qemu/util/async.c
Stefan Hajnoczi 8c6b0356b5 util/async: make bh_aio_poll() O(1)
The ctx->first_bh list contains all created BHs, including those that
are not scheduled.  The list is iterated by the event loop and therefore
has O(n) time complexity with respected to the number of created BHs.

Rewrite BHs so that only scheduled or deleted BHs are enqueued.
Only BHs that actually require action will be iterated.

One semantic change is required: qemu_bh_delete() enqueues the BH and
therefore invokes aio_notify().  The
tests/test-aio.c:test_source_bh_delete_from_cb() test case assumed that
g_main_context_iteration(NULL, false) returns false after
qemu_bh_delete() but it now returns true for one iteration.  Fix up the
test case.

This patch makes aio_compute_timeout() and aio_bh_poll() drop from a CPU
profile reported by perf-top(1).  Previously they combined to 9% CPU
utilization when AioContext polling is commented out and the guest has 2
virtio-blk,num-queues=1 and 99 virtio-blk,num-queues=32 devices.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20200221093951.1414693-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2020-02-22 08:26:47 +00:00

603 lines
15 KiB
C

/*
* Data plane event loop
*
* Copyright (c) 2003-2008 Fabrice Bellard
* Copyright (c) 2009-2017 QEMU contributors
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*/
#include "qemu/osdep.h"
#include "qapi/error.h"
#include "block/aio.h"
#include "block/thread-pool.h"
#include "qemu/main-loop.h"
#include "qemu/atomic.h"
#include "qemu/rcu_queue.h"
#include "block/raw-aio.h"
#include "qemu/coroutine_int.h"
#include "trace.h"
/***********************************************************/
/* bottom halves (can be seen as timers which expire ASAP) */
/* QEMUBH::flags values */
enum {
/* Already enqueued and waiting for aio_bh_poll() */
BH_PENDING = (1 << 0),
/* Invoke the callback */
BH_SCHEDULED = (1 << 1),
/* Delete without invoking callback */
BH_DELETED = (1 << 2),
/* Delete after invoking callback */
BH_ONESHOT = (1 << 3),
/* Schedule periodically when the event loop is idle */
BH_IDLE = (1 << 4),
};
struct QEMUBH {
AioContext *ctx;
QEMUBHFunc *cb;
void *opaque;
QSLIST_ENTRY(QEMUBH) next;
unsigned flags;
};
/* Called concurrently from any thread */
static void aio_bh_enqueue(QEMUBH *bh, unsigned new_flags)
{
AioContext *ctx = bh->ctx;
unsigned old_flags;
/*
* The memory barrier implicit in atomic_fetch_or makes sure that:
* 1. idle & any writes needed by the callback are done before the
* locations are read in the aio_bh_poll.
* 2. ctx is loaded before the callback has a chance to execute and bh
* could be freed.
*/
old_flags = atomic_fetch_or(&bh->flags, BH_PENDING | new_flags);
if (!(old_flags & BH_PENDING)) {
QSLIST_INSERT_HEAD_ATOMIC(&ctx->bh_list, bh, next);
}
aio_notify(ctx);
}
/* Only called from aio_bh_poll() and aio_ctx_finalize() */
static QEMUBH *aio_bh_dequeue(BHList *head, unsigned *flags)
{
QEMUBH *bh = QSLIST_FIRST_RCU(head);
if (!bh) {
return NULL;
}
QSLIST_REMOVE_HEAD(head, next);
/*
* The atomic_and is paired with aio_bh_enqueue(). The implicit memory
* barrier ensures that the callback sees all writes done by the scheduling
* thread. It also ensures that the scheduling thread sees the cleared
* flag before bh->cb has run, and thus will call aio_notify again if
* necessary.
