util/aio-posix.c

   1 /*
   2  * QEMU aio implementation
   3  *
   4  * Copyright IBM, Corp. 2008
   5  *
   6  * Authors:
   7  *  Anthony Liguori   <aliguori@us.ibm.com>
   8  *
   9  * This work is licensed under the terms of the GNU GPL, version 2.  See
  10  * the COPYING file in the top-level directory.
  11  *
  12  * Contributions after 2012-01-13 are licensed under the terms of the
  13  * GNU GPL, version 2 or (at your option) any later version.
  14  */
  15
  16 #include "qemu/osdep.h"
  17 #include "qemu-common.h"
  18 #include "block/block.h"
  19 #include "qemu/rcu_queue.h"
  20 #include "qemu/sockets.h"
  21 #include "qemu/cutils.h"
  22 #include "trace.h"
  23 #ifdef CONFIG_EPOLL_CREATE1
  24 #include <sys/epoll.h>
  25 #endif
  26
  27 struct AioHandler
  28 {
  29     GPollFD pfd;
  30     IOHandler *io_read;
  31     IOHandler *io_write;
  32     AioPollFn *io_poll;
  33     IOHandler *io_poll_begin;
  34     IOHandler *io_poll_end;
  35     int deleted;
  36     void *opaque;
  37     bool is_external;
  38     QLIST_ENTRY(AioHandler) node;
  39 };
  40
  41 #ifdef CONFIG_EPOLL_CREATE1
  42
  43 /* The fd number threashold to switch to epoll */
  44 #define EPOLL_ENABLE_THRESHOLD 64
  45
  46 static void aio_epoll_disable(AioContext *ctx)
  47 {
  48     ctx->epoll_available = false;
  49     if (!ctx->epoll_enabled) {
  50         return;
  51     }
  52     ctx->epoll_enabled = false;
  53     close(ctx->epollfd);
  54 }
  55
  56 static inline int epoll_events_from_pfd(int pfd_events)
  57 {
  58     return (pfd_events & G_IO_IN ? EPOLLIN : 0) |
  59            (pfd_events & G_IO_OUT ? EPOLLOUT : 0) |
  60            (pfd_events & G_IO_HUP ? EPOLLHUP : 0) |
  61            (pfd_events & G_IO_ERR ? EPOLLERR : 0);
  62 }
  63
  64 static bool aio_epoll_try_enable(AioContext *ctx)
  65 {
  66     AioHandler *node;
  67     struct epoll_event event;
  68
  69     QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
  70         int r;
  71         if (node->deleted || !node->pfd.events) {
  72             continue;
  73         }
  74         event.events = epoll_events_from_pfd(node->pfd.events);
  75         event.data.ptr = node;
  76         r = epoll_ctl(ctx->epollfd, EPOLL_CTL_ADD, node->pfd.fd, &event);
  77         if (r) {
  78             return false;
  79         }
  80     }
  81     ctx->epoll_enabled = true;
  82     return true;
  83 }
  84
  85 static void aio_epoll_update(AioContext *ctx, AioHandler *node, bool is_new)
  86 {
  87     struct epoll_event event;
  88     int r;
  89     int ctl;
  90
  91     if (!ctx->epoll_enabled) {
  92         return;
  93     }
  94     if (!node->pfd.events) {
  95         ctl = EPOLL_CTL_DEL;
  96     } else {
  97         event.data.ptr = node;
  98         event.events = epoll_events_from_pfd(node->pfd.events);
  99         ctl = is_new ? EPOLL_CTL_ADD : EPOLL_CTL_MOD;
 100     }
 101
 102     r = epoll_ctl(ctx->epollfd, ctl, node->pfd.fd, &event);
 103     if (r) {
 104         aio_epoll_disable(ctx);
 105     }
 106 }
 107
 108 static int aio_epoll(AioContext *ctx, GPollFD *pfds,
 109                      unsigned npfd, int64_t timeout)
 110 {
 111     AioHandler *node;
 112     int i, ret = 0;
 113     struct epoll_event events[128];
 114
 115     assert(npfd == 1);
 116     assert(pfds[0].fd == ctx->epollfd);
 117     if (timeout > 0) {
 118         ret = qemu_poll_ns(pfds, npfd, timeout);
 119     }
 120     if (timeout <= 0 || ret > 0) {
 121         ret = epoll_wait(ctx->epollfd, events,
 122                          sizeof(events) / sizeof(events[0]),
 123                          timeout);
 124         if (ret <= 0) {
 125             goto out;
