351                                         void *arg __unused)
 352 {
 353         epoch_record_t record;
 354         struct epoch_pcpu_state *eps;
 355         struct thread *td, *tdwait, *owner;
 356         struct turnstile *ts;
 357         struct lock_object *lock;
 358         int spincount, gen;
 359
 360         eps = arg;
 361         record = __containerof(cr, struct epoch_record, er_record);
 362         td = curthread;
 363         spincount = 0;
 364         counter_u64_add(block_count, 1);
 365         if (record->er_cpuid != curcpu) {
 366                 /*
 367                  * If the head of the list is running, we can wait for it
 368                  * to remove itself from the list and thus save us the
 369                  * overhead of a migration
 370                  */
 371                 if ((tdwait = TAILQ_FIRST(&record->er_tdlist)) != NULL &&
 372                         TD_IS_RUNNING(tdwait)) {
 373                         gen = record->er_gen;
 374                         thread_unlock(td);
 375                         do {
 376                                 cpu_spinwait();
 377                         } while (tdwait == TAILQ_FIRST(&record->er_tdlist) &&
 378                                          gen == record->er_gen && TD_IS_RUNNING(tdwait) &&
 379                                          spincount++ < MAX_ADAPTIVE_SPIN);
 380                         thread_lock(td);
 381                         return;
 382                 }
 383
 384                 /*
 385                  * Being on the same CPU as that of the record on which
 386                  * we need to wait allows us access to the thread
 387                  * list associated with that CPU. We can then examine the
 388                  * oldest thread in the queue and wait on its turnstile
 389                  * until it resumes and so on until a grace period
 390                  * elapses.
 391                  *
 392                  */
 393                 counter_u64_add(migrate_count, 1);
 394                 sched_bind(td, record->er_cpuid);
 395                 /*
 396                  * At this point we need to return to the ck code
 397                  * to scan to see if a grace period has elapsed.
 398                  * We can't move on to check the thread list, because
 399                  * in the meantime new threads may have arrived that
 400                  * in fact belong to a different epoch.
 401                  */
 402                 return;
 403         }
 404         /*
 405          * Try to find a thread in an epoch section on this CPU
 406          * waiting on a turnstile. Otherwise find the lowest
 407          * priority thread (highest prio value) and drop our priority
 408          * to match to allow it to run.
 409          */
 410         TAILQ_FOREACH(tdwait, &record->er_tdlist, td_epochq) {
 411                 /*
 412                  * Propagate our priority to any other waiters to prevent us
 413                  * from starving them. They will have their original priority
 414                  * restore on exit from epoch_wait().
 415                  */
 416                 if (!TD_IS_INHIBITED(tdwait) && tdwait->td_priority > td->td_priority) {
 417                         thread_lock(tdwait);
 418                         sched_prio(tdwait, td->td_priority);
 419                         thread_unlock(tdwait);
 420                 }
 421                 if (TD_IS_INHIBITED(tdwait) && TD_ON_LOCK(tdwait) &&
 422                         ((ts = tdwait->td_blocked) != NULL)) {
 423                         /*
 424                          * We unlock td to allow turnstile_wait to reacquire the
 425                          * the thread lock. Before unlocking it we enter a critical
 426                          * section to prevent preemption after we reenable interrupts
 427                          * by dropping the thread lock in order to prevent tdwait
 428                          * from getting to run.
 429                          */
 430                         critical_enter();
 431                         thread_unlock(td);
 432                         owner = turnstile_lock(ts, &lock);
 433                         /*
 434                          * The owner pointer indicates that the lock succeeded. Only
 435                          * in case we hold the lock and the turnstile we locked is still
 436                          * the one that tdwait is blocked on can we continue. Otherwise
 437                          * The turnstile pointer has been changed out from underneath
 438                          * us, as in the case where the lock holder has signalled tdwait,
 439                          * and we need to continue.
 440                          */
 441                         if (owner != NULL && ts == tdwait->td_blocked) {
 442                                 MPASS(TD_IS_INHIBITED(tdwait) && TD_ON_LOCK(tdwait));
 443                                 critical_exit();
 444                                 turnstile_wait(ts, owner, tdwait->td_tsqueue);
 445                                 counter_u64_add(turnstile_count, 1);
 446                                 thread_lock(td);
 447                                 return;
 448                         } else if (owner != NULL)
 449                                 turnstile_unlock(ts, lock);
 450                         thread_lock(td);
 451                         critical_exit();
 452                         KASSERT(td->td_locks == 0,
 453                                         ("%d locks held", td->td_locks));
 454                 }
 455         }
 456         /*
 457          * We didn't find any threads actually blocked on a lock
 458          * so we have nothing to do except context switch away.
