aio-posix: disable fdmon-io_uring when GSource is used

The glib event loop does not call fdmon_io_uring_wait() so fd handlers
waiting to be submitted build up in the list. There is no benefit is
using io_uring when the glib GSource is being used, so disable it
instead of implementing a more complex fix.

This fixes a memory leak where AioHandlers would build up and increasing
amounts of CPU time were spent iterating them in aio_pending(). The
symptom is that guests become slow when QEMU is built with io_uring
support.

Buglink: https://bugs.launchpad.net/qemu/+bug/1877716
Fixes: 73fd282e7b6dd4e4ea1c3bbb3d302c8db51e4ccf ("aio-posix: add io_uring fd monitoring implementation")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Oleksandr Natalenko <oleksandr@redhat.com>
Message-id: 20200511183630.279750-3-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

include/block/aio.h

@@ -701,6 +701,9 @@ void aio_context_setup(AioContext *ctx);
  */
 void aio_context_destroy(AioContext *ctx);
 
+/* Used internally, do not call outside AioContext code */
+void aio_context_use_g_source(AioContext *ctx);
+
 /**
  * aio_context_set_poll_params:
  * @ctx: the aio context

util/aio-posix.c

@@ -682,6 +682,18 @@ void aio_context_destroy(AioContext *ctx)
     aio_free_deleted_handlers(ctx);
 }
 
+void aio_context_use_g_source(AioContext *ctx)
+{
+    /*
+     * Disable io_uring when the glib main loop is used because it doesn't
+     * support mixed glib/aio_poll() usage. It relies on aio_poll() being
+     * called regularly so that changes to the monitored file descriptors are
+     * submitted, otherwise a list of pending fd handlers builds up.
+     */
+    fdmon_io_uring_destroy(ctx);
+    aio_free_deleted_handlers(ctx);
+}
+
 void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
                                  int64_t grow, int64_t shrink, Error **errp)
 {

util/aio-win32.c

@@ -414,6 +414,10 @@ void aio_context_destroy(AioContext *ctx)
 {
 }
 
+void aio_context_use_g_source(AioContext *ctx)
+{
+}
+
 void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,
                                  int64_t grow, int64_t shrink, Error **errp)
 {

util/async.c

@@ -362,6 +362,7 @@ static GSourceFuncs aio_source_funcs = {
 
 GSource *aio_get_g_source(AioContext *ctx)
 {
+    aio_context_use_g_source(ctx);
     g_source_ref(&ctx->source);
     return &ctx->source;
 }