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Edit File: ev_linuxaio.c
/* * libev linux aio fd activity backend * * Copyright (c) 2019 Marc Alexander Lehmann <libev@schmorp.de> * All rights reserved. * * Redistribution and use in source and binary forms, with or without modifica- * tion, are permitted provided that the following conditions are met: * * 1. Redistributions of source code must retain the above copyright notice, * this list of conditions and the following disclaimer. * * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MER- * CHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO * EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPE- * CIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTH- * ERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED * OF THE POSSIBILITY OF SUCH DAMAGE. * * Alternatively, the contents of this file may be used under the terms of * the GNU General Public License ("GPL") version 2 or any later version, * in which case the provisions of the GPL are applicable instead of * the above. If you wish to allow the use of your version of this file * only under the terms of the GPL and not to allow others to use your * version of this file under the BSD license, indicate your decision * by deleting the provisions above and replace them with the notice * and other provisions required by the GPL. If you do not delete the * provisions above, a recipient may use your version of this file under * either the BSD or the GPL. */ /* * general notes about linux aio: * * a) at first, the linux aio IOCB_CMD_POLL functionality introduced in * 4.18 looks too good to be true: both watchers and events can be * batched, and events can even be handled in userspace using * a ring buffer shared with the kernel. watchers can be canceled * regardless of whether the fd has been closed. no problems with fork. * ok, the ring buffer is 200% undocumented (there isn't even a * header file), but otherwise, it's pure bliss! * b) ok, watchers are one-shot, so you have to re-arm active ones * on every iteration. so much for syscall-less event handling, * but at least these re-arms can be batched, no big deal, right? * c) well, linux as usual: the documentation lies to you: io_submit * sometimes returns EINVAL because the kernel doesn't feel like * handling your poll mask - ttys can be polled for POLLOUT, * POLLOUT|POLLIN, but polling for POLLIN fails. just great, * so we have to fall back to something else (hello, epoll), * but at least the fallback can be slow, because these are * exceptional cases, right? * d) hmm, you have to tell the kernel the maximum number of watchers * you want to queue when initialising the aio context. but of * course the real limit is magically calculated in the kernel, and * is often higher then we asked for. so we just have to destroy * the aio context and re-create it a bit larger if we hit the limit. * (starts to remind you of epoll? well, it's a bit more deterministic * and less gambling, but still ugly as hell). * e) that's when you find out you can also hit an arbitrary system-wide * limit. or the kernel simply doesn't want to handle your watchers. * what the fuck do we do then? you guessed it, in the middle * of event handling we have to switch to 100% epoll polling. and * that better is as fast as normal epoll polling, so you practically * have to use the normal epoll backend with all its quirks. * f) end result of this train wreck: it inherits all the disadvantages * from epoll, while adding a number on its own. why even bother to use * it? because if conditions are right and your fds are supported and you * don't hit a limit, this backend is actually faster, doesn't gamble