*/
*flags = atomic_fetch_and(&bh->flags,
~(BH_PENDING | BH_SCHEDULED | BH_IDLE));
return bh;
}
void aio_bh_schedule_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque)
{
QEMUBH *bh;
bh = g_new(QEMUBH, 1);
*bh = (QEMUBH){
.ctx = ctx,
.cb = cb,
.opaque = opaque,
};
aio_bh_enqueue(bh, BH_SCHEDULED | BH_ONESHOT);
}
QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void *opaque)
{
QEMUBH *bh;
bh = g_new(QEMUBH, 1);
*bh = (QEMUBH){
.ctx = ctx,
.cb = cb,
.opaque = opaque,
};
return bh;
}
void aio_bh_call(QEMUBH *bh)
{
bh->cb(bh->opaque);
}
/* Multiple occurrences of aio_bh_poll cannot be called concurrently. */
int aio_bh_poll(AioContext *ctx)
{
BHListSlice slice;
BHListSlice *s;
int ret = 0;
QSLIST_MOVE_ATOMIC(&slice.bh_list, &ctx->bh_list);
QSIMPLEQ_INSERT_TAIL(&ctx->bh_slice_list, &slice, next);
while ((s = QSIMPLEQ_FIRST(&ctx->bh_slice_list))) {
QEMUBH *bh;
unsigned flags;
bh = aio_bh_dequeue(&s->bh_list, &flags);
if (!bh) {
QSIMPLEQ_REMOVE_HEAD(&ctx->bh_slice_list, next);
continue;
}
if ((flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
/* Idle BHs don't count as progress */
if (!(flags & BH_IDLE)) {
ret = 1;
}
aio_bh_call(bh);
}
if (flags & (BH_DELETED | BH_ONESHOT)) {
g_free(bh);
}
}
return ret;
}
void qemu_bh_schedule_idle(QEMUBH *bh)
{
aio_bh_enqueue(bh, BH_SCHEDULED | BH_IDLE);
}
void qemu_bh_schedule(QEMUBH *bh)
{
aio_bh_enqueue(bh, BH_SCHEDULED);
}
/* This func is async.
*/
void qemu_bh_cancel(QEMUBH *bh)
{
atomic_and(&bh->flags, ~BH_SCHEDULED);
}
/* This func is async.The bottom half will do the delete action at the finial
* end.
*/
void qemu_bh_delete(QEMUBH *bh)
{
aio_bh_enqueue(bh, BH_DELETED);
}
static int64_t aio_compute_bh_timeout(BHList *head, int timeout)
{
QEMUBH *bh;
QSLIST_FOREACH_RCU(bh, head, next) {
if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
if (bh->flags & BH_IDLE) {
/* idle bottom halves will be polled at least
* every 10ms */
timeout = 10000000;
} else {
/* non-idle bottom halves will be executed
* immediately */
return 0;
}
}
}
return timeout;
}
int64_t
aio_compute_timeout(AioContext *ctx)
{
BHListSlice *s;
int64_t deadline;
int timeout = -1;
timeout = aio_compute_bh_timeout(&ctx->bh_list, timeout);
if (timeout == 0) {
return 0;
}
QSIMPLEQ_FOREACH(s, &ctx->bh_slice_list, next) {
timeout = aio_compute_bh_timeout(&s->bh_list, timeout);
if (timeout == 0) {
return 0;
}
}
deadline = timerlistgroup_deadline_ns(&ctx->tlg);
if (deadline == 0) {
return 0;
} else {
return qemu_soonest_timeout(timeout, deadline);
}
}
static gboolean
aio_ctx_prepare(GSource *source, gint *timeout)
{
AioContext *ctx = (AioContext *) source;
atomic_or(&ctx->notify_me, 1);
/* We assume there is no timeout already supplied */
*timeout = qemu_timeout_ns_to_ms(aio_compute_timeout(ctx));
if (aio_prepare(ctx)) {
*timeout = 0;
}
return *timeout == 0;
}
static gboolean
aio_ctx_check(GSource *source)
{
AioContext *ctx = (AioContext *) source;
QEMUBH *bh;
BHListSlice *s;
atomic_and(&ctx->notify_me, ~1);