 126         }
 127         for (i = 0; i < ret; i++) {
 128             int ev = events[i].events;
 129             node = events[i].data.ptr;
 130             node->pfd.revents = (ev & EPOLLIN ? G_IO_IN : 0) |
 131                 (ev & EPOLLOUT ? G_IO_OUT : 0) |
 132                 (ev & EPOLLHUP ? G_IO_HUP : 0) |
 133                 (ev & EPOLLERR ? G_IO_ERR : 0);
 134         }
 135     }
 136 out:
 137     return ret;
 138 }
 139
 140 static bool aio_epoll_enabled(AioContext *ctx)
 141 {
 142     /* Fall back to ppoll when external clients are disabled. */
 143     return !aio_external_disabled(ctx) && ctx->epoll_enabled;
 144 }
 145
 146 static bool aio_epoll_check_poll(AioContext *ctx, GPollFD *pfds,
 147                                  unsigned npfd, int64_t timeout)
 148 {
 149     if (!ctx->epoll_available) {
 150         return false;
 151     }
 152     if (aio_epoll_enabled(ctx)) {
 153         return true;
 154     }
 155     if (npfd >= EPOLL_ENABLE_THRESHOLD) {
 156         if (aio_epoll_try_enable(ctx)) {
 157             return true;
 158         } else {
 159             aio_epoll_disable(ctx);
 160         }
 161     }
 162     return false;
 163 }
 164
 165 #else
 166
 167 static void aio_epoll_update(AioContext *ctx, AioHandler *node, bool is_new)
 168 {
 169 }
 170
 171 static int aio_epoll(AioContext *ctx, GPollFD *pfds,
 172                      unsigned npfd, int64_t timeout)
 173 {
 174     assert(false);
 175 }
 176
 177 static bool aio_epoll_enabled(AioContext *ctx)
 178 {
 179     return false;
 180 }
 181
 182 static bool aio_epoll_check_poll(AioContext *ctx, GPollFD *pfds,
 183                           unsigned npfd, int64_t timeout)
 184 {
 185     return false;
 186 }
 187
 188 #endif
 189
 190 static AioHandler *find_aio_handler(AioContext *ctx, int fd)
 191 {
 192     AioHandler *node;
 193
 194     QLIST_FOREACH(node, &ctx->aio_handlers, node) {
 195         if (node->pfd.fd == fd)
 196             if (!node->deleted)
 197                 return node;
 198     }
 199
 200     return NULL;
 201 }
 202
 203 void aio_set_fd_handler(AioContext *ctx,
 204                         int fd,
 205                         bool is_external,
 206                         IOHandler *io_read,
 207                         IOHandler *io_write,
 208                         AioPollFn *io_poll,
 209                         void *opaque)
 210 {
 211     AioHandler *node;
 212     bool is_new = false;
 213     bool deleted = false;
 214
 215     qemu_lockcnt_lock(&ctx->list_lock);
 216
 217     node = find_aio_handler(ctx, fd);
 218
 219     /* Are we deleting the fd handler? */
 220     if (!io_read && !io_write && !io_poll) {
 221         if (node == NULL) {
 222             qemu_lockcnt_unlock(&ctx->list_lock);
 223             return;
 224         }
 225
 226         /* If the GSource is in the process of being destroyed then
 227          * g_source_remove_poll() causes an assertion failure.  Skip
 228          * removal in that case, because glib cleans up its state during
 229          * destruction anyway.
 230          */
 231         if (!g_source_is_destroyed(&ctx->source)) {
 232             g_source_remove_poll(&ctx->source, &node->pfd);
 233         }
 234
 235         /* If the lock is held, just mark the node as deleted */
 236         if (qemu_lockcnt_count(&ctx->list_lock)) {
 237             node->deleted = 1;
 238             node->pfd.revents = 0;
 239         } else {
 240             /* Otherwise, delete it for real.  We can't just mark it as
 241              * deleted because deleted nodes are only cleaned up while
 242              * no one is walking the handlers list.
