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aio-posix: make AioHandler dispatch O(1) with epoll 

File descriptor monitoring is O(1) with epoll(7), but
aio_dispatch_handlers() still scans all AioHandlers instead of
dispatching just those that are ready.  This makes aio_poll() O(n) with
respect to the total number of registered handlers.

Add a local ready_list to aio_poll() so that each nested aio_poll()
builds a list of handlers ready to be dispatched.  Since file descriptor
polling is level-triggered, nested aio_poll() calls also see fds that
were ready in the parent but not yet dispatched.  This guarantees that
nested aio_poll() invocations will dispatch all fds, even those that
became ready before the nested invocation.

Since only handlers ready to be dispatched are placed onto the
ready_list, the new aio_dispatch_ready_handlers() function provides O(1)
dispatch.

Note that AioContext polling is still O(n) and currently cannot be fully
disabled.  This still needs to be fixed before aio_poll() is fully O(1).

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Message-id: 20200214171712.541358-6-stefanha@redhat.com
[Fix compilation error on macOS where there is no epoll(87).  The
aio_epoll() prototype was out of date and aio_add_ready_list() needed to
be moved outside the ifdef.
--Stefan]
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2020-02-14 17:17:12 +00:00
parent 4749079ce0
commit 7391d34c3c
1 changed files with 79 additions and 33 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