aio_notify_accept(ctx);
QSLIST_FOREACH_RCU(bh, &ctx->bh_list, next) {
if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
return true;
}
}
QSIMPLEQ_FOREACH(s, &ctx->bh_slice_list, next) {
QSLIST_FOREACH_RCU(bh, &s->bh_list, next) {
if ((bh->flags & (BH_SCHEDULED | BH_DELETED)) == BH_SCHEDULED) {
return true;
}
}
}
return aio_pending(ctx) || (timerlistgroup_deadline_ns(&ctx->tlg) == 0);
}
static gboolean
aio_ctx_dispatch(GSource *source,
GSourceFunc callback,
gpointer user_data)
{
AioContext *ctx = (AioContext *) source;
assert(callback == NULL);
aio_dispatch(ctx);
return true;
}
static void
aio_ctx_finalize(GSource *source)
{
AioContext *ctx = (AioContext *) source;
QEMUBH *bh;
unsigned flags;
thread_pool_free(ctx->thread_pool);
#ifdef CONFIG_LINUX_AIO
if (ctx->linux_aio) {
laio_detach_aio_context(ctx->linux_aio, ctx);
laio_cleanup(ctx->linux_aio);
ctx->linux_aio = NULL;
}
#endif
#ifdef CONFIG_LINUX_IO_URING
if (ctx->linux_io_uring) {
luring_detach_aio_context(ctx->linux_io_uring, ctx);
luring_cleanup(ctx->linux_io_uring);
ctx->linux_io_uring = NULL;
}
#endif
assert(QSLIST_EMPTY(&ctx->scheduled_coroutines));
qemu_bh_delete(ctx->co_schedule_bh);
/* There must be no aio_bh_poll() calls going on */
assert(QSIMPLEQ_EMPTY(&ctx->bh_slice_list));
while ((bh = aio_bh_dequeue(&ctx->bh_list, &flags))) {
/* qemu_bh_delete() must have been called on BHs in this AioContext */
assert(flags & BH_DELETED);
g_free(bh);
}
aio_set_event_notifier(ctx, &ctx->notifier, false, NULL, NULL);
event_notifier_cleanup(&ctx->notifier);
qemu_rec_mutex_destroy(&ctx->lock);
qemu_lockcnt_destroy(&ctx->list_lock);
timerlistgroup_deinit(&ctx->tlg);
aio_context_destroy(ctx);
}
static GSourceFuncs aio_source_funcs = {
aio_ctx_prepare,
aio_ctx_check,
aio_ctx_dispatch,
aio_ctx_finalize
};
GSource *aio_get_g_source(AioContext *ctx)
{
g_source_ref(&ctx->source);
return &ctx->source;
}
ThreadPool *aio_get_thread_pool(AioContext *ctx)
{
if (!ctx->thread_pool) {
ctx->thread_pool = thread_pool_new(ctx);
}
return ctx->thread_pool;
}
#ifdef CONFIG_LINUX_AIO
LinuxAioState *aio_setup_linux_aio(AioContext *ctx, Error **errp)
{
if (!ctx->linux_aio) {
ctx->linux_aio = laio_init(errp);
if (ctx->linux_aio) {
laio_attach_aio_context(ctx->linux_aio, ctx);
}
}
return ctx->linux_aio;
}
LinuxAioState *aio_get_linux_aio(AioContext *ctx)
{
assert(ctx->linux_aio);
return ctx->linux_aio;
}
#endif
#ifdef CONFIG_LINUX_IO_URING
LuringState *aio_setup_linux_io_uring(AioContext *ctx, Error **errp)
{
if (ctx->linux_io_uring) {
return ctx->linux_io_uring;
}
ctx->linux_io_uring = luring_init(errp);
if (!ctx->linux_io_uring) {
return NULL;
}
luring_attach_aio_context(ctx->linux_io_uring, ctx);
return ctx->linux_io_uring;
}
LuringState *aio_get_linux_io_uring(AioContext *ctx)
{
assert(ctx->linux_io_uring);
return ctx->linux_io_uring;
}
#endif
void aio_notify(AioContext *ctx)
{
/* Write e.g. bh->scheduled before reading ctx->notify_me. Pairs
* with atomic_or in aio_ctx_prepare or atomic_add in aio_poll.