 243              */
 244             QLIST_REMOVE(node, node);
 245             deleted = true;
 246         }
 247
 248         if (!node->io_poll) {
 249             ctx->poll_disable_cnt--;
 250         }
 251     } else {
 252         if (node == NULL) {
 253             /* Alloc and insert if it's not already there */
 254             node = g_new0(AioHandler, 1);
 255             node->pfd.fd = fd;
 256             QLIST_INSERT_HEAD_RCU(&ctx->aio_handlers, node, node);
 257
 258             g_source_add_poll(&ctx->source, &node->pfd);
 259             is_new = true;
 260
 261             ctx->poll_disable_cnt += !io_poll;
 262         } else {
 263             ctx->poll_disable_cnt += !io_poll - !node->io_poll;
 264         }
 265
 266         /* Update handler with latest information */
 267         node->io_read = io_read;
 268         node->io_write = io_write;
 269         node->io_poll = io_poll;
 270         node->opaque = opaque;
 271         node->is_external = is_external;
 272
 273         node->pfd.events = (io_read ? G_IO_IN | G_IO_HUP | G_IO_ERR : 0);
 274         node->pfd.events |= (io_write ? G_IO_OUT | G_IO_ERR : 0);
 275     }
 276
 277     aio_epoll_update(ctx, node, is_new);
 278     qemu_lockcnt_unlock(&ctx->list_lock);
 279     aio_notify(ctx);
 280
 281     if (deleted) {
 282         g_free(node);
 283     }
 284 }
 285
 286 void aio_set_fd_poll(AioContext *ctx, int fd,
 287                      IOHandler *io_poll_begin,
 288                      IOHandler *io_poll_end)
 289 {
 290     AioHandler *node = find_aio_handler(ctx, fd);
 291
 292     if (!node) {
 293         return;
 294     }
 295
 296     node->io_poll_begin = io_poll_begin;
 297     node->io_poll_end = io_poll_end;
 298 }
 299
 300 void aio_set_event_notifier(AioContext *ctx,
 301                             EventNotifier *notifier,
 302                             bool is_external,
 303                             EventNotifierHandler *io_read,
 304                             AioPollFn *io_poll)
 305 {
 306     aio_set_fd_handler(ctx, event_notifier_get_fd(notifier), is_external,
 307                        (IOHandler *)io_read, NULL, io_poll, notifier);
 308 }
 309
 310 void aio_set_event_notifier_poll(AioContext *ctx,
 311                                  EventNotifier *notifier,
 312                                  EventNotifierHandler *io_poll_begin,
 313                                  EventNotifierHandler *io_poll_end)
 314 {
 315     aio_set_fd_poll(ctx, event_notifier_get_fd(notifier),
 316                     (IOHandler *)io_poll_begin,
 317                     (IOHandler *)io_poll_end);
 318 }
 319
 320 static void poll_set_started(AioContext *ctx, bool started)
 321 {
 322     AioHandler *node;
 323
 324     if (started == ctx->poll_started) {
 325         return;
 326     }
 327
 328     ctx->poll_started = started;
 329
 330     qemu_lockcnt_inc(&ctx->list_lock);
 331     QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
 332         IOHandler *fn;
 333
 334         if (node->deleted) {
 335             continue;
 336         }
 337
 338         if (started) {
 339             fn = node->io_poll_begin;
 340         } else {
 341             fn = node->io_poll_end;
 342         }
 343
 344         if (fn) {
 345             fn(node->opaque);
 346         }
 347     }
 348     qemu_lockcnt_dec(&ctx->list_lock);
 349 }
 350
 351
 352 bool aio_prepare(AioContext *ctx)
 353 {
 354     /* Poll mode cannot be used with glib's event loop, disable it. */
 355     poll_set_started(ctx, false);
 356
 357     return false;
 358 }
 359
 360 bool aio_pending(AioContext *ctx)
 361 {
 362     AioHandler *node;
 363     bool result = false;
 364
 365     /*
 366      * We have to walk very carefully in case aio_set_fd_handler is
 367      * called while we're walking.