with * your fds, batches watchers and events and doesn't require costly state * recreates. well, until it does. * g) all of this makes this backend use almost twice as much code as epoll. * which in turn uses twice as much code as poll. and that#s not counting * the fact that this backend also depends on the epoll backend, making * it three times as much code as poll, or kqueue. * h) bleah. why can't linux just do kqueue. sure kqueue is ugly, but by now * it's clear that whatever linux comes up with is far, far, far worse. */ #include <sys/time.h> /* actually linux/time.h, but we must assume they are compatible */ #include <poll.h> #include <linux/aio_abi.h> /*****************************************************************************/ /* syscall wrapdadoop - this section has the raw api/abi definitions */ #include <sys/syscall.h> /* no glibc wrappers */ /* aio_abi.h is not versioned in any way, so we cannot test for its existance */ #define IOCB_CMD_POLL 5 /* taken from linux/fs/aio.c. yup, that's a .c file. * not only is this totally undocumented, not even the source code * can tell you what the future semantics of compat_features and * incompat_features are, or what header_length actually is for. */ #define AIO_RING_MAGIC 0xa10a10a1 #define EV_AIO_RING_INCOMPAT_FEATURES 0 struct aio_ring { unsigned id; /* kernel internal index number */ unsigned nr; /* number of io_events */ unsigned head; /* Written to by userland or by kernel. */ unsigned tail; unsigned magic; unsigned compat_features; unsigned incompat_features; unsigned header_length; /* size of aio_ring */ struct io_event io_events[0]; }; inline_size int evsys_io_setup (unsigned nr_events, aio_context_t *ctx_idp) { return ev_syscall2 (SYS_io_setup, nr_events, ctx_idp); } inline_size int evsys_io_destroy (aio_context_t ctx_id) { return ev_syscall1 (SYS_io_destroy, ctx_id); } inline_size int evsys_io_submit (aio_context_t ctx_id, long nr, struct iocb *cbp[]) { return ev_syscall3 (SYS_io_submit, ctx_id, nr, cbp); } inline_size int evsys_io_cancel (aio_context_t ctx_id, struct iocb *cbp, struct io_event *result) { return ev_syscall3 (SYS_io_cancel, ctx_id, cbp, result); } inline_size int evsys_io_getevents (aio_context_t ctx_id, long min_nr, long nr, struct io_event *events, struct timespec *timeout) { return ev_syscall5 (SYS_io_getevents, ctx_id, min_nr, nr, events, timeout); } /*****************************************************************************/ /* actual backed implementation */ ecb_cold static int linuxaio_nr_events (EV_P) { /* we start with 16 iocbs and incraese from there * that's tiny, but the kernel has a rather low system-wide * limit that can be reached quickly, so let's be parsimonious * with this resource. * Rest assured, the kernel generously rounds up small and big numbers * in different ways (but doesn't seem to charge you for it). * The 15 here is because the kernel usually has a power of two as aio-max-nr, * and this helps to take advantage of that limit. */ /* we try to fill 4kB pages exactly. * the ring buffer header is 32 bytes, every io event is 32 bytes. * the kernel takes the io requests number, doubles it, adds 2 * and adds the ring buffer. * the way we use this is by starting low, and then roughly doubling the * size each time we hit a limit. */ int requests = 15 << linuxaio_iteration; int one_page = (4096 / sizeof (struct io_event) ) / 2; /* how many fit into one page */ int first_page = ((4096 - sizeof (struct aio_ring)) / sizeof (struct io_event) - 2) / 2; /* how many fit into the first page */ /* if everything fits into one page, use count exactly */ if (requests > first_page) /* otherwise, round down to full pages and add the first page */ requests = requests / one_page * one_page + first_page; return requests; } /* we use out own wrapper structure in case we ever want to do something "clever" */ typedef struct aniocb { struct iocb io; /*int inuse;*/ } *ANIOCBP; inline_size void linuxaio_array_needsize_iocbp (ANIOCBP *base, int offset, int count) { while (count--) { /* TODO: quite the overhead to allocate every iocb separately, maybe use our own allocator? */ ANIOCBP iocb = (ANIOCBP)ev_malloc (sizeof (*iocb)); /* full zero initialise is probably not required at the moment, but * this is not well documented, so we better do it. */ memset (iocb, 0, sizeof (*iocb)); iocb->io.aio_lio_opcode = IOCB_CMD_POLL; iocb->io.aio_fildes = offset; base [offset++] = iocb; } } ecb_cold static void linuxaio_free_iocbp (EV_P) { while (linuxaio_iocbpmax--) ev_free (linuxaio_iocbps [linuxaio_iocbpmax]); linuxaio_iocbpmax = 0; /* next resize will completely reallocate the array, at some overhead */ } static void linuxaio_modify (EV_P_ int fd, int oev, int nev) { array_needsize (ANIOCBP, linuxaio_iocbps, linuxaio_iocbpmax, fd + 1, linuxaio_array_needsize_iocbp); ANIOCBP iocb = linuxaio_iocbps [fd]; ANFD *anfd = &anfds [fd]; if (ecb_expect_false (iocb->io.aio_reqprio < 0)) { /* we handed this fd over to epoll, so undo this first */ /* we do it manually because the optimisations on epoll_modify won't do us any good */ epoll_ctl (backend_fd, EPOLL_CTL_DEL, fd, 0); anfd->emask = 0; iocb->io.aio_reqprio = 0; } else if (ecb_expect_false (iocb->io.aio_buf)) { /* iocb active, so cancel it first before resubmit */ /* this assumes we only ever get one call per fd per loop iteration */ for (;;) { /* on all relevant kernels, io_cancel fails with EINPROGRESS on "success" */ if (ecb_expect_false (evsys_io_cancel (linuxaio_ctx, &iocb->io, (struct io_event *)0) == 0)) break; if (ecb_expect_true (errno == EINPROGRESS)) break; /* the EINPROGRESS test is for nicer error message. clumsy. */ if (errno != EINTR) { assert (("libev: linuxaio unexpected io_cancel failed", errno != EINTR && errno != EINPROGRESS)); break; } } /* increment generation counter to avoid handling old events */ ++anfd->egen; } iocb->io.aio_buf = (nev & EV_READ ? POLLIN : 0) | (nev & EV_WRITE ? POLLOUT : 0); if (nev) { iocb->io.aio_data = (uint32_t)fd | ((__u64)(uint32_t)anfd->egen << 32); /* queue iocb up for io_submit */ /* this assumes we only ever get one call per fd per loop iteration */ ++linuxaio_submitcnt; array_needsize (struct iocb *, linuxaio_submits, linuxaio_submitmax, linuxaio_submitcnt, array_needsize_noinit); linuxaio_submits [linuxaio_submitcnt - 1] = &iocb->io; } } static void linuxaio_epoll_cb (EV_P_ struct ev_io *w, int revents) { epoll_poll (EV_A_ 0); } inline_speed void linuxaio_fd_rearm (EV_P_ int fd) { anfds [fd].events = 0; linuxaio_iocbps [fd]->io.aio_buf = 0; fd_change (EV_A_ fd, EV_ANFD_REIFY); } static void linuxaio_parse_events (EV_P_ struct io_event *ev, int nr) { while (nr) { int fd = ev->data & 0xffffffff; uint32_t gen = ev->data >> 32; int res = ev->res; assert (("libev: iocb fd must be in-bounds", fd >= 0 && fd < anfdmax)); /* only accept events if generation counter matches */ if (ecb_expect_true (gen == (uint32_t)anfds [fd].egen)) { /* feed events, we do not expect or handle POLLNVAL */ fd_event ( EV_A_ fd, (res & (POLLOUT | POLLERR | POLLHUP) ? EV_WRITE : 0) | (res & (POLLIN | POLLERR | POLLHUP) ? EV_READ : 0) ); /* linux aio is oneshot: rearm fd. TODO: this does more work than strictly needed */ linuxaio_fd_rearm (EV_A_ fd); } --nr; ++ev; } } /* get any events from ring buffer, return true if any were handled */ static int linuxaio_get_events_from_ring (EV_P) { struct aio_ring *ring = (struct aio_ring *)linuxaio_ctx; unsigned head, tail; /* the kernel reads and writes both of these variables, */ /* as a C extension, we assume that volatile use here */ /* both makes reads atomic and once-only */ head = *(volatile unsigned *)&ring->head; ECB_MEMORY_FENCE_ACQUIRE; tail = *(volatile unsigned *)&ring->tail; if (head == tail) return 0; /* parse all available events, but only once, to avoid starvation */ if (ecb_expect_true (tail > head)) /* normal case around */ linuxaio_parse_events (EV_A_ ring->io_events + head, tail - head); else /* wrapped around */ { linuxaio_parse_events (EV_A_ ring->io_events + head, ring->nr - head); linuxaio_parse_events (EV_A_ ring->io_events, tail); } ECB_MEMORY_FENCE_RELEASE; /* as an extension to C, we hope that the volatile will make this atomic and once-only */ *(volatile unsigned *)&ring->head = tail; return 1; } inline_size int linuxaio_ringbuf_valid (EV_P) { struct aio_ring *ring = (struct aio_ring *)linuxaio_ctx; return ecb_expect_true (ring->magic == AIO_RING_MAGIC) && ring->incompat_features == EV_AIO_RING_INCOMPAT_FEATURES && ring->header_length == sizeof (struct aio_ring); /* TODO: or use it to find io_event[0]? */ } /* read at least one event from kernel, or timeout */ inline_size void linuxaio_get_events (EV_P_ ev_tstamp timeout) { struct timespec ts; struct io_event ioev[8]; /* 256 octet stack space */ int want = 1; /* how many events to request */ int ringbuf_valid = linuxaio_ringbuf_valid (EV_A); if (ecb_expect_true (ringbuf_valid)) { /* if the ring buffer has any events, we don't wait or call the kernel at all */ if (linuxaio_get_events_from_ring (EV_A)) return; /* if the ring buffer is empty, and we don't have a timeout, then don't call the kernel */ if (!timeout) return; } else /* no ringbuffer, request slightly larger batch */ want = sizeof (ioev) / sizeof (ioev [0]); /* no events, so wait for some * for fairness reasons, we do this in a loop, to fetch all events */ for (;;) { int res; EV_RELEASE_CB; EV_TS_SET (ts, timeout); res = evsys_io_getevents (linuxaio_ctx, 1, want, ioev, &ts); EV_ACQUIRE_CB; if (res < 0) if (errno == EINTR) /* ignored, retry */; else ev_syserr ("(libev) linuxaio io_getevents"); else if (res) { /* at least one event available, handle them */ linuxaio_parse_events (EV_A_ ioev, res); if (ecb_expect_true (ringbuf_valid)) { /* if we have a ring buffer, handle any remaining events in it */ linuxaio_get_events_from_ring (EV_A); /* at this point, we should have handled all outstanding events */ break; } else if (res < want) /* otherwise, if there were fewere events than we wanted, we assume there are no more */ break; } else break; /* no events from the kernel, we are done */ timeout = EV_TS_CONST (0.); /* only wait in the first iteration */ } } inline_size int linuxaio_io_setup (EV_P) { linuxaio_ctx = 0; return evsys_io_setup (linuxaio_nr_events (EV_A), &linuxaio_ctx); } static void linuxaio_poll (EV_P_ ev_tstamp timeout) { int submitted; /* first phase: submit new iocbs */ /* io_submit might return less than the requested number of iocbs */ /* this is, afaics, only because of errors, but we go by the book and use a loop, */ /* which allows us to pinpoint the erroneous iocb */ for (submitted = 0; submitted < linuxaio_submitcnt; ) { int res = evsys_io_submit (linuxaio_ctx, linuxaio_submitcnt - submitted, linuxaio_submits + submitted); if (ecb_expect_false (res < 0)) if (errno == EINVAL) { /* This happens for unsupported fds, officially, but in my testing, * also randomly happens for supported fds. We fall back to good old * poll() here, under the assumption that this is a very rare case. * See https://lore.kernel.org/patchwork/patch/1047453/ to see * discussion about such a case (ttys) where polling for POLLIN * fails but POLLIN|POLLOUT works. */ struct iocb *iocb = linuxaio_submits [submitted]; epoll_modify (EV_A_ iocb->aio_fildes, 0, anfds [iocb->aio_fildes].events); iocb->aio_reqprio = -1; /* mark iocb as epoll */ res = 1; /* skip this iocb - another iocb, another chance */ } else if (errno == EAGAIN) { /* This happens when the ring buffer is full, or some other shit we * don't know and isn't documented. Most likely because we have too * many requests and linux aio can't be assed to handle them. * In this case, we try to allocate a larger ring buffer, freeing * ours first. This might fail, in which case we have to fall back to 100% * epoll. * God, how I hate linux not getting its act together. Ever. */ evsys_io_destroy (linuxaio_ctx); linuxaio_submitcnt = 0; /* rearm all fds with active iocbs */ { int fd; for (fd = 0; fd < linuxaio_iocbpmax; ++fd) if (linuxaio_iocbps [fd]->io.aio_buf) linuxaio_fd_rearm (EV_A_ fd); } ++linuxaio_iteration; if (linuxaio_io_setup (EV_A) < 0) { /* TODO: rearm all and recreate epoll backend from scratch */ /* TODO: might be more prudent? */ /* to bad, we can't get a new aio context, go 100% epoll */ linuxaio_free_iocbp (EV_A); ev_io_stop (EV_A_ &linuxaio_epoll_w); ev_ref (EV_A); linuxaio_ctx = 0; backend = EVBACKEND_EPOLL; backend_modify = epoll_modify; backend_poll = epoll_poll; } timeout = EV_TS_CONST (0.); /* it's easiest to handle this mess in another iteration */ return; } else if (errno == EBADF) { assert (("libev: event loop rejected bad fd", errno != EBADF)); fd_kill (EV_A_ linuxaio_submits [submitted]->aio_fildes); res = 1; /* skip this iocb */ } else if (errno == EINTR) /* not seen in reality, not documented */ res = 0; /* silently ignore and retry */ else { ev_syserr ("(libev) linuxaio io_submit"); res = 0; } submitted += res; } linuxaio_submitcnt = 0; /* second phase: fetch and parse events */ linuxaio_get_events (EV_A_ timeout); } inline_size int linuxaio_init (EV_P_ int flags) { /* would be great to have a nice test for IOCB_CMD_POLL instead */ /* also: test some semi-common fd types, such as files and ttys in recommended_backends */ /* 4.18 introduced IOCB_CMD_POLL, 4.19 made epoll work, and we need that */ if (ev_linux_version () < 0x041300) return 0; if (!epoll_init (EV_A_ 0)) return 0; linuxaio_iteration = 0; if (linuxaio_io_setup (EV_A) < 0) { epoll_destroy (EV_A); return 0; } ev_io_init (&linuxaio_epoll_w, linuxaio_epoll_cb, backend_fd, EV_READ); ev_set_priority (&linuxaio_epoll_w, EV_MAXPRI); ev_io_start (EV_A_ &linuxaio_epoll_w); ev_unref (EV_A); /* watcher should not keep loop alive */ backend_modify = linuxaio_modify; backend_poll = linuxaio_poll; linuxaio_iocbpmax = 0; linuxaio_iocbps = 0; linuxaio_submits = 0; linuxaio_submitmax = 0; linuxaio_submitcnt = 0; return EVBACKEND_LINUXAIO; } inline_size void linuxaio_destroy (EV_P) { epoll_destroy (EV_A); linuxaio_free_iocbp (EV_A); evsys_io_destroy (linuxaio_ctx); /* fails in child, aio context is destroyed */ } ecb_cold static void linuxaio_fork (EV_P) { linuxaio_submitcnt = 0; /* all pointers were invalidated */ linuxaio_free_iocbp (EV_A); /* this frees all iocbs, which is very heavy-handed */ evsys_io_destroy (linuxaio_ctx); /* fails in child, aio context is destroyed */ linuxaio_iteration = 0; /* we start over in the child */ while (linuxaio_io_setup (EV_A) < 0) ev_syserr ("(libev) linuxaio io_setup"); /* forking epoll should also effectively unregister all fds from the backend */ epoll_fork (EV_A); /* epoll_fork already did this. hopefully */ /*fd_rearm_all (EV_A);*/ ev_io_stop (EV_A_ &linuxaio_epoll_w); ev_io_set (EV_A_ &linuxaio_epoll_w, backend_fd, EV_READ); ev_io_start (EV_A_ &linuxaio_epoll_w); }