path: root/async.c
diff options
authorPaolo Bonzini <pbonzini@redhat.com>2014-07-07 15:18:04 +0200
committerKevin Wolf <kwolf@redhat.com>2014-07-09 15:50:11 +0200
commit0ceb849bd336a5f9b6e1ed56d45cf5773d251ad8 (patch)
tree970ee4f04a48eba0894602d28d2f70d23aeb532a /async.c
parentef508f427b348c7f0ef2bfe7c080fe5fcaee9f6b (diff)
AioContext: speed up aio_notify
In many cases, the call to event_notifier_set in aio_notify is unnecessary. In particular, if we are executing aio_dispatch, or if aio_poll is not blocking, we know that we will soon get to the next loop iteration (if necessary); the thread that hosts the AioContext's event loop does not need any nudging. The patch includes a Promela formal model that shows that this really works and does not need any further complication such as generation counts. It needs a memory barrier though. The generation counts are not needed because any change to ctx->dispatching after the memory barrier is okay for aio_notify. If it changes from zero to one, it is the right thing to skip event_notifier_set. If it changes from one to zero, the event_notifier_set is unnecessary but harmless. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Diffstat (limited to 'async.c')
1 files changed, 18 insertions, 1 deletions
diff --git a/async.c b/async.c
index 5b6fe6b4cc..34af0b25ca 100644
--- a/async.c
+++ b/async.c
@@ -26,6 +26,7 @@
#include "block/aio.h"
#include "block/thread-pool.h"
#include "qemu/main-loop.h"
+#include "qemu/atomic.h"
/* bottom halves (can be seen as timers which expire ASAP) */
@@ -247,9 +248,25 @@ ThreadPool *aio_get_thread_pool(AioContext *ctx)
return ctx->thread_pool;
+void aio_set_dispatching(AioContext *ctx, bool dispatching)
+ ctx->dispatching = dispatching;
+ if (!dispatching) {
+ /* Write ctx->dispatching before reading e.g. bh->scheduled.
+ * Optimization: this is only needed when we're entering the "unsafe"
+ * phase where other threads must call event_notifier_set.
+ */
+ smp_mb();
+ }
void aio_notify(AioContext *ctx)
- event_notifier_set(&ctx->notifier);
+ /* Write e.g. bh->scheduled before reading ctx->dispatching. */
+ smp_mb();
+ if (!ctx->dispatching) {
+ event_notifier_set(&ctx->notifier);
+ }
static void aio_timerlist_notify(void *opaque)