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- /*
- * 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);
- }
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