112
util/aio-posix.c
View File

@ -35,9 +35,20 @@ struct AioHandler
void *opaque;
bool is_external;
QLIST_ENTRY(AioHandler) node;
QLIST_ENTRY(AioHandler) node_ready; /* only used during aio_poll() */
QLIST_ENTRY(AioHandler) node_deleted;
};
/* Add a handler to a ready list */
static void add_ready_handler(AioHandlerList *ready_list,
AioHandler *node,
int revents)
{
QLIST_SAFE_REMOVE(node, node_ready); /* remove from nested parent's list */
node->pfd.revents = revents;
QLIST_INSERT_HEAD(ready_list, node, node_ready);
}
#ifdef CONFIG_EPOLL_CREATE1
/* The fd number threshold to switch to epoll */
@ -105,7 +116,8 @@ static void aio_epoll_update(AioContext *ctx, AioHandler *node, bool is_new)
}
}
static int aio_epoll(AioContext *ctx, int64_t timeout)
static int aio_epoll(AioContext *ctx, AioHandlerList *ready_list,
int64_t timeout)
{
GPollFD pfd = {
.fd = ctx->epollfd,
@ -130,11 +142,13 @@ static int aio_epoll(AioContext *ctx, int64_t timeout)
}
for (i = 0; i < ret; i++) {
int ev = events[i].events;
int revents = (ev & EPOLLIN ? G_IO_IN : 0) |
(ev & EPOLLOUT ? G_IO_OUT : 0) |
(ev & EPOLLHUP ? G_IO_HUP : 0) |
(ev & EPOLLERR ? G_IO_ERR : 0);
node = events[i].data.ptr;
node->pfd.revents = (ev & EPOLLIN ? G_IO_IN : 0) |
(ev & EPOLLOUT ? G_IO_OUT : 0) |
(ev & EPOLLHUP ? G_IO_HUP : 0) |
(ev & EPOLLERR ? G_IO_ERR : 0);
add_ready_handler(ready_list, node, revents);
}
}
out:
@ -172,8 +186,8 @@ static void aio_epoll_update(AioContext *ctx, AioHandler *node, bool is_new)
{
}
static int aio_epoll(AioContext *ctx, GPollFD *pfds,
unsigned npfd, int64_t timeout)
static int aio_epoll(AioContext *ctx, AioHandlerList *ready_list,
int64_t timeout)
{
assert(false);
}
@ -438,35 +452,62 @@ static void aio_free_deleted_handlers(AioContext *ctx)
qemu_lockcnt_inc_and_unlock(&ctx->list_lock);
}
static bool aio_dispatch_handler(AioContext *ctx, AioHandler *node)
{
bool progress = false;
int revents;
revents = node->pfd.revents & node->pfd.events;
node->pfd.revents = 0;
if (!QLIST_IS_INSERTED(node, node_deleted) &&
(revents & (G_IO_IN | G_IO_HUP | G_IO_ERR)) &&
aio_node_check(ctx, node->is_external) &&
node->io_read) {
node->io_read(node->opaque);
/* aio_notify() does not count as progress */
if (node->opaque != &ctx->notifier) {
progress = true;
}
}
if (!QLIST_IS_INSERTED(node, node_deleted) &&
(revents & (G_IO_OUT | G_IO_ERR)) &&
aio_node_check(ctx, node->is_external) &&
node->io_write) {
node->io_write(node->opaque);
progress = true;
}
return progress;
}
/*
* If we have a list of ready handlers then this is more efficient than
* scanning all handlers with aio_dispatch_handlers().
*/
static bool aio_dispatch_ready_handlers(AioContext *ctx,
AioHandlerList *ready_list)
{
bool progress = false;
AioHandler *node;
while ((node = QLIST_FIRST(ready_list))) {
QLIST_SAFE_REMOVE(node, node_ready);
progress = aio_dispatch_handler(ctx, node) || progress;
}
return progress;
}
/* Slower than aio_dispatch_ready_handlers() but only used via glib */
static bool aio_dispatch_handlers(AioContext *ctx)
{
AioHandler *node, *tmp;
bool progress = false;
QLIST_FOREACH_SAFE_RCU(node, &ctx->aio_handlers, node, tmp) {
int revents;
revents = node->pfd.revents & node->pfd.events;
node->pfd.revents = 0;
if (!QLIST_IS_INSERTED(node, node_deleted) &&
(revents & (G_IO_IN | G_IO_HUP | G_IO_ERR)) &&
aio_node_check(ctx, node->is_external) &&
node->io_read) {
node->io_read(node->opaque);
/* aio_notify() does not count as progress */
if (node->opaque != &ctx->notifier) {
progress = true;
}
}
if (!QLIST_IS_INSERTED(node, node_deleted) &&
(revents & (G_IO_OUT | G_IO_ERR)) &&
aio_node_check(ctx, node->is_external) &&
node->io_write) {
node->io_write(node->opaque);
progress = true;
}
progress = aio_dispatch_handler(ctx, node) || progress;
}
return progress;
@ -639,6 +680,7 @@ static bool try_poll_mode(AioContext *ctx, int64_t *timeout)
bool aio_poll(AioContext *ctx, bool blocking)
{
AioHandlerList ready_list = QLIST_HEAD_INITIALIZER(ready_list);
AioHandler *node;
int i;
int ret = 0;
@ -689,7 +731,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
/* wait until next event */
if (aio_epoll_check_poll(ctx, pollfds, npfd, timeout)) {
npfd = 0; /* pollfds[] is not being used */
ret = aio_epoll(ctx, timeout);
ret = aio_epoll(ctx, &ready_list, timeout);
} else {
ret = qemu_poll_ns(pollfds, npfd, timeout);
}
@ -744,7 +786,11 @@ bool aio_poll(AioContext *ctx, bool blocking)
/* if we have any readable fds, dispatch event */
if (ret > 0) {
for (i = 0; i < npfd; i++) {
nodes[i]->pfd.revents = pollfds[i].revents;
int revents = pollfds[i].revents;
if (revents) {
add_ready_handler(&ready_list, nodes[i], revents);
}
}
}
@ -753,7 +799,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
progress |= aio_bh_poll(ctx);
if (ret > 0) {
progress |= aio_dispatch_handlers(ctx);
progress |= aio_dispatch_ready_handlers(ctx, &ready_list);
}
aio_free_deleted_handlers(ctx);