*/
smp_mb();
if (ctx->notify_me) {
event_notifier_set(&ctx->notifier);
atomic_mb_set(&ctx->notified, true);
}
}
void aio_notify_accept(AioContext *ctx)
{
if (atomic_xchg(&ctx->notified, false)
#ifdef WIN32
|| true
#endif
) {
event_notifier_test_and_clear(&ctx->notifier);
}
}
static void aio_timerlist_notify(void *opaque, QEMUClockType type)
{
aio_notify(opaque);
}
static void event_notifier_dummy_cb(EventNotifier *e)
{
}
/* Returns true if aio_notify() was called (e.g. a BH was scheduled) */
static bool event_notifier_poll(void *opaque)
{
EventNotifier *e = opaque;
AioContext *ctx = container_of(e, AioContext, notifier);
return atomic_read(&ctx->notified);
}
static void co_schedule_bh_cb(void *opaque)
{
AioContext *ctx = opaque;
QSLIST_HEAD(, Coroutine) straight, reversed;
QSLIST_MOVE_ATOMIC(&reversed, &ctx->scheduled_coroutines);
QSLIST_INIT(&straight);
while (!QSLIST_EMPTY(&reversed)) {
Coroutine *co = QSLIST_FIRST(&reversed);
QSLIST_REMOVE_HEAD(&reversed, co_scheduled_next);
QSLIST_INSERT_HEAD(&straight, co, co_scheduled_next);
}
while (!QSLIST_EMPTY(&straight)) {
Coroutine *co = QSLIST_FIRST(&straight);
QSLIST_REMOVE_HEAD(&straight, co_scheduled_next);
trace_aio_co_schedule_bh_cb(ctx, co);
aio_context_acquire(ctx);
/* Protected by write barrier in qemu_aio_coroutine_enter */
atomic_set(&co->scheduled, NULL);
qemu_aio_coroutine_enter(ctx, co);
aio_context_release(ctx);
}
}
AioContext *aio_context_new(Error **errp)
{
int ret;
AioContext *ctx;
ctx = (AioContext *) g_source_new(&aio_source_funcs, sizeof(AioContext));
QSLIST_INIT(&ctx->bh_list);
QSIMPLEQ_INIT(&ctx->bh_slice_list);
aio_context_setup(ctx);
ret = event_notifier_init(&ctx->notifier, false);
if (ret < 0) {
error_setg_errno(errp, -ret, "Failed to initialize event notifier");
goto fail;
}
g_source_set_can_recurse(&ctx->source, true);
qemu_lockcnt_init(&ctx->list_lock);
ctx->co_schedule_bh = aio_bh_new(ctx, co_schedule_bh_cb, ctx);
QSLIST_INIT(&ctx->scheduled_coroutines);
aio_set_event_notifier(ctx, &ctx->notifier,
false,
event_notifier_dummy_cb,
event_notifier_poll);
#ifdef CONFIG_LINUX_AIO
ctx->linux_aio = NULL;
#endif
#ifdef CONFIG_LINUX_IO_URING
ctx->linux_io_uring = NULL;
#endif
ctx->thread_pool = NULL;
qemu_rec_mutex_init(&ctx->lock);
timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
ctx->poll_ns = 0;
ctx->poll_max_ns = 0;
ctx->poll_grow = 0;
ctx->poll_shrink = 0;
return ctx;
fail:
g_source_destroy(&ctx->source);
return NULL;
}
void aio_co_schedule(AioContext *ctx, Coroutine *co)
{
trace_aio_co_schedule(ctx, co);
const char *scheduled = atomic_cmpxchg(&co->scheduled, NULL,
__func__);
if (scheduled) {
fprintf(stderr,
"%s: Co-routine was already scheduled in '%s'\n",
__func__, scheduled);
abort();
}
/* The coroutine might run and release the last ctx reference before we
* invoke qemu_bh_schedule(). Take a reference to keep ctx alive until
* we're done.
*/
aio_context_ref(ctx);
QSLIST_INSERT_HEAD_ATOMIC(&ctx->scheduled_coroutines,
co, co_scheduled_next);
qemu_bh_schedule(ctx->co_schedule_bh);
aio_context_unref(ctx);
}
void aio_co_wake(struct Coroutine *co)
{
AioContext *ctx;
/* Read coroutine before co->ctx. Matches smp_wmb in
* qemu_coroutine_enter.
*/
smp_read_barrier_depends();
ctx = atomic_read(&co->ctx);
aio_co_enter(ctx, co);
}
void aio_co_enter(AioContext *ctx, struct Coroutine *co)
{
if (ctx != qemu_get_current_aio_context()) {
aio_co_schedule(ctx, co);
return;
}
if (qemu_in_coroutine()) {
Coroutine *self = qemu_coroutine_self();
assert(self != co);
QSIMPLEQ_INSERT_TAIL(&self->co_queue_wakeup, co, co_queue_next);
} else {
aio_context_acquire(ctx);
qemu_aio_coroutine_enter(ctx, co);
aio_context_release(ctx);
}
}
void aio_context_ref(AioContext *ctx)
{
g_source_ref(&ctx->source);
}
void aio_context_unref(AioContext *ctx)
{
g_source_unref(&ctx->source);
}
void aio_context_acquire(AioContext *ctx)
{
qemu_rec_mutex_lock(&ctx->lock);
}
void aio_context_release(AioContext *ctx)
{
qemu_rec_mutex_unlock(&ctx->lock);
}