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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 /include
parentef508f427b348c7f0ef2bfe7c080fe5fcaee9f6b (diff)
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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 'include')
-rw-r--r--include/block/aio.h9
1 files changed, 9 insertions, 0 deletions
diff --git a/include/block/aio.h b/include/block/aio.h
index d81250c..433e7ff 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -60,8 +60,14 @@ struct AioContext {
*/
int walking_handlers;
+ /* Used to avoid unnecessary event_notifier_set calls in aio_notify.
+ * Writes protected by lock or BQL, reads are lockless.
+ */
+ bool dispatching;
+
/* lock to protect between bh's adders and deleter */
QemuMutex bh_lock;
+
/* Anchor of the list of Bottom Halves belonging to the context */
struct QEMUBH *first_bh;
@@ -83,6 +89,9 @@ struct AioContext {
QEMUTimerListGroup tlg;
};
+/* Used internally to synchronize aio_poll against qemu_bh_schedule. */
+void aio_set_dispatching(AioContext *ctx, bool dispatching);
+
/**
* aio_context_new: Allocate a new AioContext.
*