 368      */
 369     qemu_lockcnt_inc(&ctx->list_lock);
 370
 371     QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
 372         int revents;
 373
 374         revents = node->pfd.revents & node->pfd.events;
 375         if (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR) && node->io_read &&
 376             aio_node_check(ctx, node->is_external)) {
 377             result = true;
 378             break;
 379         }
 380         if (revents & (G_IO_OUT | G_IO_ERR) && node->io_write &&
 381             aio_node_check(ctx, node->is_external)) {
 382             result = true;
 383             break;
 384         }
 385     }
 386     qemu_lockcnt_dec(&ctx->list_lock);
 387
 388     return result;
 389 }
 390
 391 static bool aio_dispatch_handlers(AioContext *ctx)
 392 {
 393     AioHandler *node, *tmp;
 394     bool progress = false;
 395
 396     QLIST_FOREACH_SAFE_RCU(node, &ctx->aio_handlers, node, tmp) {
 397         int revents;
 398
 399         revents = node->pfd.revents & node->pfd.events;
 400         node->pfd.revents = 0;
 401
 402         if (!node->deleted &&
 403             (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR)) &&
 404             aio_node_check(ctx, node->is_external) &&
 405             node->io_read) {
 406             node->io_read(node->opaque);
 407
 408             /* aio_notify() does not count as progress */
 409             if (node->opaque != &ctx->notifier) {
 410                 progress = true;
 411             }
 412         }
 413         if (!node->deleted &&
 414             (revents & (G_IO_OUT | G_IO_ERR)) &&
 415             aio_node_check(ctx, node->is_external) &&
 416             node->io_write) {
 417             node->io_write(node->opaque);
 418             progress = true;
 419         }
 420
 421         if (node->deleted) {
 422             if (qemu_lockcnt_dec_if_lock(&ctx->list_lock)) {
 423                 QLIST_REMOVE(node, node);
 424                 g_free(node);
 425                 qemu_lockcnt_inc_and_unlock(&ctx->list_lock);
 426             }
 427         }
 428     }
 429
 430     return progress;
 431 }
 432
 433 void aio_dispatch(AioContext *ctx)
 434 {
 435     qemu_lockcnt_inc(&ctx->list_lock);
 436     aio_bh_poll(ctx);
 437     aio_dispatch_handlers(ctx);
 438     qemu_lockcnt_dec(&ctx->list_lock);
 439
 440     timerlistgroup_run_timers(&ctx->tlg);
 441 }
 442
 443 /* These thread-local variables are used only in a small part of aio_poll
 444  * around the call to the poll() system call.  In particular they are not
 445  * used while aio_poll is performing callbacks, which makes it much easier
 446  * to think about reentrancy!
 447  *
 448  * Stack-allocated arrays would be perfect but they have size limitations;
 449  * heap allocation is expensive enough that we want to reuse arrays across
 450  * calls to aio_poll().  And because poll() has to be called without holding
 451  * any lock, the arrays cannot be stored in AioContext.  Thread-local data
 452  * has none of the disadvantages of these three options.
 453  */
 454 static __thread GPollFD *pollfds;
 455 static __thread AioHandler **nodes;
 456 static __thread unsigned npfd, nalloc;
 457 static __thread Notifier pollfds_cleanup_notifier;
 458
 459 static void pollfds_cleanup(Notifier *n, void *unused)
 460 {
 461     g_assert(npfd == 0);
 462     g_free(pollfds);
 463     g_free(nodes);
 464     nalloc = 0;
 465 }
 466
 467 static void add_pollfd(AioHandler *node)
 468 {
 469     if (npfd == nalloc) {
 470         if (nalloc == 0) {
 471             pollfds_cleanup_notifier.notify = pollfds_cleanup;
 472             qemu_thread_atexit_add(&pollfds_cleanup_notifier);
 473             nalloc = 8;
 474         } else {
 475             g_assert(nalloc <= INT_MAX);
 476             nalloc *= 2;
 477         }
 478         pollfds = g_renew(GPollFD, pollfds, nalloc);
 479         nodes = g_renew(AioHandler *, nodes, nalloc);
 480     }
 481     nodes[npfd] = node;
 482     pollfds[npfd] = (GPollFD) {
 483         .fd = node->pfd.fd,
 484         .events = node->pfd.events,
 485     };
 486     npfd++;
 487 }
 488
 489 static bool run_poll_handlers_once(AioContext *ctx)
 490 {
 491     bool progress = false;
 492     AioHandler *node;
 493
 494     QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
 495         if (!node->deleted && node->io_poll &&
 496             aio_node_check(ctx, node->is_external) &&
 497             node->io_poll(node->opaque)) {
 498             progress = true;
 499         }
 500
 501         /* Caller handles freeing deleted nodes.  Don't do it here. */
 502     }
 503
 504     return progress;
 505 }
 506
 507 /* run_poll_handlers:
 508  * @ctx: the AioContext
 509  * @max_ns: maximum time to poll for, in nanoseconds
 510  *
 511  * Polls for a given time.
