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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 (EV_A_ &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);
}