aio: add flag to skip fds to aio_dispatch()

Polling mode will not call ppoll(2)/epoll_wait(2).  Therefore we know
there are no fds ready and should avoid looping over fd handlers in
aio_dispatch().
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-id: 20161201192652.9509-2-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

aio-posix.c

@@ -290,9 +290,13 @@ bool aio_pending(AioContext *ctx)
     return false;
 }
 
-bool aio_dispatch(AioContext *ctx)
+/*
+ * Note that dispatch_fds == false has the side-effect of post-poning the
+ * freeing of deleted handlers.
+ */
+bool aio_dispatch(AioContext *ctx, bool dispatch_fds)
 {
-    AioHandler *node;
+    AioHandler *node = NULL;
     bool progress = false;
 
     /*
@@ -308,7 +312,9 @@ bool aio_dispatch(AioContext *ctx)
      * We have to walk very carefully in case aio_set_fd_handler is
      * called while we're walking.
      */
-    node = QLIST_FIRST(&ctx->aio_handlers);
+    if (dispatch_fds) {
+        node = QLIST_FIRST(&ctx->aio_handlers);
+    }
     while (node) {
         AioHandler *tmp;
         int revents;
@@ -473,7 +479,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
     ctx->walking_handlers--;
 
     /* Run dispatch even if there were no readable fds to run timers */
-    if (aio_dispatch(ctx)) {
+    if (aio_dispatch(ctx, ret > 0)) {
         progress = true;
     }
 

aio-win32.c

@@ -271,12 +271,14 @@ static bool aio_dispatch_handlers(AioContext *ctx, HANDLE event)
     return progress;
 }
 
-bool aio_dispatch(AioContext *ctx)
+bool aio_dispatch(AioContext *ctx, bool dispatch_fds)
 {
     bool progress;
 
     progress = aio_bh_poll(ctx);
-    progress |= aio_dispatch_handlers(ctx, INVALID_HANDLE_VALUE);
+    if (dispatch_fds) {
+        progress |= aio_dispatch_handlers(ctx, INVALID_HANDLE_VALUE);
+    }
     progress |= timerlistgroup_run_timers(&ctx->tlg);
     return progress;
 }

async.c

@@ -251,7 +251,7 @@ aio_ctx_dispatch(GSource     *source,
     AioContext *ctx = (AioContext *) source;
 
     assert(callback == NULL);
-    aio_dispatch(ctx);
+    aio_dispatch(ctx, true);
     return true;
 }
 

include/block/aio.h

@@ -295,8 +295,12 @@ bool aio_pending(AioContext *ctx);
 /* Dispatch any pending callbacks from the GSource attached to the AioContext.
  *
  * This is used internally in the implementation of the GSource.
+ *
+ * @dispatch_fds: true to process fds, false to skip them
+ *                (can be used as an optimization by callers that know there
+ *                are no fds ready)
  */
-bool aio_dispatch(AioContext *ctx);
+bool aio_dispatch(AioContext *ctx, bool dispatch_fds);
 
 /* Progress in completing AIO work to occur.  This can issue new pending
  * aio as a result of executing I/O completion or bh callbacks.