 512  *
 513  * Note that ctx->notify_me must be non-zero so this function can detect
 514  * aio_notify().
 515  *
 516  * Note that the caller must have incremented ctx->list_lock.
 517  *
 518  * Returns: true if progress was made, false otherwise
 519  */
 520 static bool run_poll_handlers(AioContext *ctx, int64_t max_ns)
 521 {
 522     bool progress;
 523     int64_t end_time;
 524
 525     assert(ctx->notify_me);
 526     assert(qemu_lockcnt_count(&ctx->list_lock) > 0);
 527     assert(ctx->poll_disable_cnt == 0);
 528
 529     trace_run_poll_handlers_begin(ctx, max_ns);
 530
 531     end_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) + max_ns;
 532
 533     do {
 534         progress = run_poll_handlers_once(ctx);
 535     } while (!progress && qemu_clock_get_ns(QEMU_CLOCK_REALTIME) < end_time);
 536
 537     trace_run_poll_handlers_end(ctx, progress);
 538
 539     return progress;
 540 }
 541
 542 /* try_poll_mode:
 543  * @ctx: the AioContext
 544  * @blocking: busy polling is only attempted when blocking is true
 545  *
 546  * ctx->notify_me must be non-zero so this function can detect aio_notify().
 547  *
 548  * Note that the caller must have incremented ctx->list_lock.
 549  *
 550  * Returns: true if progress was made, false otherwise
 551  */
 552 static bool try_poll_mode(AioContext *ctx, bool blocking)
 553 {
 554     if (blocking && ctx->poll_max_ns && ctx->poll_disable_cnt == 0) {
 555         /* See qemu_soonest_timeout() uint64_t hack */
 556         int64_t max_ns = MIN((uint64_t)aio_compute_timeout(ctx),
 557                              (uint64_t)ctx->poll_ns);
 558
 559         if (max_ns) {
 560             poll_set_started(ctx, true);
 561
 562             if (run_poll_handlers(ctx, max_ns)) {
 563                 return true;
 564             }
 565         }
 566     }
 567
 568     poll_set_started(ctx, false);
 569
 570     /* Even if we don't run busy polling, try polling once in case it can make
 571      * progress and the caller will be able to avoid ppoll(2)/epoll_wait(2).
 572      */
 573     return run_poll_handlers_once(ctx);
 574 }
 575
 576 bool aio_poll(AioContext *ctx, bool blocking)
 577 {
 578     AioHandler *node;
 579     int i;
 580     int ret = 0;
 581     bool progress;
 582     int64_t timeout;
 583     int64_t start = 0;
 584
 585     /* aio_notify can avoid the expensive event_notifier_set if
 586      * everything (file descriptors, bottom halves, timers) will
 587      * be re-evaluated before the next blocking poll().  This is
 588      * already true when aio_poll is called with blocking == false;
 589      * if blocking == true, it is only true after poll() returns,
 590      * so disable the optimization now.
