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[Qemu-devel] [PATCH 17/40] iothread: release AioContext around aio_poll
From: |
Paolo Bonzini |
Subject: |
[Qemu-devel] [PATCH 17/40] iothread: release AioContext around aio_poll |
Date: |
Tue, 24 Nov 2015 19:01:08 +0100 |
This is the first step towards having fine-grained critical sections in
dataplane threads, which resolves lock ordering problems between
address_space_* functions (which need the BQL when doing MMIO, even
after we complete RCU-based dispatch) and the AioContext.
Because AioContext does not use contention callbacks anymore, the
unit test has to be changed.
Previously applied as a0710f7995f914e3044e5899bd8ff6c43c62f916 and
then reverted.
Reviewed-by: Stefan Hajnoczi <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
---
async.c | 22 +++-------------------
docs/multiple-iothreads.txt | 25 +++++++++++++++----------
include/block/aio.h | 3 ---
iothread.c | 11 ++---------
tests/test-aio.c | 19 +++++++++++--------
5 files changed, 31 insertions(+), 49 deletions(-)
diff --git a/async.c b/async.c
index e106072..f70d23a 100644
--- a/async.c
+++ b/async.c
@@ -84,8 +84,8 @@ int aio_bh_poll(AioContext *ctx)
* aio_notify again if necessary.
*/
if (!bh->deleted && atomic_xchg(&bh->scheduled, 0)) {
- /* Idle BHs and the notify BH don't count as progress */
- if (!bh->idle && bh != ctx->notify_dummy_bh) {
+ /* Idle BHs don't count as progress */
+ if (!bh->idle) {
ret = 1;
}
bh->idle = 0;
@@ -237,7 +237,6 @@ aio_ctx_finalize(GSource *source)
{
AioContext *ctx = (AioContext *) source;
- qemu_bh_delete(ctx->notify_dummy_bh);
thread_pool_free(ctx->thread_pool);
qemu_mutex_lock(&ctx->bh_lock);
@@ -304,19 +303,6 @@ static void aio_timerlist_notify(void *opaque)
aio_notify(opaque);
}
-static void aio_rfifolock_cb(void *opaque)
-{
- AioContext *ctx = opaque;
-
- /* Kick owner thread in case they are blocked in aio_poll() */
- qemu_bh_schedule(ctx->notify_dummy_bh);
-}
-
-static void notify_dummy_bh(void *opaque)
-{
- /* Do nothing, we were invoked just to force the event loop to iterate */
-}
-
static void event_notifier_dummy_cb(EventNotifier *e)
{
}
@@ -345,11 +331,9 @@ AioContext *aio_context_new(Error **errp)
event_notifier_dummy_cb);
ctx->thread_pool = NULL;
qemu_mutex_init(&ctx->bh_lock);
- rfifolock_init(&ctx->lock, aio_rfifolock_cb, ctx);
+ rfifolock_init(&ctx->lock, NULL, NULL);
timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
- ctx->notify_dummy_bh = aio_bh_new(ctx, notify_dummy_bh, NULL);
-
return ctx;
fail:
g_source_destroy(&ctx->source);
diff --git a/docs/multiple-iothreads.txt b/docs/multiple-iothreads.txt
index 40b8419..723cc7e 100644
--- a/docs/multiple-iothreads.txt
+++ b/docs/multiple-iothreads.txt
@@ -105,13 +105,10 @@ a BH in the target AioContext beforehand and then call
qemu_bh_schedule(). No
acquire/release or locking is needed for the qemu_bh_schedule() call. But be
sure to acquire the AioContext for aio_bh_new() if necessary.
-The relationship between AioContext and the block layer
--------------------------------------------------------
-The AioContext originates from the QEMU block layer because it provides a
-scoped way of running event loop iterations until all work is done. This
-feature is used to complete all in-flight block I/O requests (see
-bdrv_drain_all()). Nowadays AioContext is a generic event loop that can be
-used by any QEMU subsystem.
+AioContext and the block layer
+------------------------------
+The AioContext originates from the QEMU block layer, even though nowadays
+AioContext is a generic event loop that can be used by any QEMU subsystem.
The block layer has support for AioContext integrated. Each BlockDriverState
is associated with an AioContext using bdrv_set_aio_context() and
@@ -122,9 +119,17 @@ Block layer code must therefore expect to run in an
IOThread and avoid using
old APIs that implicitly use the main loop. See the "How to program for
IOThreads" above for information on how to do that.
-If main loop code such as a QMP function wishes to access a BlockDriverState it
-must first call aio_context_acquire(bdrv_get_aio_context(bs)) to ensure the
-IOThread does not run in parallel.
+If main loop code such as a QMP function wishes to access a BlockDriverState
+it must first call aio_context_acquire(bdrv_get_aio_context(bs)) to ensure
+that callbacks in the IOThread do not run in parallel.
+
+Code running in the monitor typically uses bdrv_drain() to ensure that
+past requests from the guest are completed. When a block device is
+running in an IOThread, the IOThread can also process requests from
+the guest (via ioeventfd). These requests have to be blocked with
+aio_disable_clients() before calling bdrv_drain(). You can then reenable
+guest requests with aio_enable_clients() before finally releasing the
+AioContext and completing the monitor command.