 459          */
 460         counter_u64_add(switch_count, 1);
 461         mi_switch(SW_VOL | SWT_RELINQUISH, NULL);
 462
 463         /*
 464          * Release the thread lock while yielding to
 465          * allow other threads to acquire the lock
 466          * pointed to by TDQ_LOCKPTR(td). Else a
 467          * deadlock like situation might happen. (HPS)
 468          */
 469         thread_unlock(td);
 470         thread_lock(td);
 471 }
 472
 473 void
 474 epoch_wait(epoch_t epoch)
 475 {
 476         struct thread *td;
 477         int was_bound;
 478         int old_cpu;
 479         int old_pinned;
 480         u_char old_prio;
 481 #ifdef INVARIANTS
 482         int locks;
 483
 484         locks = curthread->td_locks;
 485 #endif
 486         INIT_CHECK(epoch);
 487
 488         WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL,
 489             "epoch_wait() can sleep");
 490
 491         td = curthread;
 492         KASSERT(td->td_epochnest == 0, ("epoch_wait() in the middle of an epoch section"));
 493         thread_lock(td);
 494
 495         DROP_GIANT();
 496
 497         old_cpu = PCPU_GET(cpuid);
 498         old_pinned = td->td_pinned;
 499         old_prio = td->td_priority;
 500         was_bound = sched_is_bound(td);
 501         sched_unbind(td);
 502         td->td_pinned = 0;
 503         sched_bind(td, old_cpu);
 504
 505         ck_epoch_synchronize_wait(&epoch->e_epoch, epoch_block_handler, NULL);
 506
 507         /* restore CPU binding, if any */
 508         if (was_bound != 0) {
 509                 sched_bind(td, old_cpu);
 510         } else {
 511                 /* get thread back to initial CPU, if any */
 512                 if (old_pinned != 0)
 513                         sched_bind(td, old_cpu);
 514                 sched_unbind(td);
 515         }
 516         /* restore pinned after bind */
 517         td->td_pinned = old_pinned;
 518
 519         /* restore thread priority */
 520         sched_prio(td, old_prio);
 521         thread_unlock(td);
 522         PICKUP_GIANT();
 523         KASSERT(td->td_locks == locks,
 524                         ("%d residual locks held", td->td_locks - locks));
 525 }
 526
 527 void
 528 epoch_call(epoch_t epoch, epoch_context_t ctx, void (*callback) (epoch_context_t))
 529 {
 530         struct epoch_pcpu_state *eps;
 531         epoch_cb_t cb;
 532
 533         cb = (void *)ctx;
 534
 535         MPASS(callback);
 536         /* too early in boot to have epoch set up */
 537         if (__predict_false(epoch == NULL)) {
 538                 callback(ctx);
 539                 return;
 540         }
 541         MPASS(cb->ec_callback == NULL);
 542         MPASS(cb->ec_link.stqe_next == NULL);
 543         cb->ec_callback = callback;
 544         counter_u64_add(epoch->e_frees, 1);
 545
 546         critical_enter();
 547         eps = epoch->e_pcpu[curcpu];
 548         STAILQ_INSERT_HEAD(&eps->eps_cblist, cb, ec_link);
 549         critical_exit();
 550 }
 551
 552 static void
 553 epoch_call_task(void *context)
 554 {
 555         struct epoch_pcpu_state *eps;
 556         epoch_t epoch;
 557         epoch_cb_t cb;
 558         struct thread *td;
 559         int cpu;
 560         STAILQ_HEAD(, epoch_cb) tmp_head;
 561
 562         epoch = context;
 563         STAILQ_INIT(&tmp_head);
 564         td = curthread;
 565         thread_lock(td);
 566         CPU_FOREACH(cpu) {
 567                 sched_bind(td, cpu);
 568                 eps = epoch->e_pcpu[cpu];
 569                 if (!STAILQ_EMPTY(&eps->eps_cblist))
 570                         STAILQ_CONCAT(&tmp_head, &eps->eps_cblist);
 571         }
 572         sched_unbind(td);
 573         thread_unlock(td);
 574         epoch_wait(epoch);
 575
 576         while ((cb = STAILQ_FIRST(&tmp_head)) != NULL) {
 577                 STAILQ_REMOVE_HEAD(&tmp_head, ec_link);
 578                 cb->ec_callback((void*)cb);
 579         }
 580 }
 581
 582 int
 583 in_epoch(void)
 584 {
 585         return (curthread->td_epochnest != 0);
 586 }