 591      */
 592     if (blocking) {
 593         atomic_add(&ctx->notify_me, 2);
 594     }
 595
 596     qemu_lockcnt_inc(&ctx->list_lock);
 597
 598     if (ctx->poll_max_ns) {
 599         start = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
 600     }
 601
 602     progress = try_poll_mode(ctx, blocking);
 603     if (!progress) {
 604         assert(npfd == 0);
 605
 606         /* fill pollfds */
 607
 608         if (!aio_epoll_enabled(ctx)) {
 609             QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
 610                 if (!node->deleted && node->pfd.events
 611                     && aio_node_check(ctx, node->is_external)) {
 612                     add_pollfd(node);
 613                 }
 614             }
 615         }
 616
 617         timeout = blocking ? aio_compute_timeout(ctx) : 0;
 618
 619         /* wait until next event */
 620         if (aio_epoll_check_poll(ctx, pollfds, npfd, timeout)) {
 621             AioHandler epoll_handler;
 622
 623             epoll_handler.pfd.fd = ctx->epollfd;
 624             epoll_handler.pfd.events = G_IO_IN | G_IO_OUT | G_IO_HUP | G_IO_ERR;
 625             npfd = 0;
 626             add_pollfd(&epoll_handler);
 627             ret = aio_epoll(ctx, pollfds, npfd, timeout);
 628         } else  {
 629             ret = qemu_poll_ns(pollfds, npfd, timeout);
 630         }
 631     }
 632
 633     if (blocking) {
 634         atomic_sub(&ctx->notify_me, 2);
 635     }
 636
 637     /* Adjust polling time */
 638     if (ctx->poll_max_ns) {
 639         int64_t block_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start;
 640
 641         if (block_ns <= ctx->poll_ns) {
 642             /* This is the sweet spot, no adjustment needed */
 643         } else if (block_ns > ctx->poll_max_ns) {
 644             /* We'd have to poll for too long, poll less */
 645             int64_t old = ctx->poll_ns;
 646
 647             if (ctx->poll_shrink) {
 648                 ctx->poll_ns /= ctx->poll_shrink;
 649             } else {
 650                 ctx->poll_ns = 0;
 651             }
 652
 653             trace_poll_shrink(ctx, old, ctx->poll_ns);
 654         } else if (ctx->poll_ns < ctx->poll_max_ns &&
 655                    block_ns < ctx->poll_max_ns) {
 656             /* There is room to grow, poll longer */
 657             int64_t old = ctx->poll_ns;
 658             int64_t grow = ctx->poll_grow;
 659
 660             if (grow == 0) {
 661                 grow = 2;
 662             }
 663
 664             if (ctx->poll_ns) {
 665                 ctx->poll_ns *= grow;
 666             } else {
 667                 ctx->poll_ns = 4000; /* start polling at 4 microseconds */
 668             }
 669
 670             if (ctx->poll_ns > ctx->poll_max_ns) {
 671                 ctx->poll_ns = ctx->poll_max_ns;
 672             }
 673
 674             trace_poll_grow(ctx, old, ctx->poll_ns);
 675         }
 676     }
 677
 678     aio_notify_accept(ctx);
 679
 680     /* if we have any readable fds, dispatch event */
 681     if (ret > 0) {
 682         for (i = 0; i < npfd; i++) {
 683             nodes[i]->pfd.revents = pollfds[i].revents;
 684         }
 685     }
 686
 687     npfd = 0;
 688
 689     progress |= aio_bh_poll(ctx);
 690
 691     if (ret > 0) {
 692         progress |= aio_dispatch_handlers(ctx);
 693     }
 694
 695     qemu_lockcnt_dec(&ctx->list_lock);
 696
 697     progress |= timerlistgroup_run_timers(&ctx->tlg);
 698
 699     return progress;
 700 }
 701
 702 void aio_context_setup(AioContext *ctx)
 703 {
 704 #ifdef CONFIG_EPOLL_CREATE1
 705     assert(!ctx->epollfd);
 706     ctx->epollfd = epoll_create1(EPOLL_CLOEXEC);
 707     if (ctx->epollfd == -1) {
 708         fprintf(stderr, "Failed to create epoll instance: %s", strerror(errno));
 709         ctx->epoll_available = false;
 710     } else {
 711         ctx->epoll_available = true;
 712     }
 713 #endif
 714 }
 715
 716 void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
 717                                  int64_t grow, int64_t shrink, Error **errp)
 718 {
 719     /* No thread synchronization here, it doesn't matter if an incorrect value
 720      * is used once.
 721      */
 722     ctx->poll_max_ns = max_ns;
 723     ctx->poll_ns = 0;
 724     ctx->poll_grow = grow;
 725     ctx->poll_shrink = shrink;
 726
 727     aio_notify(ctx);
 728 }