Long-running jobs (usually in the form of coroutines) are best scheduled in the
BlockDriverState's AioContext to avoid the need to acquire/release around each
diff --git a/include/block/aio.h b/include/block/aio.h
index e086e3b..b41e291 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -114,9 +114,6 @@ struct AioContext {
bool notified;
EventNotifier notifier;
- /* Scheduling this BH forces the event loop it iterate */
- QEMUBH *notify_dummy_bh;
-
/* Thread pool for performing work and receiving completion callbacks */
struct ThreadPool *thread_pool;
diff --git a/iothread.c b/iothread.c
index 1b8c2bb..df30f5e 100644
--- a/iothread.c
+++ b/iothread.c
@@ -30,7 +30,6 @@ typedef ObjectClass IOThreadClass;
static void *iothread_run(void *opaque)
{
IOThread *iothread = opaque;
- bool blocking;
rcu_register_thread();
@@ -39,14 +38,8 @@ static void *iothread_run(void *opaque)
qemu_cond_signal(&iothread->init_done_cond);
qemu_mutex_unlock(&iothread->init_done_lock);
- while (!iothread->stopping) {
- aio_context_acquire(iothread->ctx);
- blocking = true;
- while (!iothread->stopping && aio_poll(iothread->ctx, blocking)) {
- /* Progress was made, keep going */
- blocking = false;
- }
- aio_context_release(iothread->ctx);
+ while (!atomic_read(&iothread->stopping)) {
+ aio_poll(iothread->ctx, true);
}
rcu_unregister_thread();
diff --git a/tests/test-aio.c b/tests/test-aio.c
index 1623803..cb64e3b 100644
--- a/tests/test-aio.c
+++ b/tests/test-aio.c
@@ -99,6 +99,7 @@ static void event_ready_cb(EventNotifier *e)
typedef struct {
QemuMutex start_lock;
+ EventNotifier notifier;
bool thread_acquired;
} AcquireTestData;
@@ -110,6 +111,8 @@ static void *test_acquire_thread(void *opaque)
qemu_mutex_lock(&data->start_lock);
qemu_mutex_unlock(&data->start_lock);
+ g_usleep(500000);
+ event_notifier_set(&data->notifier);
aio_context_acquire(ctx);
aio_context_release(ctx);
@@ -124,20 +127,19 @@ static void set_event_notifier(AioContext *ctx,
EventNotifier *notifier,
aio_set_event_notifier(ctx, notifier, false, handler);
}
-static void dummy_notifier_read(EventNotifier *unused)
+static void dummy_notifier_read(EventNotifier *n)
{
- g_assert(false); /* should never be invoked */
+ event_notifier_test_and_clear(n);
}
static void test_acquire(void)
{
QemuThread thread;
- EventNotifier notifier;
AcquireTestData data;
/* Dummy event notifier ensures aio_poll() will block */
- event_notifier_init(¬ifier, false);
- set_event_notifier(ctx, ¬ifier, dummy_notifier_read);
+ event_notifier_init(&data.notifier, false);
+ set_event_notifier(ctx, &data.notifier, dummy_notifier_read);
g_assert(!aio_poll(ctx, false)); /* consume aio_notify() */
qemu_mutex_init(&data.start_lock);
@@ -151,12 +153,13 @@ static void test_acquire(void)
/* Block in aio_poll(), let other thread kick us and acquire context */
aio_context_acquire(ctx);
qemu_mutex_unlock(&data.start_lock); /* let the thread run */
- g_assert(!aio_poll(ctx, true));
+ g_assert(aio_poll(ctx, true));
+ g_assert(!data.thread_acquired);
aio_context_release(ctx);
qemu_thread_join(&thread);
- set_event_notifier(ctx, ¬ifier, NULL);
- event_notifier_cleanup(¬ifier);
+ set_event_notifier(ctx, &data.notifier, NULL);
+ event_notifier_cleanup(&data.notifier);
g_assert(data.thread_acquired);
}
--
1.8.3.1
- [Qemu-devel] [PATCH 05/40] virtio: read/write the VirtQueueElement a field at a time, (continued)
- [Qemu-devel] [PATCH 05/40] virtio: read/write the VirtQueueElement a field at a time, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 12/40] virtio: export vring_notify as virtio_should_notify, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 10/40] virtio: combine the read of a descriptor, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 11/40] virtio: add AioContext-specific function for host notifiers, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 14/40] virtio-blk: do not use vring in dataplane, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 15/40] virtio-scsi: do not use vring in dataplane, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 16/40] vring: remove, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 20/40] aio: rename bh_lock to list_lock, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 22/40] aio: make ctx->list_lock a QemuLockCnt, subsuming ctx->walking_bh, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 19/40] aio: convert from RFifoLock to QemuRecMutex, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 17/40] iothread: release AioContext around aio_poll,
Paolo Bonzini <=
- [Qemu-devel] [PATCH 18/40] qemu-thread: introduce QemuRecMutex, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 24/40] aio: tweak walking in dispatch phase, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 27/40] aio: document locking, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 23/40] qemu-thread: optimize QemuLockCnt with futexes on Linux, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 21/40] qemu-thread: introduce QemuLockCnt, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 26/40] aio-win32: remove walking_handlers, protecting AioHandler list with list_lock, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 25/40] aio-posix: remove walking_handlers, protecting AioHandler list with list_lock, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 29/40] quorum: use atomics for rewrite_count, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 30/40] quorum: split quorum_fifo_aio_cb from quorum_aio_cb, Paolo Bonzini, 2015/11/24
- [Qemu-devel] [PATCH 28/40] aio: push aio_context_acquire/release down to dispatching, Paolo Bonzini, 2015/11/24